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vate, which can then form glucose to increase blood sugar levels. 45. What is the evolutionary significance of glycogen production? a. Excess ATP and glucose produce glycogen, which can be used at a later point in time to act as co-factor if, for example, a good source is scarce. b. Excess proteins and glucose produce glycogen, which can be used at a later point in time to produce energy if, for example, food is scarce. c. Excess ATP and glucose produce glycogen, which can be used at a later point in time to produce energy if, for example, food is scarce. d. Excess proteins and fats produce glycogen, which can be used at a later point in time to act as source of nitrogen if, for example, a good source is scarce. 46. How can eating too much bread and pasta physiologically promote obesity? 1098 Chapter 25 | Animal Nutrition and the Digestive System 49. What happens to undigested food after the water is reabsorbed? a. Undigested food is moved through the colon, where intestinal flora aid in digestion by peristalsis, and then stored in the rectum until elimination through the anus. b. Undigested food is moved through the colon, where intestinal flora aid in digestion by peristalsis; further absorption takes place in the rectum, after which it stores the food until elimination through the anus. c. Undigested food is moved through the colon, where intestinal flora aid in digestion by segmentation, and then it is stored in the rectum until elimination through the anus. d. Undigested food is moved through the ileum, where intestinal flora aid in digestion by peristalsis, and then it is stored in the rectum until elimination through the anus. 50. a. What are micelles? b. Why are micelles integral to lipid absorption? a. a. Micelles are lipoproteins designed for the transport of lipids that enter lacteals. b. Micelles facilitate absorption by microvilli, where the fatty acids and proteins diffuse out to form lipoproteins. b. a. Micelles are lipoproteins designed for the transport of lipids that enter lacteals. b. Micelles facilitate absorption by microvilli, where the fatty acids and monoglycerides diffuse out to form triglycerides. c. a. Micelles are |
bile salt–surrounded fatty acids and phospholipids. b. Micelles facilitate absorption by microvilli, where the fatty acids and monoglycerides diffuse out to form triglycerides. d. a. Micelles are bile salt–surrounded fatty acids and monoglycerides. b. Micelles facilitate absorption by microvilli, where the fatty acids and monoglycerides diffuse out to form triglycerides. 51. On a cellular level, why must food be broken down? a. Excess blood glucose increases the amount of urea, which is converted into fatty acids. Fatty acids are stored in areolar cells, which increase the amount of body fat. b. Excess blood glucose increases the amount of pyruvate, which is converted into fatty acids. Fatty acids are stored in adipose cells, which increase the amount of body fat. c. Bread and pasta are rich in fats. Their digestion produces fatty acids and glycerol. Fatty acids are stored in adipose cells, which increase the amount of body fat. d. Bread and pasta are rich in fats. Their digestion produces fatty acids and glycerol. Fatty acids are stored in areolar cells, which increase the amount of body fat. 47. How do ingestion and digestion differ? a. b. c. d. Ingestion is taking food in through mouth, where mechanical digestion begins. Chemical digestion begins in the stomach, where food is further broken down into smaller molecules that can be absorbed and used by the body. Ingestion is the process of taking in food through the mouth, where mechanical and chemical digestion begins to break down the food into smaller molecules that can be absorbed and used by the body. Ingestion is taking food in through the mouth, where mechanical and chemical digestion begins. Digestion in the stomach breaks down proteins and fats present in food into smaller molecules that can be absorbed and used by the body. Ingestion is the transfer of food from the mouth to the esophagus, where mechanical and chemical digestion begin to break down the food into smaller molecules that can be absorbed and used by the body. 48. Why are some dietary lipids a necessary part of a balanced diet? a. Dietary lipids aid in the absorption of water- soluble vitamins, including B and C, which are needed for various bodily functions. b. Dietary lipids aid in the absorption of some minerals, including folic acid, iron, and magnesium, |
which are needed for various bodily functions. c. Dietary lipids aid in the absorption of vitamins, including A, B, C, D, E, and K, which are needed for various bodily functions d. Dietary lipids aid in the absorption of fat-soluble vitamins, including A, D, E, and K, which are needed for various bodily functions. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1099 a. Large molecules present in intact food pass 53. How do hormones regulate digestion? a. Hormones regulate aspects of digestion such as increasing the peristaltic movements in the esophagus when food is sensed. b. Hormones regulate digestion by signaling when the stomach is full or empty so that an individual will consume food or stop eating. c. Hormones like gastrin, secretin, adrenocorticotropic are released from the pituitary to regulate which digestive secretions are released. d. Hormones regulate aspects of digestion such as which digestive secretions are released as well as when they are released. 54. When you are eating a meal, how do you know when you are full? a. The pituitary gland release hormones when the stomach is full, which therefore reduces hunger. b. The brain signals when the stomach is full that you are satiated, which therefore reduces hunger. c. The stomach signals when it is full, which therefore reduces hunger. d. Low blood sugar levels stimulate a neurotransmitter, which sends a signal to the brain when the stomach is full and therefore reduces hunger. through the digestive epithelium and enter the cell through the membrane, thereby damaging the nuclear membrane. Hence it must be broken down. b. Fats present in intact food contain very large molecules, which cannot pass through cell membranes. Fats need to be passed through the digestive epithelium to be utilized. c. Large molecules present in intact food cannot pass through cell membranes. Nutrients need to be passed through the digestive epithelium to be utilized. d. Large molecules, if not broken down, produce toxic substances that pass through the epithelium of the digestive tract and are utilized by the cells. This can be lethal to the cell. 52. What is the importance of neural responses to food stimuli? a. Neural responses facilitate secretion of fumarase needed for chemical digestion of food as well as other |
involuntary responses like peristalsis. b. Neural responses facilitate secretion of enzymes that are needed to digest or break down food as well as other involuntary responses like segmentation in stomach. c. Neural responses facilitate secretion of enzymes needed to digest or break down food as well as other involuntary responses like peristalsis. d. Neural responses facilitate secretion of salivary amylase needed to digest or break down food as well as secretion of hormones like secretin and gastrin. TEST PREP FOR AP® COURSES 55. Simple cuboidal epithelial cells line the ducts of certain human exocrine glands. Various materials are transported into or out of the cells by diffusion. (The formula for the surface area of a cube is 6 × S2, and the formula for the volume of a cube is S3, where S = the length of a side of a cube.) Which of the following cubeshaped cells would be most efficient in removing waste by diffusion? 1100 Chapter 25 | Animal Nutrition and the Digestive System a. Nutrients can enter the bloodstream through the blood vessels that are located in middle of the microvilli. b. Larger microvilli have more surface area over which more nutrients are absorbed. c. The microvilli projections aid in mechanical digestion of food particles. d. The finger-like projections prevent large particles of food from passing through the digestive system. 58. Microvilli greatly increase the efficiency of nutrient uptake in the small intestines. How do the size and shape of microvilli promote this efficiency? a. They have a greater surface area-to-volume ratio than larger cells. The finger-like projection shape provides more surface area over the small intestines from which they absorb nutrients and contains blood vessels so nutrients passing through them can enter the bloodstream readily. b. They have a greater surface area-to-volume ratio than larger cells. The finger-like projection shape is present in the middle of microvilli, which have more surface area over the small intestines from which they absorb nutrients and also contains blood vessels so nutrients can enter the blood easily. c. They have a greater surface area-to-volume ratio than larger cells. The finger-like projections prevent large particles of food from passing through the digestive system and also contain blood vessels so nutrients passed through them can readily enter the bloodstream. d. They have a greater surface area-to-volume ratio than larger cells. The finger-like projections aid in mechanical digestion of food particles and contain blood vessels |
so nutrients passing through them can enter the bloodstream readily. 59. Birds have several unique physical differences from other vertebrates, and several pertain to how birds process food. Some differences are obvious, such as the presence of a beak and no teeth, whereas other differences can be observed in their internal features. For example, birds have a monogastric digestive system like most other vertebrates, but their digestive system structure differs from that of most other monogastric vertebrates. Which of the following is true about how birds process food? a. 10 µm b. 20 µm c. 30 µm d. 40 µm 56. Celiac disease is dangerous in affected individuals, because ingesting gluten damages the villi of the small intestines. Why is this potentially life threatening? a. The villi aid in mechanical digestion of food particles. When they are damaged, nutrients cannot be digested properly in the body. b. Villi increase the surface area of the small intestine, which aids in the absorption of bile salts. This nutrient cannot be absorbed when they are damaged. c. Villi decrease the surface area of the small intestine available for absorption. Nutrients cannot properly enter the bloodstream when they are damaged. d. Villi increase the surface area available for nutrient absorption. When villi are damaged, nutrients cannot properly enter the bloodstream. 57. One of the key features of villi and microvilli in the digestive system is their finger-like projection shape. Which of the following is an example of how the shape of microvilli can enhance nutrient absorption? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1101 a. Beak emergence coincided with insect inclusion in the bird diet. b. The gizzard is the primary site of mechanical digestion. c. Birds excrete nitrogenous waste and feces through separate openings. d. Birds digest plant material more slowly than ruminants. 60. As shown in this figure, the oral cavity has several components that contribute to ingestion and the initial stages of digestion. How do the components of the oral cavity work together to complete the first step of food processing? a. The teeth and jaw mechanically chew the food, and saliva from the salivary glands moistens the food and begins chemical digestion. The tongue then physically moves the food to the pharynx, where peristalsis moves the food into the stomach |
. b. The teeth and jaw mechanically chew the food, and saliva from the salivary glands moistens the food and initiates mechanical and chemical digestion. The tongue then physically moves the food to the pharynx, where peristalsis moves the food into the stomach. c. The teeth and jaw mechanically chew the food, and saliva from the salivary glands moistens the food and begins chemical digestion. The tongue then physically moves the food to the larynx, where peristalsis moves the food into the stomach. d. The teeth and jaw mechanically chew the food, and saliva from the salivary glands moistens the food and initiates mechanical and chemical digestion. The tongue then physically moves the food to the pharynx, where segmentation moves the food into the stomach. 61. Most mammals have a monogastric digestive system, which means they have one stomach chamber. Ruminants and pseudo-ruminants consume a large amount of plant material and have polygastric digestive systems, which means they have more than one stomach chamber. Why is an increased number of stomach chambers beneficial for ruminants and pseudo-ruminants? a. Microbes in the chambers break down and ferment plant material. b. Extended exposure to stomach acid breaks down more cellulose. c. Increased amounts of peristalsis crush more of the plant fibers. d. Having more stomach chambers increases exposure for nutrients to be absorbed. 62. This figure shows the majority of the digestive tracts of two organisms that consume different food sources. a. Which digestive tract belongs to the herbivore? b. How did you determine this? 1102 Chapter 25 | Animal Nutrition and the Digestive System a. a. The digestive tract shown at the bottom belongs to the herbivore. b. Herbivores have a shorter intestinal tract, which allows stronger smooth muscle contractions called peristalsis in a shorter area, providing more opportunity for nutrients to be obtained and absorbed. b. a. The digestive tract shown at the top belongs to the herbivore. b. Herbivores have a longer intestinal tract, which provides more opportunity for nutrients to be obtained and absorbed, since plant material is difficult for animals to break down. c. a. The digestive tract shown at the bottom belongs to the herbivore. b. Herbivores have a longer intestinal tract, which provides more opportunity for the nutrients to react with the intestinal enzymes for better absorption, since plant material is difficult for animals to break |
down. d. a. The digestive tract shown at the bottom belongs to the herbivore. b. Herbivores have a shorter intestinal tract, which provides more opportunity for nutrients to be obtained and absorbed, since plant material is difficult for animals to break down. 63. The ruminant digestive system has evolved several differences from the traditional mammalian monogastric digestive system because they consume large amounts of plant material. Which of the following is NOT a component of the ruminant digestive system that has evolved to more efficiently digest plant fibers? a. omasum b. abomasum c. reticulum d. gizzard 64. This figure shows the three main components of the large intestine. How do these three parts contribute to processing as food material passes through the large intestine? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1103 a. The cecum receives semi-solid waste from the small intestine and absorbs water, vitamins, and minerals. Then the colon further digests some material. The rectum stores the fecal matter until it is excreted. b. The cecum receives semi-solid waste from the small intestine. Then the colon digests some materials. The rectum absorbs water and some vitamins and minerals and then stores the fecal matter until it is excreted. c. The cecum receives semi-solid waste from small intestine. Then, the colon absorbs water and some vitamins and minerals, and further digests some material. The rectum stores the fecal matter until it is excreted. d. The cecum receives semi-solid waste from the small intestine. The colon is the only region where absorption of vitamins takes place in the digestive system. The rectum stores the fecal matter until it is excreted. 65. This figure shows involuntary muscle movement in part of the digestive system. What stimulates this involuntary response? a. b. smelling food seeing food c. chewing food d. swallowing food 66. This image shows the digestive system of a ruminant animal. How does this polygastric digestive system enhance digestion efficiency in ruminants? a. Multiple stomach chambers in ruminant animals contain microbes that have cellulase, which breaks down plant material. Plant material is difficult to digest because animals lack cellulase to break down cellulose. b. Multiple stomach chambers in ruminant animals allow stronger |
smooth muscle contractions, which break down plant material. Plant material is difficult to digest because animals lack cellulase to break down cellulose. c. Multiple stomach chambers present in ruminant animals contain cellulase, which break down plant material. Plant material is difficult to digest because animals lack cellulase to break down cellulose. d. Multiple stomach chambers in ruminant animals allow the food to stay in the stomach for a longer time so that peristaltic movements and the action of enzymes on food particles occurs for a longer time. 67. According to these data, mice at 10°C demonstrated 1104 Chapter 25 | Animal Nutrition and the Digestive System greater oxygen consumption per gram of tissue than mice at 25°C. Which of the following statements best explains the observation? a. The mice at 10°C had a higher rate of ATP production than the mice at 25°C. b. The mice at 10°C had a lower metabolic rate than the mice at 25°C. c. The mice at 25°C weighed less than the mice at 10°C. d. The mice at 25°C were more active than the mice at 10°C. 68. ATP is essential for organisms because it provides energy to cells. How does ATP provide this energy on a physiological level? a. When energy is needed, ATP is converted to ADP and a phosphate group. Energy is released from the breaking of the phosphodiester bonds. b. When energy is needed, ATP is converted to ADP and a phosphate group. Energy is released from the breaking of the glycosidic bonds. c. When energy is needed, ATP is formed from ADP and a phosphate group. Energy is released from the breaking of the phosphodiester bonds. d. When energy is needed, ATP is formed from ADP and a phosphate group. Energy is released from the breaking of the phosphoanhydride bonds. 69. An omnivore comes across potatoes, avocados, kale, and eggs and craves only the eggs. In what nutrient is the animal likely deficient? a. carbohydrates b. protein c. d. fiber fatty acids 70. Carbohydrates often get a bad reputation for their role in promoting weight gain when consumed in excess. However, carbohydrates are necessary for biological functions. Why is it important to consume carbohydrates? a. Carbohydrates are broken down into glucose, which provides energy as ATP through metabolic pathways. ATP helps to maintain connective tissue. b. Carbohydrates are broken |
down into glucose, which is essential for blood clotting. c. Carbohydrates, along with proteins, help maintain connective tissue and are essential to blood clotting. d. Carbohydrates are broken down into glucose, which provides energy as ATP through metabolic pathways. ATP is required for proper cellular function. 71. Excess ATP is combined with excess glucose and stored as glycogen in the liver and skeletal muscle. Under what circumstance would glycogen storage in skeletal muscle prove beneficial for a rabbit? a. A rabbit has not eaten recently and its blood sugar drops. b. There is an overabundance of food available to a rabbit. c. A rabbit spots a coyote and flees in response. d. A young rabbit with an adequate food source is developing into an adult rabbit. SCIENCE PRACTICE CHALLENGE QUESTIONS 72. E. coli colonize the human gastrointestinal tract. The temperature of that environment is tightly regulated. However, the pH ranges from the highly acidic stomach (pH 4.5) to the relatively basic lower intestine (pH 9). Over the entire pH range of the environment the pH of the E. coli cytoplasm is maintained in a narrow range between 7.2 and 7.8. Wilks and Slonczewski (Journal of Bacteriology, 189, 2007) used a fluorescent dye to follow the recovery of cytoplasmic pH after an acid shock comparable to what occurs in the human stomach. They found that the pH within the cell recovered in approximately 2 minutes. Rapid restoration of cytoplasmic pH does not occur in the presence of ATPase inhibitors. Construct an explanation for the mechanisms that maintain homeostasis with a model of exchange of hydrogen ions (H+) between the cell and its extracellular environment. 73. We need an explanation of the common experience that an "upset stomach" (functional dyspepsia) or constipation can result from stress. Irritable bowel syndrome is a chronic gastrointestinal disease and is treated with the neurotransmitter serotonin. Serotonin receptors are located on the cell membranes of neurons and activate second messenger cascades that regulate gene expression. In humans most serotonin is synthesized in neurons that enervate smooth muscle cells lining the gastrointestinal tract. There is an association of serotonin with a sense of well-being. A. Based on these data, justify the claim that timing of the passage of food in the gastrointestinal tract is regulated by serotonin. The effect of serotonin on smooth muscle is a |
clue but it doesn't provide a mechanism connecting stress to the symptoms of functional dyspepsia. Serotonin is This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1105 synthesized by all Bilateria (animals with bilateral symmetry, including humans) and is released as a response to stress (Puglisi-Allegra and Andolini, Behavioral Brain Research, 277, 2015). Serotonin is also synthesized by plants to regulate root growth. B. Describe the role for serotonin that is indicated across domains. C. Evaluate the effect that stress produces in serotonin production, the association of stress with functional dyspepsia, and the role of serotonin in the regulation of expression in smooth muscle cells in terms of evidence of a negative feedback produced by serotonin as a medication. D. Justify your evaluation of the stress and the role of serotonin as a response to the stress in the form of a feedback loop diagrammatically. Quorum sensing coordinates bacterial expression, stimulating virulence factors, and behavior, inducing the formation of biofilms. Knecht et al. (EBioMedicine, 9, 2016) have demonstrated that serotonin functions as a quorum sensing messenger among bacteria in the gastrointestinal tract of mice. E. Construct an explanation of the effect of serotonin as a treatment of functional dyspepsia. 1106 Chapter 25 | Animal Nutrition and the Digestive System This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1107 26 | THE NERVOUS SYSTEM Figure 26.1 An athlete’s nervous system is hard at work during the planning and execution of a movement as precise as a high jump. Parts of the nervous system are involved in determining how hard to push off and when to turn, as well as controlling the muscles throughout the body that make this complicated movement possible without knocking the bar down—all in just a few seconds. (credit: modification of work by Shane T. McCoy, U.S. Navy) Chapter Outline 26.1: Neurons and Glial Cells 26.2: How Neurons Communicate 26.3: The Central Nervous System 26.4: The Peripheral Nervous System 26.5: Nervous System Disorders Introduction While you’ |
re reading this book, your nervous system is performing several functions simultaneously. The visual system is processing what is seen on the page, the motor system controls the turn of the pages (or click of the mouse), and the prefrontal cortex maintains attention. Even fundamental functions, like breathing and regulation of body temperature, are controlled by the nervous system. A nervous system is an organism’s control center: it processes sensory information from outside (and inside) the body and controls all behaviors—from eating to sleeping to finding a mate. Scientists have even discovered that certain individual neurons (a type of nerve cell) can multitask. Neuroscientists often use the model organism C. elegans (a worm) to study neurons. While studying these worms, it was recently discovered that one type of neuron called AIY regulates both speed and direction of movement. Even though humans have billions of neurons compared to the 302 in C. elegans, it is thought that many perform multiple functions. You can read more about this research at the Science Daily website (http://openstaxcollege.org/l/32neurons). [1] 1. University of Michigan. First peek at how neurons multitask. Science Daily, 6 November 2014. www.sciencedaily.com/releases/2014/11/141106131520.htm. 1108 Chapter 26 | The Nervous System 26.1 | Neurons and Glial Cells In this section, you will explore the following questions: • What are the functions of the structural components of a neuron? • What are the four main types of neurons? • What are the functions of different types of glial cells? Connection for AP® Courses Much information about the various organ systems of animals is not within the scope of AP®. The nervous system, however, was selected for in-depth study because an animal’s ability to detect, transmit, and respond to information is critical for survival. The nervous system interacts with all other organ systems to coordinate responses. The information in this chapter allows us to apply concepts we explored previously, including structure and function relationships, homeostasis, the movement of substances across cell membranes, cell signaling and communication, and the use of ATP. Nervous systems in animals range from relatively simple nerve nets in jellyfish to a complex brain, spinal cord, and peripheral nervous in humans. (For the purpose of AP®, you do not need to have detailed information about myriad types of nervous systems in other animals. Instead, |
just focus on the complex nervous system of humans.) The basic structure of the nervous system that reflects function is the neuron, of which there are three types: sensory, motor, and interneuron. A typical neuron consists of dendrites, a cell body, and an axon to detect, generate, transmit, and integrate signal information. Many neurons are surrounded by Schwann cells (a type of glial cell) that form a myelin sheath, which acts as an electrical insulator, like the plastic wrap that surrounds the copper wires in a household appliance cord. The Schwann cells are separated by gaps of unmyelinated fibers called nodes of Ranvier over which the nerve impulse travels as the signal passes along the neuron, increasing the speed of transmission. (As we will learn in the Nervous System Disorders section, some diseases of the nervous system result from the loss of myelin.) Glial cells—often thought of as the “supporting cast” of the nervous system—outnumber neurons and play a role in the development of neurons, buffer harmful ions and chemicals, and provide myelin sheaths around neurons. Most brain tumors are caused by mutations in glial cells. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 3 Enduring Understanding 3.E Living systems store, retrieve, transmit and respond to information essential to life processes. Transmission of information results in changes within and between biological systems. Essential Knowledge 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain. Learning Objective Essential Knowledge 3.44 The student is able to describe how nervous systems detect external and internal signals. 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. Learning Objective 3.48 The student is able to create a visual representation to describe how nervous systems detect external and internal |
signals. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1109 Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 26.2. Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves. Flatworms of the phylum Platyhelminthes have both a central nervous system (CNS), made up of a small “brain” and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species. Figure 26.2 Nervous systems vary in structure and complexity. In (a) cnidarians, nerve cells form a decentralized nerve net. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, (d) arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. In (f) vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of the body comprise the peripheral nervous system. (credit e: modification of work by Michael Vecchione, Clyde F.E. Roper, and Michael J. Sweeney, NOAA; credit f: modification of work by NIH) Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, |
they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves. One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally whereas the vertebrate spinal cords are located dorsally. There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates. 1110 Chapter 26 | The Nervous System Watch this video (http://openstaxcollege.org/l/vertebrate_evol) of biologist Mark Kirschner discussing the “flipping” phenomenon of vertebrate evolution. The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. A neuron can be compared to an electrical wire—it transmits a signal from one place to another. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. Although glia have been compared to workers, recent evidence suggests that also usurp some of the signaling functions of neurons. There is great diversity in the types of neurons and glia that are present in different parts of the nervous system. There are four major types of neurons, and they share several important cellular components. Neurons The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus. A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors—from basic reflexes to more complicated behaviors like finding food and courting mates. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors. Most neurons share the same cellular components. But neurons are also highly specialized—different types of neurons have different sizes and shapes that relate to their functional roles. Parts of a Neuron Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Gol |
gi apparatus, mitochondria, and other cellular components. Neurons also contain unique structures, illustrated in Figure 26.3 for receiving and sending the electrical signals that make neuronal communication possible. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Although some neurons do not have any dendrites, some types of neurons have multiple dendrites. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections. Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter—from the base of the spine to the toes. The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along the axon there are periodic gaps in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is “recharged” as it travels along the axon. It is important to note that a single neuron does not act alone—neuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The N |
ervous System 1111 Figure 26.3 Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons. Types of Neurons There are different types of neurons, and the functional role of a given neuron is intimately dependent on its structure. There is an amazing diversity of neuron shapes and sizes found in different parts of the nervous system (and across species), as illustrated by the neurons shown in Figure 26.4. 1112 Chapter 26 | The Nervous System Figure 26.4 There is great diversity in the size and shape of neurons throughout the nervous system. Examples include (a) a pyramidal cell from the cerebral cortex, (b) a Purkinje cell from the cerebellar cortex, and (c) olfactory cells from the olfactory epithelium and olfactory bulb. While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. Figure 26.5 illustrates these four basic neuron types. Unipolar neurons have only one structure that extends away from the soma. These neurons are not found in vertebrates but are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite extending from the soma. An example of a bipolar neuron is a retinal bipolar cell, which receives signals from photoreceptor cells that are sensitive to light and transmits these signals to ganglion cells that carry the signal to the brain. Multipolar neurons are the most common type of neuron. Each multipolar neuron contains one axon and multiple dendrites. Multipolar neurons can be found in the central nervous system (brain and spinal cord). An example of a multipolar neuron is a Purkinje cell in the cerebellum, which has many branching dendrites but only one axon. Pseudounipolar cells share characteristics with both unipolar and bipolar cells. A pseudounipolar cell has a single process that extends from the soma, like a unipolar cell, but this process later branches into two distinct structures, like a bipolar cell. Most sensory neurons are pseudounipolar and have an axon that branches into two extensions: one connected to dendrites that receive sensory information and another that transmits this information to the spinal cord. This OpenStax book is available for free at http://cnx |
.org/content/col12078/1.6 Chapter 26 | The Nervous System 1113 Figure 26.5 Neurons are broadly divided into four main types based on the number and placement of axons: (1) unipolar, (2) bipolar, (3) multipolar, and (4) pseudounipolar. 1114 Chapter 26 | The Nervous System Neurogenesis At one time, scientists believed that people were born with all the neurons they would ever have. Research performed during the last few decades indicates that neurogenesis, the birth of new neurons, continues into adulthood. Neurogenesis was first discovered in songbirds that produce new neurons while learning songs. For mammals, new neurons also play an important role in learning: about 1000 new neurons develop in the hippocampus (a brain structure involved in learning and memory) each day. While most of the new neurons will die, researchers found that an increase in the number of surviving new neurons in the hippocampus correlated with how well rats learned a new task. Interestingly, exercise also promotes neurogenesis in the hippocampus. Stress has the opposite effect. While neurogenesis is quite limited compared to regeneration in other tissues, research in this area may lead to new treatments for disorders such as Alzheimer’s, stroke, and epilepsy. How do scientists identify new neurons? A researcher can inject a compound called bromodeoxyuridine (BrdU) into the brain of an animal. While all cells will be exposed to BrdU, BrdU will only be incorporated into the DNA of newly generated cells that are in S phase. A technique called immunohistochemistry can be used to attach a fluorescent label to the incorporated BrdU, and a researcher can use fluorescent microscopy to visualize the presence of BrdU, and thus new neurons, in brain tissue. Figure 26.6 is a micrograph which shows fluorescently labeled neurons in the hippocampus of a rat. Figure 26.6 This micrograph shows fluorescently labeled new neurons in a rat hippocampus. Cells that are actively dividing have bromodoxyuridine (BrdU) incorporated into their DNA and are labeled in red. Cells that express glial fibrillary acidic protein (GFAP) are labeled in green. Astrocytes, but not neurons, express GFAP. Thus, cells that are labeled both red and green are actively dividing astrocytes, whereas cells labeled red only are actively dividing neurons. (credit: modification of work by Dr |
. Maryam Faiz, et. al., University of Barcelona; scale-bar data from Matt Russell) This site (http://openstaxcollege.org/l/neurogenesis) contains more information about neurogenesis, including an interactive laboratory simulation and a video that explains how BrdU labels new cells. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1115 Think About It How does the unique structure of the neuron allow it to detect (and ultimately transmit) incoming signals? Glia While glia are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain actually outnumbers the number of neurons by a factor of ten. Neurons would be unable to function without the vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have recently discovered that they also play a role in responding to nerve activity and modulating communication between nerve cells. Types of Glia There are several different types of glia with different functions, two of which are shown in Figure 26.7. Astrocytes, shown in Figure 26.8a make contact with both capillaries and neurons in the CNS. They provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses. Astrocytes also form the blood-brain barrier—a structure that blocks entrance of toxic substances into the brain. Astrocytes, in particular, have been shown through calcium imaging experiments to become active in response to nerve activity, transmit calcium waves between astrocytes, and modulate the activity of surrounding synapses. Satellite glia provide nutrients and structural support for neurons in the PNS. Microglia scavenge and degrade dead cells and protect the brain from invading microorganisms. Oligodendrocytes, shown in Figure 26.8b form myelin sheaths around axons in the CNS. One axon can be myelinated by several oligodendrocytes, and one oligodendrocyte can provide myelin for multiple neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the |
axon. Radial glia serve as scaffolds for developing neurons as they migrate to their end destinations. Ependymal cells line fluid-filled ventricles of the brain and the central canal of the spinal cord. They are involved in the production of cerebrospinal fluid, which serves as a cushion for the brain, moves the fluid between the spinal cord and the brain, and is a component for the choroid plexus. Figure 26.7 Glial cells support neurons and maintain their environment. Glial cells of the (a) central nervous system include oligodendrocytes, astrocytes, ependymal cells, and microglial cells. Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, and provide structural support. Microglia scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions the neurons. Glial cells of the (b) peripheral nervous system include Schwann cells, which form the myelin sheath, and satellite cells, which provide nutrients and structural support to neurons. 1116 Chapter 26 | The Nervous System Figure 26.8 (a) Astrocytes and (b) oligodendrocytes are glial cells of the central nervous system. (credit a: modification of work by Uniformed Services University; credit b: modification of work by Jurjen Broeke; scale-bar data from Matt Russell) 26.2 | How Neurons Communicate In this section, you will explore the following questions: • What is the basis of the resting membrane potential? • What are the stages of an action potential, and how are action potentials propagated? • What are the similarities and differences between chemical and electrical synapses? • What is long-term potentiation and long-term depression, and how do both relate to transmission of impulses across synapses? Connection for AP® Courses The neuron is a great example of a structure-function relationship at the cellular level. Information flow along a neuron is usually from dendrite to axon and from neuron to neuron or from neuron to a cell of a target organ. Like other eukaryotic cells, neurons consist of a cell membrane, nucleus, and organelles, including mitochondria. Action potentials propagate impulses along neurons. When an axon is at rest, the membrane is said to be polarized |
; that is, there is an electrochemical gradient across it, with the inside of the membrane being more negatively charged than the outside. We explored the formation of electrochemical gradients using H+ when we studied photosynthesis and cellular respiration. The neuron, however, uses Na+ and K+ to establish a gradient. It is also important to recall that ions cannot diffuse across the lipid bilayer of the cell membrane and must use transport proteins; in this case, the transport proteins are voltage-gated Na+ and K+ channels. At rest, the Na+/K+ pump, powered by ATP, maintains this gradient, known as resting membrane potential. In response to a stimulus, such as an odorant molecule, membrane potential changes, and an action potential is generated along the membrane as the voltage-gated Na+ and K+ channels open sequentially, causing the membrane to depolarize. In depolarization, the inside of the membrane becomes more positive than the outside as Na+ flows to the inside. Repolarization occurs when K+ flows across the membrane to the outside. In myelinated neurons, action potentials “jump” between gaps of unmyelinated axons (nodes of Ranvier), a phenomenon called saltatory conduction. Transmission of a nerve impulse from one neuron to another or to another type of cell such as a muscle cell occurs across a junction called a synapse. Synaptic vesicles at the axon terminal of the presynaptic neuron release chemical messengers called neurotransmitters into the junction; neurotransmitters then bind to receptors embedded in the membrane of the postsynaptic neuron. Neurotransmitters may be either excitatory (such as acetylcholine or epinephrine) or inhibitory (such as serotonin or GABA) as they either increase or decrease the change of an action potential in the postsynaptic neuron. Many drugs, including both pharmaceuticals and drugs of abuse, can induce changes in synaptic transmission; for example, tetrahydrocannabinol (more commonly known as THC) in marijuana binds to a naturally occurring neurotransmitter This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1117 important to short-term memory. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the |
Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 3 Enduring Understanding 3.E Living systems store, retrieve, transmit and respond to information essential to life processes. Transmission of information results in changes within and between biological systems. Essential Knowledge 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Science Practice Learning Objective 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 3.43 The student is able to construct an explanation, based on scientific theories and models, about how nervous systems detect external and internal signals, transmit and integrate information, and produce responses. Essential Knowledge 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Science Practice Learning Objective 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain. 3.45 The student is able to describe how nervous systems transmit information. Essential Knowledge 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Science Practice Learning Objective 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 3.47 The student is able to create a visual representation of complex nervous systems to describe/explain how these systems detect external and internal signals, transmit and integrate information, and produce responses. All functions performed by the nervous system—from a simple motor reflex to more advanced functions like making a memory or a decision—require neurons to communicate with one another. While humans use words and body language to communicate, neurons use electrical and chemical signals. Just like a person in a committee, one neuron usually receives and synthesizes messages from multiple other neurons before “making the decision” to send the message on to other neurons. Nerve Impulse Transmission within a Neuron For the nervous system to function, neurons must be able to send and receive signals. These signals are possible because each neuron has a charged cellular membrane (a voltage difference between the inside and the outside), and the charge of this membrane can change in response to neurotransmitter molecules released from other neurons and environmental stimuli. To understand how neurons communicate, one must first understand the basis of the baseline or ‘rest |
ing’ membrane charge. Neuronal Charged Membranes The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane. Ion channels have different configurations: open, closed, and inactive, as illustrated in Figure 26.9. Some ion channels need to be activated in order to open and allow ions to pass into or out of the cell. These ion channels are sensitive to the environment and can change their shape accordingly. Ion channels that change their structure in response to voltage changes are called voltage-gated ion 1118 Chapter 26 | The Nervous System channels. Voltage-gated ion channels regulate the relative concentrations of different ions inside and outside the cell. The difference in total charge between the inside and outside of the cell is called the membrane potential. Figure 26.9 Voltage-gated ion channels open in response to changes in membrane voltage. After activation, they become inactivated for a brief period and will no longer open in response to a signal. This video (http://openstaxcollege.org/l/resting_neuron) discusses the basis of the resting membrane potential. Resting Membrane Potential A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV, note that this number varies by neuron type and by species). This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium. Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell, as shown in Table 26.1. The difference in the number of positively charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential (Figure 26.10). When the membrane is at rest, K+ ions accumulate inside the cell due to a net movement with the concentration gradient. The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell (in the extracellular fluid) relative to inside the cell (in the cytoplasm). The negative charge within the cell is created by the cell membrane being more permeable to potassium ion movement than sodium ion movement. In neurons, potassium ions are maintained at high |
concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell. The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. The actions of the sodium potassium pump help to maintain the resting potential, once established. Recall that sodium potassium pumps brings two K+ ions into the cell while removing three Na+ ions per ATP consumed. As more cations are expelled from the cell than taken in, the inside of the cell remains negatively charged relative to the extracellular fluid. It should be noted that chloride ions (Cl–) tend to accumulate outside of the cell because they are repelled by negatively-charged proteins within the cytoplasm. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1119 Ion Concentration Inside and Outside Neurons Ion Extracellular concentration (mM) Intracellular concentration (mM) Ratio outside/ inside Na+ K+ Cl− Organic anions (A−) 145 4 120 — 12 155 4 100 12 0.026 30 Table 26.1 The resting membrane potential is a result of different concentrations inside and outside the cell. 1120 Chapter 26 | The Nervous System Figure 26.10 The (a) resting membrane potential is a result of different concentrations of Na+ and K+ ions inside and outside the cell. A nerve impulse causes Na+ to enter the cell, resulting in (b) depolarization. At the peak action potential, K+ channels open and the cell becomes (c) hyperpolarized. Action Potential A neuron can receive input from other neurons and, if this input is strong enough, send the signal to downstream neurons. Transmission of a signal between neurons is generally carried by a chemical called a neurotransmitter. Transmission of a signal within a neuron (from dendrite to axon terminal) is carried by a brief reversal of the resting membrane potential called an action potential. When neurotransmitter molecules bind to receptors located on a neuron’s dendrites, ion channels This OpenStax book is available for free at http://cn |
x.org/content/col12078/1.6 Chapter 26 | The Nervous System 1121 open. At excitatory synapses, this opening allows positive ions to enter the neuron and results in depolarization of the membrane—a decrease in the difference in voltage between the inside and outside of the neuron. A stimulus from a sensory cell or another neuron depolarizes the target neuron to its threshold potential (-55 mV). Na+ channels in the axon hillock open, allowing positive ions to enter the cell (Figure 26.10 and Figure 26.11). Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV. Action potentials are considered an "all-or nothing" event, in that, once the threshold potential is reached, the neuron always completely depolarizes. Once depolarization is complete, the cell must now "reset" its membrane voltage back to the resting potential. To accomplish this, the Na+ channels close and cannot be opened. This begins the neuron's refractory period, in which it cannot produce another action potential because its sodium channels will not open. At the same time, voltage-gated K+ channels open, allowing K+ to leave the cell. As K+ ions leave the cell, the membrane potential once again becomes negative. The diffusion of K+ out of the cell actually hyperpolarizes the cell, in that the membrane potential becomes more negative than the cell's normal resting potential. At this point, the sodium channels will return to their resting state, meaning they are ready to open again if the membrane potential again exceeds the threshold potential. Eventually the extra K+ ions diffuse out of the cell through the potassium leakage channels, bringing the cell from its hyperpolarized state, back to its resting membrane potential. Figure 26.11 The formation of an action potential can be divided into five steps: (1) A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. (2) If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes. (3) At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close. (4) The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire. |
(5) The K+ channels close and the Na+/K+ transporter restores the resting potential. 1122 Chapter 26 | The Nervous System Figure 26.12 The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes. This video (http://openstaxcollege.org/l/actionpotential) presents an overview of action potential. Myelin and the Propagation of the Action Potential For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release. The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon’s resistance to current leak. Myelin acts as an insulator that prevents current from leaving the axon; this increases the speed of action potential conduction. In demyelinating diseases like multiple sclerosis, action potential conduction slows because current leaks from previously insulated axon areas. The nodes of Ranvier, illustrated in Figure 26.13 are gaps in the myelin sheath along the axon. These unmyelinated spaces are about one micrometer long and contain voltage gated Na+ and K+ channels. Flow of ions through these channels, particularly the Na+ channels, regenerates the action potential over and over again along the axon. This ‘jumping’ of the action potential from one node to the next is called saltatory conduction. If nodes of Ranvier were not present along an axon, the action potential would propagate very slowly since Na+ and K+ channels would have to continuously regenerate action potentials at every point along the axon instead of at specific points. Nodes of Ranvier also save energy for the neuron since the channels only need to be present at the nodes and not along the entire axon. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1123 Figure 26.13 Nodes of Ranvier are gaps in myelin coverage along axons. Nodes contain voltage-gated K+ and Na+ channels. Action potentials travel down the axon by jumping from one node to the next. Synaptic Transmission The synapse or “gap” is the place where information is transmitted from one neuron to another. Synapses usually form between axon terminals |
and dendritic spines, but this is not universally true. There are also axon-to-axon, dendrite-todendrite, and axon-to-cell body synapses. The neuron transmitting the signal is called the presynaptic neuron, and the neuron receiving the signal is called the postsynaptic neuron. Note that these designations are relative to a particular synapse—most neurons are both presynaptic and postsynaptic. There are two types of synapses: chemical and electrical. Chemical Synapse When an action potential reaches the axon terminal it depolarizes the membrane and opens voltage-gated Na+ channels. Na+ ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated Ca2+ channels to open. Calcium ions entering the cell initiate a signaling cascade that causes small membrane-bound vesicles, called synaptic vesicles, containing neurotransmitter molecules to fuse with the presynaptic membrane. Synaptic vesicles are shown in Figure 26.14, which is an image from a scanning electron microscope. Figure 26.14 This pseudocolored image taken with a scanning electron microscope shows an axon terminal that was broken open to reveal synaptic vesicles (blue and orange) inside the neuron. (credit: modification of work by Tina Carvalho, NIH-NIGMS; scale-bar data from Matt Russell) Fusion of a vesicle with the presynaptic membrane causes neurotransmitter to be released into the synaptic cleft, the extracellular space between the presynaptic and postsynaptic membranes, as illustrated in Figure 26.15. The neurotransmitter diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane. 1124 Chapter 26 | The Nervous System Figure 26.15 Communication at chemical synapses requires release of neurotransmitters. When the presynaptic membrane is depolarized, voltage-gated Ca2+ channels open and allow Ca2+ to enter the cell. The calcium entry causes synaptic vesicles to fuse with the membrane and release neurotransmitter molecules into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron. The binding of a specific neurotransmitter causes particular ion channels, in this case lig |
and-gated channels, on the postsynaptic membrane to open. Neurotransmitters can either have excitatory or inhibitory effects on the postsynaptic membrane, as detailed in Table 26.1. For example, when acetylcholine is released at the synapse between a nerve and muscle (called the neuromuscular junction) by a presynaptic neuron, it causes postsynaptic Na+ channels to open. Na+ enters the postsynaptic cell and causes the postsynaptic membrane to depolarize. This depolarization is called an excitatory postsynaptic potential (EPSP) and makes the postsynaptic neuron more likely to fire an action potential. Release of neurotransmitter at inhibitory synapses causes inhibitory postsynaptic potentials (IPSPs), a hyperpolarization of the presynaptic membrane. For example, when the neurotransmitter GABA (gamma-aminobutyric acid) is released from a presynaptic neuron, it binds to and opens Cl- channels. Cl- ions enter the cell and hyperpolarizes the membrane, making the This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1125 neuron less likely to fire an action potential. Once neurotransmission has occurred, the neurotransmitter must be removed from the synaptic cleft so the postsynaptic membrane can “reset” and be ready to receive another signal. This can be accomplished in three ways: the neurotransmitter can diffuse away from the synaptic cleft, it can be degraded by enzymes in the synaptic cleft, or it can be recycled (sometimes called reuptake) by the presynaptic neuron. Several drugs act at this step of neurotransmission. For example, some drugs that are given to Alzheimer’s patients work by inhibiting acetylcholinesterase, the enzyme that degrades acetylcholine. This inhibition of the enzyme essentially increases neurotransmission at synapses that release acetylcholine. Once released, the acetylcholine stays in the cleft and can continually bind and unbind to postsynaptic receptors. Neurotransmitter Examples and Location Neurotransmitter Example Acetylcholine — Biogenic amine Dopamine, serotonin, norepinephrine Location CNS and/or PNS CNS and/or PNS Amino acid Glycine, glutamate, aspartate, gamma |
aminobutyric acid CNS Neuropeptide Substance P, endorphins CNS and/or PNS Table 26.2 Electrical Synapse While electrical synapses are fewer in number than chemical synapses, they are found in all nervous systems and play important and unique roles. The mode of neurotransmission in electrical synapses is quite different from that in chemical synapses. In an electrical synapse, the presynaptic and postsynaptic membranes are very close together and are actually physically connected by channel proteins forming gap junctions. Gap junctions allow current to pass directly from one cell to the next. In addition to the ions that carry this current, other molecules, such as ATP, can diffuse through the large gap junction pores. There are key differences between chemical and electrical synapses. Because chemical synapses depend on the release of neurotransmitter molecules from synaptic vesicles to pass on their signal, there is an approximately one millisecond delay between when the axon potential reaches the presynaptic terminal and when the neurotransmitter leads to opening of postsynaptic ion channels. Additionally, this signaling is unidirectional. Signaling in electrical synapses, in contrast, is virtually instantaneous (which is important for synapses involved in key reflexes), and some electrical synapses are bidirectional. Electrical synapses are also more reliable as they are less likely to be blocked, and they are important for synchronizing the electrical activity of a group of neurons. For example, electrical synapses in the thalamus are thought to regulate slow-wave sleep, and disruption of these synapses can cause seizures. Signal Summation Sometimes a single excitatory postsynaptic potential (EPSP) is strong enough to induce an action potential in the postsynaptic neuron, but often multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential. This process is called summation and occurs at the axon hillock, as illustrated in Figure 26.16. Additionally, one neuron often has inputs from many presynaptic neurons—some excitatory and some inhibitory—so IPSPs can cancel out EPSPs and vice versa. It is the net change in postsynaptic membrane voltage that determines whether the postsynaptic cell has reached its threshold of excitation needed to fire an action potential. Together, synaptic summation and the threshold for excitation act as a filter so that random “noise” in the system |
is not transmitted as important information. 1126 Chapter 26 | The Nervous System Figure 26.16 A single neuron can receive both excitatory and inhibitory inputs from multiple neurons, resulting in local membrane depolarization (EPSP input) and hyperpolarization (IPSP input). All these inputs are added together at the axon hillock. If the EPSPs are strong enough to overcome the IPSPs and reach the threshold of excitation, the neuron will fire. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1127 Brain-computer interface Amyotrophic lateral sclerosis (ALS, also called Lou Gehrig’s Disease) is a neurological disease characterized by the degeneration of the motor neurons that control voluntary movements. The disease begins with muscle weakening and lack of coordination and eventually destroys the neurons that control speech, breathing, and swallowing; in the end, the disease can lead to paralysis. At that point, patients require assistance from machines to be able to breathe and to communicate. Several special technologies the world. One have been developed to allow “locked-in” patients to communicate with the rest of technology, for example, allows patients to type out sentences by twitching their cheek. These sentences can then be read aloud by a computer. A relatively new line of research for helping paralyzed patients, including those with ALS, to communicate and retain a degree of self-sufficiency is called brain-computer interface (BCI) technology and is illustrated in Figure 26.17. This technology sounds like something out of science fiction: it allows paralyzed patients to control a computer using only their thoughts. There are several forms of BCI. Some forms use EEG recordings from electrodes taped onto the skull. These recordings contain information from large populations of neurons that can be decoded by a computer. Other forms of BCI require the implantation of an array of electrodes smaller than a postage stamp in the arm and hand area of the motor cortex. This form of BCI, while more invasive, is very powerful as each electrode can record actual action potentials from one or more neurons. These signals are then sent to a computer, which has been trained to decode the signal and feed it to a tool—such as a cursor on a computer screen. This means that a patient with ALS can use e-mail, read the Internet, and communicate with others by thinking of moving his or her hand |
or arm (even though the paralyzed patient cannot make that bodily movement). Recent advances have allowed a paralyzed locked-in patient who suffered a stroke 15 years ago to control a robotic arm and even to feed herself coffee using BCI technology. Despite the amazing advancements in BCI technology, it also has limitations. The technology can require many hours of training and long periods of intense concentration for the patient; it can also require brain surgery to implant the devices. Figure 26.17 With brain-computer interface technology, neural signals from a paralyzed patient are collected, decoded, and then fed to a tool, such as a computer, a wheelchair, or a robotic arm. 1128 Chapter 26 | The Nervous System Watch this video (http://openstaxcollege.org/l/paralyzation) in which a paralyzed woman use a brain-controlled robotic arm to bring a drink to her mouth, among other images of brain-computer interface technology in action. Synaptic Plasticity Synapses are not static structures. They can be weakened or strengthened. They can be broken, and new synapses can be made. Synaptic plasticity allows for these changes, which are all needed for a functioning nervous system. In fact, synaptic plasticity is the basis of learning and memory. Two processes in particular, long-term potentiation (LTP) and long-term depression (LTD) are important forms of synaptic plasticity that occur in synapses in the hippocampus, a brain region that is involved in storing memories. Long-term Potentiation (LTP) Long-term potentiation (LTP) is a persistent strengthening of a synaptic connection. LTP is based on the Hebbian principle: cells that fire together wire together. There are various mechanisms, none fully understood, behind the synaptic strengthening seen with LTP. One known mechanism involves a type of postsynaptic glutamate receptor, called NMDA (NMethyl-D-aspartate) receptors, shown in Figure 26.18. These receptors are normally blocked by magnesium ions; however, when the postsynaptic neuron is depolarized by multiple presynaptic inputs in quick succession (either from one neuron or multiple neurons), the magnesium ions are forced out allowing Ca2+ ions to pass into the postsynaptic cell. Next, Ca2+ ions entering the cell initiate a signaling cascade that causes a different type of glutamate receptor, called AMPA (α-amino-3-hydroxy-5-methyl-4-isox |
azolepropionic acid) receptors, to be inserted into the postsynaptic membrane, since activated AMPA receptors allow positive ions to enter the cell. So, the next time glutamate is released from the presynaptic membrane, it will have a larger excitatory effect (EPSP) on the postsynaptic cell because the binding of glutamate to these AMPA receptors will allow more positive ions into the cell. The insertion of additional AMPA receptors strengthens the synapse and means that the postsynaptic neuron is more likely to fire in response to presynaptic neurotransmitter release. Long-term Depression (LTD) Long-term depression (LTD) is essentially the reverse of LTP: it is a long-term weakening of a synaptic connection. One mechanism known to cause LTD also involves AMPA receptors. In this situation, calcium that enters through NMDA receptors initiates a different signaling cascade, which results in the removal of AMPA receptors from the postsynaptic membrane, as illustrated in Figure 26.18. The decrease in AMPA receptors in the membrane makes the postsynaptic neuron less responsive to glutamate released from the presynaptic neuron. While it may seem counterintuitive, LTD may be just as important for learning and memory as LTP. The weakening and pruning of unused synapses allows for unimportant connections to be lost and makes the synapses that have undergone LTP that much stronger by comparison. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1129 Figure 26.18 Calcium entry through postsynaptic NMDA receptors can initiate two different forms of synaptic plasticity: long-term potentiation (LTP) and long-term depression (LTD). LTP arises when a single synapse is repeatedly stimulated. This stimulation causes a calcium- and CaMKII-dependent cellular cascade, which results in the insertion of more AMPA receptors into the postsynaptic membrane. The next time glutamate is released from the presynaptic cell, it will bind to both NMDA and the newly inserted AMPA receptors, thus depolarizing the membrane more efficiently. LTD occurs when few glutamate molecules bind to NMDA receptors at a synapse (due to a low firing rate of the presynaptic neuron). The calcium that does flow through NMDA receptors initiates a different calcineurin and protein phosphatase 1-dependent cascade, which results in the end |
ocytosis of AMPA receptors. This makes the postsynaptic neuron less responsive to glutamate released from the presynaptic neuron. Activity Don’t Eat the Fugu: Understanding the Neuron. Create a model of a neuron to explain how the vertebrate nervous system detects signals and transmits information. Then use the model to predict how abnormal cell structure, drugs, and toxins (such as tetrodotoxin found in fugu/pufferfish) can affect impulse transmission. Think About It Potassium channel blockers, such as procainamide, are often used to treat abnormal activity in the heart. These channel blocks impede the movement of K+ through voltage-gated K+ channels. What is the likely effect(s) of these medications on action potentials? 1130 Chapter 26 | The Nervous System 26.3 | The Central Nervous System In this section, you will explore the following questions: • What are the major areas of the brain? • What are the primary functions of the spinal cord, cerebral lobes, cerebellum, and brainstem? Connection for AP® Courses The central nervous system (CNS) consists of the brain and spinal cord, both of which are protected by the skull and vertebral column, respectively. The CNS receives sensory information, integrates this information, and initiates a motor response, with the brain serving as the control center for processing sensory information and directing responses. Different parts of the vertebrate brain (including ours) have different functions, and brain development in animals reveals a unique evolutionary progression. You do not have to memorize all the different parts of the brain and their functions for AP. However, as a student of biology, you should have a general understanding of the three major parts of the brain and their functions. In mammals, the parts of the brain include the cerebrum or cortex (which can be broken down into four primary lobes: frontal, temporal, occipital, and parietal), basal ganglia, thalamus, hypothalamus, limbic system, cerebellum, and brainstem. Information traveling up the spinal cord to the brain is directed to one of the specialized areas of the cerebrum; for example, association areas for hearing are localized in the temporal lobe. The cerebellum helps coordinate skeletal muscle activity, and the medulla oblongata and pons in the brainstem are centers for vital functions, such as breathing and heart rate. Although localization of functions occurs, most complex functions, like |
language, involve neurons in multiple brain regions. In terms of energy, since the brain consumes about 20 percent of the body’s resources (ATP), is it any wonder that you’re exhausted after taking an AP® test? Information from the brain travels down the spinal cord, making connections with peripheral nerves; thus, the spinal cord transmits sensory and motor input and controls motor reflexes, like the automatic responses when the pupils of your eye constrict in bright sunlight or when your jerk your hand away from something hot. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 3 Enduring Understanding 3.E Living systems store, retrieve, transmit and respond to information essential to life processes. Transmission of information results in changes within and between biological systems. Essential Knowledge 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. Learning Objective Essential Knowledge 3.49 The student is able to create a visual representation to describe how the vertebrate brain integrates information to produce a response. 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain. Learning Objective 3.46 The student is able to describe how the vertebrate brain integrates information to produce a response. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1131 Essential Knowledge 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. Learning Objective 3.50 The student is able to create a visual representation to describe how the vertebrate brain integrates information to produce a response. As mentioned above, the central nervous system (CNS |
) is made up of the brain, a part of which is shown in Figure 26.19 and spinal cord and is covered with three layers of protective coverings called meninges (from the Greek word for membrane). The outermost layer is the dura mater (Latin for “hard mother”). As the Latin suggests, the primary function for this thick layer is to protect the brain and spinal cord. The dura mater also contains vein-like structures that carry blood from the brain back to the heart. The middle layer is the web-like arachnoid mater. The last layer is the pia mater (Latin for “soft mother”), which directly contacts and covers the brain and spinal cord like plastic wrap. The space between the arachnoid and pia maters is filled with cerebrospinal fluid (CSF). CSF is produced by a tissue called choroid plexus in fluid-filled compartments in the CNS called ventricles. The brain floats in CSF, which acts as a cushion and shock absorber and makes the brain neutrally buoyant. CSF also functions to circulate chemical substances throughout the brain and into the spinal cord. The entire brain contains only about 8.5 tablespoons of CSF, but CSF is constantly produced in the ventricles. This creates a problem when a ventricle is blocked—the CSF builds up and creates swelling and the brain is pushed against the skull. This swelling condition is called hydrocephalus (“water head”) and can cause seizures, cognitive problems, and even death if a shunt is not inserted to remove the fluid and pressure. Figure 26.19 The cerebral cortex is covered by three layers of meninges: the dura, arachnoid, and pia maters. (credit: modification of work by Gray’s Anatomy) Brain The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. It includes the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, and cerebellum. There are three different ways that a brain can be sectioned in order to view internal structures: a sagittal section cuts the brain left to right, as shown in Figure 26.21b, a coronal section cuts the brain front to back, as shown in Figure 26.20a, and a horizontal section cuts the brain top to bottom. Cerebral Cortex |
The outermost part of the brain is a thick piece of nervous system tissue called the cerebral cortex, which is folded into hills called gyri (singular: gyrus) and valleys called sulci (singular: sulcus). The cortex is made up of two hemispheres—right and left—which are separated by a large sulcus. A thick fiber bundle called the corpus callosum (Latin: “tough body”) connects the two hemispheres and allows information to be passed from one side to the other. Although there are some 1132 Chapter 26 | The Nervous System brain functions that are localized more to one hemisphere than the other, the functions of the two hemispheres are largely redundant. In fact, sometimes (very rarely) an entire hemisphere is removed to treat severe epilepsy. While patients do suffer some deficits following the surgery, they can have surprisingly few problems, especially when the surgery is performed on children who have very immature nervous systems. Figure 26.20 These illustrations show the (a) coronal and (b) sagittal sections of the human brain. (a) (b) In other surgeries to treat severe epilepsy, the corpus callosum is cut instead of removing an entire hemisphere. This causes a condition called split-brain, which gives insights into unique functions of the two hemispheres. For example, when an object is presented to patients’ left visual field, they may be unable to verbally name the object (and may claim to not have seen an object at all). This is because the visual input from the left visual field crosses and enters the right hemisphere and cannot then signal to the speech center, which generally is found in the left side of the brain. Remarkably, if a split-brain patient is asked to pick up a specific object out of a group of objects with the left hand, the patient will be able to do so but will still be unable to vocally identify it. See this website (http://openstaxcollege.org/l/split-brain) to learn more about split-brain patients and to play a game where you can model the split-brain experiments yourself. Each cortical hemisphere contains regions called lobes that are involved in different functions. Scientists use various techniques to determine what brain areas are involved in different functions: they examine patients who have had injuries or diseases that affect specific areas and see how those areas are related to functional deficits. They also conduct animal studies where they stimulate brain areas and see if there are |
any behavioral changes. They use a technique called transmagnetic stimulation (TMS) to temporarily deactivate specific parts of the cortex using strong magnets placed outside the head; and they use functional magnetic resonance imaging (fMRI) to look at changes in oxygenated blood flow in particular brain regions that correlate with specific behavioral tasks. These techniques, and others, have given great insight into the functions of different brain regions but have also showed that any given brain area can be involved in more than one behavior or process, and any given behavior or process generally involves neurons in multiple brain areas. That being said, each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital. Figure 26.21 illustrates these four lobes of the human cerebral cortex. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1133 Figure 26.21 The human cerebral cortex includes the frontal, parietal, temporal, and occipital lobes. The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which processes smells. The frontal lobe also contains the motor cortex, which is important for planning and implementing movement. Areas within the motor cortex map to different muscle groups, and there is some organization to this map, as shown in Figure 26.22. For example, the neurons that control movement of the fingers are next to the neurons that control movement of the hand. Neurons in the frontal lobe also control cognitive functions like maintaining attention, speech, and decisionmaking. Studies of humans who have damaged their frontal lobes show that parts of this area are involved in personality, socialization, and assessing risk. Figure 26.22 Different parts of the motor cortex control different muscle groups. Muscle groups that are neighbors in the body are generally controlled by neighboring regions of the motor cortex as well. For example, the neurons that control finger movement are near the neurons that control hand movement. The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and also reading. Two of the parietal lobe’s main functions are processing somatosensation—touch sensations like pressure, pain, heat, cold—and processing proprioception—the sense of how parts of the body are oriented in space. The parietal lobe contains |
a somatosensory map of the body similar to the motor cortex. The occipital lobe is located at the back of the brain. It is primarily involved in vision—seeing, recognizing, and identifying the visual world. The temporal lobe is located at the base of the brain by your ears and is primarily involved in processing and interpreting sounds. It also contains the hippocampus (Greek for “seahorse”)—a structure that processes memory formation. The hippocampus is illustrated in Figure 26.24. The role of the hippocampus in memory was partially determined by studying one famous epileptic patient, HM, who had both sides of his hippocampus removed in an attempt to cure his epilepsy. His seizures went away, but he could no longer form new memories (although he could remember some facts from before his 1134 Chapter 26 | The Nervous System surgery and could learn new motor tasks). Cerebral Cortex Compared to other vertebrates, mammals have exceptionally large brains for their body size. An entire alligator’s brain, for example, would fill about one and a half teaspoons. This increase in brain to body size ratio is especially pronounced in apes, whales, and dolphins. While this increase in overall brain size doubtlessly played a role in the evolution of complex behaviors unique to mammals, it does not tell the whole story. Scientists have found a relationship between the relatively high surface area of the cortex and the intelligence and complex social behaviors exhibited by some mammals. This increased surface area is due, in part, to increased folding of the cortical sheet (more sulci and gyri). For example, a rat cortex is very smooth with very few sulci and gyri. Cat and sheep cortices have more sulci and gyri. Chimps, humans, and dolphins have even more. Figure 26.23 Mammals have larger brain-to-body ratios than other vertebrates. Within mammals, increased cortical folding and surface area is correlated with complex behavior. Basal Ganglia Interconnected brain areas called the basal ganglia (or basal nuclei), shown in Figure 26.20b, play important roles in movement control and posture. Damage to the basal ganglia, as in Parkinson’s disease, leads to motor impairments like a shuffling gait when walking. The basal ganglia also regulate motivation. For example, when a wasp sting led to bilateral basal ganglia damage in a 25-year-old businessman, he began to spend all his days in bed and showed no interest in anything or anybody. |
But when he was externally stimulated—as when someone asked to play a card game with him—he was able to function normally. Interestingly, he and other similar patients do not report feeling bored or frustrated by their state. Thalamus The thalamus (Greek for “inner chamber”), illustrated in Figure 26.24, acts as a gateway to and from the cortex. It receives sensory and motor inputs from the body and also receives feedback from the cortex. This feedback mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and arousal state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states. A rare genetic disorder called fatal familial insomnia causes the degeneration of thalamic neurons and glia. This disorder prevents affected patients from being able to sleep, among other symptoms, and is eventually fatal. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1135 Figure 26.24 The limbic system regulates emotion and other behaviors. It includes parts of the cerebral cortex located near the center of the brain, including the cingulate gyrus and the hippocampus as well as the thalamus, hypothalamus and amygdala. Hypothalamus Below the thalamus is the hypothalamus, shown in Figure 26.24. The hypothalamus controls the endocrine system by sending signals to the pituitary gland, a pea-sized endocrine gland that releases several different hormones that affect other glands as well as other cells. This relationship means that the hypothalamus regulates important behaviors that are controlled by these hormones. The hypothalamus is the body’s thermostat—it makes sure key functions like food and water intake, energy expenditure, and body temperature are kept at appropriate levels. Neurons within the hypothalamus also regulate circadian rhythms, sometimes called sleep cycles. Limbic System The limbic system is a connected set of structures that regulates emotion, as well as behaviors related to fear and motivation. It plays a role in memory formation and includes parts of the thalamus and hypothalamus as well as the hippocampus. One important structure within the limbic system is a temporal lobe structure called the amygdala (Greek for “almond”), illustrated in Figure 26.24. The two amygdala are important both for the sensation of fear and for recognizing fearful faces. The cingulate gyrus helps regulate emotions and pain. Cerebell |
um The cerebellum (Latin for “little brain”), shown in Figure 26.21, sits at the base of the brain on top of the brainstem. The cerebellum controls balance and aids in coordinating movement and learning new motor tasks. Brainstem The brainstem, illustrated in Figure 26.21, connects the rest of the brain with the spinal cord. It consists of the midbrain, medulla oblongata, and the pons. Motor and sensory neurons extend through the brainstem allowing for the relay of signals between the brain and spinal cord. Ascending neural pathways cross in this section of the brain allowing the left hemisphere of the cerebrum to control the right side of the body and vice versa. The brainstem coordinates motor control signals sent from the brain to the body. The brainstem controls several important functions of the body including alertness, arousal, breathing, blood pressure, digestion, heart rate, swallowing, walking, and sensory and motor information integration. 1136 Chapter 26 | The Nervous System Activity Create a representation to illustrate what parts of the brain allow you to perform a favorite daily activity, like kicking a soccer ball, learning a new dance move, or reading the information in this section of the text and jotting down a few notes. Spinal Cord Connecting to the brainstem and extending down the body through the spinal column is the spinal cord, shown in Figure 26.21. The spinal cord is a thick bundle of nerve tissue that carries information about the body to the brain and from the brain to the body. The spinal cord is contained within the bones of the vertebrate column but is able to communicate signals to and from the body through its connections with spinal nerves (part of the peripheral nervous system). A cross-section of the spinal cord looks like a white oval containing a gray butterfly-shape, as illustrated in Figure 26.25. Myelinated axons make up the “white matter” and neuron and glial cell bodies make up the “gray matter.” Gray matter is also composed of interneurons, which connect two neurons each located in different parts of the body. Axons and cell bodies in the dorsal (facing the back of the animal) spinal cord convey mostly sensory information from the body to the brain. Axons and cell bodies in the ventral (facing the front of the animal) spinal cord primarily transmit signals controlling movement from the brain to the body. The spinal cord also controls motor reflexes. These reflex |
es are quick, unconscious movements—like automatically removing a hand from a hot object. Reflexes are so fast because they involve local synaptic connections. For example, the knee reflex that a doctor tests during a routine physical is controlled by a single synapse between a sensory neuron and a motor neuron. While a reflex may only require the involvement of one or two synapses, synapses with interneurons in the spinal column transmit information to the brain to convey what happened (the knee jerked, or the hand was hot). In the United States, there around 10,000 spinal cord injuries each year. Because the spinal cord is the information superhighway connecting the brain with the body, damage to the spinal cord can lead to paralysis. The extent of the paralysis depends on the location of the injury along the spinal cord and whether the spinal cord was completely severed. For example, if the spinal cord is damaged at the level of the neck, it can cause paralysis from the neck down, whereas damage to the spinal column further down may limit paralysis to the legs. Spinal cord injuries are notoriously difficult to treat because spinal nerves do not regenerate, although ongoing research suggests that stem cell transplants may be able to act as a bridge to reconnect severed nerves. Researchers are also looking at ways to prevent the inflammation that worsens nerve damage after injury. One such treatment is to pump the body with cold saline to induce hypothermia. This cooling can prevent swelling and other processes that are thought to worsen spinal cord injuries. Figure 26.25 A cross-section of the spinal cord shows gray matter (containing cell bodies and interneurons) and white matter (containing axons). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1137 26.4 | The Peripheral Nervous System In this section, you will explore the following questions: • What are the organization and functions of the sympathetic and parasympathetic nervous systems? • What is the organization and function of the sensory-somatic nervous system? Connection for AP® Courses The information about the peripheral nervous system is not within the scope of AP®. However, it is important to note that the peripheral nervous system (PNS) is the connection between the central nervous system and the rest of the body. The PNS can be broken down into the autonomic nervous system, which controls bodily functions without conscious control, and the sensory-s |
omatic nervous system, which transmits sensory information from the skin, muscles, and sensory organs to the CNS and motor commands from the CNS to the muscles. The autonomic nervous system can be further divided into the parasympathetic and sympathetic pathways. “Rest and digest” responses are activated by the parasympathetic division, whereas “fight or flight” responses are activated by the sympathetic division. In other words, these two systems often have opposing effects on target organs; for example, activation of the parasympathetic system slows heart rate, whereas activation of the sympathetic system increases heart rate. (If, as you’re reading this information, a Tyrannosaurus rex barged into the room, which division would be activated?) The sensory-somatic nervous system is made up of cranial and spinal nerves with both sensory and motor neurons. The peripheral nervous system (PNS) is the connection between the central nervous system and the rest of the body. The CNS is like the power plant of the nervous system. It creates the signals that control the functions of the body. The PNS is like the wires that go to individual houses. Without those “wires,” the signals produced by the CNS could not control the body (and the CNS would not be able to receive sensory information from the body either). The PNS can be broken down into the autonomic nervous system, which controls bodily functions without conscious control, and the sensory-somatic nervous system, which transmits sensory information from the skin, muscles, and sensory organs to the CNS and sends motor commands from the CNS to the muscles. 1138 Chapter 26 | The Nervous System Autonomic Nervous System Figure 26.26 In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a postganglionic neuron of the PNS. The postganglionic neuron, in turn, acts on a target organ. Autonomic responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one another. The sympathetic system activates the “fight or flight” response, while the parasympathetic system activates the “rest and digest” response. Which of the following statements about the autonomic nervous system is true? a. The sympathetic pathway is responsible for resting the body, whereas the parasympathetic pathway is responsible for preparing for an emergency. b. Most preganglionic neurons in the sympathetic pathway originate in the spinal |
cord. c. Slowing of the heartbeat is a sympathetic response. d. Parasympathetic neurons are responsible for releasing norepinephrine on the target organ, whereas sympathetic neurons are responsible for releasing acetylcholine. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1139 The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control; it can continuously monitor the conditions of these different systems and implement changes as needed. Signaling to the target tissue usually involves two synapses: a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ, as illustrated in Figure 26.26. There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system. Sympathetic Nervous System The sympathetic nervous system is responsible for the “fight or flight” response that occurs when an animal encounters a dangerous situation. One way to remember this is to think of the surprise a person feels when encountering a snake (“snake” and “sympathetic” both begin with “s”). Examples of functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare an organism’s body for the physical strain required to escape a potentially dangerous situation or to fend off a predator. Figure 26.27 The sympathetic and parasympathetic nervous systems often have opposing effects on target organs. Most preganglionic neurons in the sympathetic nervous system originate in the spinal cord, as illustrated in Figure 26.27. The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic ganglia (the sympathetic ganglia form a chain that extends alongside the spinal cord). The acetylcholine activates the postganglionic neurons. Postganglionic neurons then release norepinephrine onto target organs. As anyone who has ever felt a rush before a big test, speech, or athletic event can attest, the effects of the sympathetic nervous system are quite pervasive. This is both beca<|endoftext|>use one pregang |
lionic neuron synapses on multiple postganglionic neurons, amplifying the effect of the original synapse, and because the adrenal gland also releases norepinephrine (and the closely related hormone epinephrine) into the blood stream. The physiological effects of this norepinephrine release include dilating the trachea and bronchi (making it easier for the animal to breathe), increasing heart rate, and moving blood from the skin to the heart, muscles, and brain (so 1140 Chapter 26 | The Nervous System the animal can think and run). The strength and speed of the sympathetic response helps an organism avoid danger, and scientists have found evidence that it may also increase LTP—allowing the animal to remember the dangerous situation and avoid it in the future. Parasympathetic Nervous System While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous system allows an animal to “rest and digest.” One way to remember this is to think that during a restful situation like a picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with “p”). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the bottom) spinal cord, as shown in Figure 26.27. The axons of the preganglionic neurons release acetylcholine on the postganglionic neurons, which are generally located very near the target organs. Most postganglionic neurons release acetylcholine onto target organs, although some release nitric oxide. The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated (the common adrenaline dump you feel after a ‘fight-or-flight’ event). Effects of acetylcholine release on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion. Sensory-Somatic Nervous System The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Without its sensorysomatic nervous system, an animal would be unable to process any information about its environment (what it sees, feels, hears, and so on |
) and could not control motor movements. Unlike the autonomic nervous system, which has two synapses between the CNS and the target organ, sensory and motor neurons have only one synapse—one ending of the neuron is at the organ and the other directly contacts a CNS neuron. Acetylcholine is the main neurotransmitter released at these synapses. Humans have 12 cranial nerves, nerves that emerge from or enter the skull (cranium), as opposed to the spinal nerves, which emerge from the vertebral column. Each cranial nerve is accorded a name, which are detailed in Figure 26.28. Some cranial nerves transmit only sensory information. For example, the olfactory nerve transmits information about smells from the nose to the brainstem. Other cranial nerves transmit almost solely motor information. For example, the oculomotor nerve controls the opening and closing of the eyelid and some eye movements. Other cranial nerves contain a mix of sensory and motor fibers. For example, the glossopharyngeal nerve has a role in both taste (sensory) and swallowing (motor). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1141 Figure 26.28 The human brain contains 12 cranial nerves that receive sensory input and control motor output for the head and neck. Spinal nerves transmit sensory and motor information between the spinal cord and the rest of the body. Each of the 31 spinal nerves (in humans) contains both sensory and motor axons. The sensory neuron cell bodies are grouped in structures called dorsal root ganglia and are shown in Figure 26.29. Each sensory neuron has one projection—with a sensory receptor ending in skin, muscle, or sensory organs—and another that synapses with a neuron in the dorsal spinal cord. Motor neurons have cell bodies in the ventral gray matter of the spinal cord that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the spinal cord but are sometimes directly stimulated by sensory neurons. Figure 26.29 Spinal nerves contain both sensory and motor axons. The somas of sensory neurons are located in dorsal root ganglia. The somas of motor neurons are found in the ventral portion of the gray matter of the spinal cord. 1142 Chapter 26 | The Nervous System 26.5 | Nervous System Disorders In this section, you will |
explore the following questions: • What are examples of symptoms, causes, and treatments for several examples of nervous system disorders? Connection for AP® Courses Information about disorders of the nervous system is out of scope for AP®. A nervous system that functions correctly is a complex and well-oiled machine—synapses fire appropriately, muscle move when needed, memories are formed and stored, and emotions are well regulated. You can now appreciate what it takes for you to be able to read and comprehend the information in this textbook. Unfortunately, each year millions of people in the United States deal with some sort of disorder involving the nervous system. Neurodegenerative disorders are characterized by loss of nervous system functioning usually caused by the death of neurons; examples include Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS). Neurodevelopmental disorders occur when the development of the nervous system is disturbed; examples include autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), schizophrenia, and major depression. Epilepsy and stroke also have neurological origins. Although scientists have discovered potential causes of many of these diseases, and effective treatments for some, ongoing research into the prevention and treatment of these disorders continues. A nervous system that functions correctly is a fantastically complex, well-oiled machine—synapses fire appropriately, muscles move when needed, memories are formed and stored, and emotions are well regulated. Unfortunately, each year millions of people in the United States deal with some sort of nervous system disorder. While scientists have discovered potential causes of many of these diseases, and viable treatments for some, ongoing research seeks to find ways to better prevent and treat all of these disorders. Neurodegenerative Disorders Neurodegenerative disorders are illnesses characterized by a loss of nervous system functioning that are usually caused by neuronal death. These diseases generally worsen over time as more and more neurons die. The symptoms of a particular neurodegenerative disease are related to where in the nervous system the death of neurons occurs. Spinocerebellar ataxia, for example, leads to neuronal death in the cerebellum. The death of these neurons causes problems in balance and walking. Neurodegenerative disorders include Huntington’s disease, amyotrophic lateral sclerosis, Alzheimer’s disease and other types of dementia disorders, and Parkinson’s disease. Here, Alzheimer’s and Parkinson’s disease will be discussed in more depth. Alzheimer’s Disease Alzheimer’s disease is |
the most common cause of dementia in the elderly. In 2012, an estimated 5.4 million Americans suffered from Alzheimer’s disease, and payments for their care are estimated at $200 billion. Roughly one in every eight people age 65 or older has the disease. Due to the aging of the baby-boomer generation, there are projected to be as many as 13 million Alzheimer’s patients in the United States in the year 2050. Symptoms of Alzheimer’s disease include disruptive memory loss, confusion about time or place, difficulty planning or executing tasks, poor judgment, and personality changes. Problems smelling certain scents can also be indicative of Alzheimer’s disease and may serve as an early warning sign. Many of these symptoms are also common in people who are aging normally, so it is the severity and longevity of the symptoms that determine whether a person is suffering from Alzheimer’s. Alzheimer’s disease was named for Alois Alzheimer, a German psychiatrist who published a report in 1911 about a woman who showed severe dementia symptoms. Along with his colleagues, he examined the woman’s brain following her death and reported the presence of abnormal clumps, which are now called amyloid plaques, along with tangled brain fibers called neurofibrillary tangles. Amyloid plaques, neurofibrillary tangles, and an overall shrinking of brain volume are commonly seen in the brains of Alzheimer’s patients. Loss of neurons in the hippocampus is especially severe in advanced Alzheimer’s patients. Figure 26.30 compares a normal brain to the brain of an Alzheimer’s patient. Many research groups are examining the causes of these hallmarks of the disease. One form of the disease is usually caused by mutations in one of three known genes. This rare form of early onset Alzheimer’s disease affects fewer than five percent of patients with the disease and causes dementia beginning between the ages of 30 and 60. The more prevalent, late-onset form of the disease likely also has a genetic component. One particular This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1143 gene, apolipoprotein E (APOE) has a variant (E4) that increases a carrier’s likelihood of getting the disease. Many other genes have been identified that might be involved in the pathology. Visit this website (http://openstaxcollege.org/ |
l/alzheimers) for video links discussing genetics and Alzheimer’s disease. What statement about risk genes in Alzheimer’s disease is true? a. Risk genes do not guarantee that a person will get Alzheimer’s disease. b. Risk genes account for <5% of Alzheimer’s cases. c. Risk genes directly cause a disease. d. Individuals with risk genes have symptoms that develop when an individual is in their 40s or 50s. Unfortunately, there is no cure for Alzheimer’s disease. Current treatments focus on managing the symptoms of the disease. Because decrease in the activity of cholinergic neurons (neurons that use the neurotransmitter acetylcholine) is common in Alzheimer’s disease, several drugs used to treat the disease work by increasing acetylcholine neurotransmission, often by inhibiting the enzyme that breaks down acetylcholine in the synaptic cleft. Other clinical interventions focus on behavioral therapies like psychotherapy, sensory therapy, and cognitive exercises. Since Alzheimer’s disease appears to hijack the normal aging process, research into prevention is prevalent. Smoking, obesity, and cardiovascular problems may be risk factors for the disease, so treatments for those may also help to prevent Alzheimer’s disease. Some studies have shown that people who remain intellectually active by playing games, reading, playing musical instruments, and being socially active in later life have a reduced risk of developing the disease. Figure 26.30 Compared to a normal brain (left), the brain from a patient with Alzheimer’s disease (right) shows a dramatic neurodegeneration, particularly within the ventricles and hippocampus. (credit: modification of work by “Garrando”/Wikimedia Commons based on original images by ADEAR: "Alzheimer's Disease Education and Referral Center, a service of the National Institute on Aging”) Parkinson’s Disease Like Alzheimer’s disease, Parkinson’s disease is a neurodegenerative disease. It was first characterized by James Parkinson in 1817. Each year, 50,000-60,000 people in the United States are diagnosed with the disease. Parkinson’s disease causes the loss of dopamine neurons in the substantia nigra, a midbrain structure that regulates movement. Loss of these neurons causes many symptoms including tremor (shaking of fingers or a limb), slowed movement, speech changes, balance and posture problems, and rigid muscles. The combination of |
these symptoms often causes a characteristic slow hunched 1144 Chapter 26 | The Nervous System shuffling walk, illustrated in Figure 26.31. Patients with Parkinson’s disease can also exhibit psychological symptoms, such as dementia or emotional problems. Although some patients have a form of the disease known to be caused by a single mutation, for most patients the exact causes of Parkinson’s disease remain unknown: the disease likely results from a combination of genetic and environmental factors (similar to Alzheimer’s disease). Post-mortem analysis of brains from Parkinson’s patients shows the presence of Lewy bodies—abnormal protein clumps—in dopaminergic neurons. The prevalence of these Lewy bodies often correlates with the severity of the disease. There is no cure for Parkinson’s disease, and treatment is focused on easing symptoms. One of the most commonly prescribed drugs for Parkinson’s is L-DOPA, which is a chemical that is converted into dopamine by neurons in the brain. This conversion increases the overall level of dopamine neurotransmission and can help compensate for the loss of dopaminergic neurons in the substantia nigra. Other drugs work by inhibiting the enzyme that breaks down dopamine. Figure 26.31 Parkinson’s patients often have a characteristic hunched walk. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1145 Figure 26.32 Amyotrophic lateral sclerosis, also known as ALS or Lou Gehrig’s disease, is a rapidly progressing disease that attacks the neurons that control voluntary muscles. Stephen Hawking, one of the greatest scientists in modern times, suffers from ALS. Credit: ²°¹°° The roots of the term amyotrophic lateral sclerosis (ALS) provide clues as to what happens to an individual with this disease. “Amyotrophic” means “no muscle nourishment,” “lateral” refers to the part of the spine that tells muscles what to do, and “sclerosis” indicates that the lateral portion of the spine that controls muscle movement hardens. Explain how ALS causes a decline in voluntary muscle control over time. a. ALS degenerates sensory neurons that control voluntary muscle movement. As the lateral portion of the spine that controls muscle movement hardens, signals are no longer sent to muscles. Initially, muscles weaken but coordination is not effected, and eventually paralysis occurs. b |
. ALS degenerates motor neurons that control voluntary muscle movement. As the lateral portion of the spine that controls muscle movement hardens, signals are no longer sent to muscles. Initially, muscles strengthen and coordination is effected, and eventually paralysis occurs. c. ALS degenerates sensory neurons that control voluntary muscle movement. As the lateral portion of the spine that controls muscle movement hardens, signals are no longer sent to muscles. Initially, muscles weaken and coordination is effected, and eventually paralysis occurs. d. ALS degenerates motor neurons that control voluntary muscle movement. As the lateral portion of the spine that controls muscle movement hardens, signals are no longer sent to muscles. Initially, muscles weaken and coordination is effected, and eventually paralysis occurs. Neurodevelopmental Disorders Neurodevelopmental disorders occur when the development of the nervous system is disturbed. There are several different classes of neurodevelopmental disorders. Some, like Down Syndrome, cause intellectual deficits. Others specifically affect communication, learning, or the motor system. Some disorders like autism spectrum disorder and attention deficit/ hyperactivity disorder have complex symptoms. 1146 Autism Chapter 26 | The Nervous System Autism spectrum disorder (ASD) is a neurodevelopmental disorder. Its severity differs from person to person. Estimates for the prevalence of the disorder have changed rapidly in the past few decades. Current estimates suggest that one in 88 children will develop the disorder. ASD is four times more prevalent in males than females. This video (http://openstaxcollege.org/l/autism) discusses possible reasons why there has been a recent increase in the number of people diagnosed with autism. Which of the following is a partial explanation of the increase in autism spectrum disorder diagnosis between 1992 and 2005? a. diagnostic criteria changed b. increased vaccination rates c. more young parents reproducing d. decreased ascertainment across classes A characteristic symptom of ASD is impaired social skills. Children with autism may have difficulty making and maintaining eye contact and reading social cues. They also may have problems feeling empathy for others. Other symptoms of ASD include repetitive motor behaviors (such as rocking back and forth), preoccupation with specific subjects, strict adherence to certain rituals, and unusual language use. Up to 30 percent of patients with ASD develop epilepsy, and patients with some forms of the disorder (like Fragile X) also have intellectual disability. Because it is a spectrum disorder, other ASD patients are very functional and have good-to-excellent language skills. Many of these patients do not feel that they suffer from a disorder and instead think that their |
brains just process information differently. Except for some well-characterized, clearly genetic forms of autism (like Fragile X and Rett’s Syndrome), the causes of ASD are largely unknown. Variants of several genes correlate with the presence of ASD, but for any given patient, many different mutations in different genes may be required for the disease to develop. At a general level, ASD is thought to be a disease of “incorrect” wiring. Accordingly, brains of some ASD patients lack the same level of synaptic pruning that occurs in non-affected people. In the 1990s, a research paper linked autism to a common vaccine given to children. This paper was retracted when it was discovered that the author falsified data, and follow-up studies showed no connection between vaccines and autism. Treatment for autism usually combines behavioral therapies and interventions, along with medications to treat other disorders common to people with autism (depression, anxiety, obsessive compulsive disorder). Although early interventions can help mitigate the effects of the disease, there is currently no cure for ASD. Attention Deficit Hyperactivity Disorder (ADHD) Approximately three to five percent of children and adults are affected by attention deficit/hyperactivity disorder (ADHD). Like ASD, ADHD is more prevalent in males than females. Symptoms of the disorder include inattention (lack of focus), executive functioning difficulties, impulsivity, and hyperactivity beyond what is characteristic of the normal developmental stage. Some patients do not have the hyperactive component of symptoms and are diagnosed with a subtype of ADHD: attention deficit disorder (ADD). Many people with ADHD also show comorbitity, in that they develop secondary disorders in addition to ADHD. Examples include depression or obsessive compulsive disorder (OCD). Figure 26.33 provides some statistics concerning comorbidity with ADHD. The cause of ADHD is unknown, although research points to a delay and dysfunction in the development of the prefrontal cortex and disturbances in neurotransmission. According to studies of twins, the disorder has a strong genetic component. There are several candidate genes that may contribute to the disorder, but no definitive links have been discovered. Environmental factors, including exposure to certain pesticides, may also contribute to the development of ADHD in some This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1147 patients. Treatment for ADHD often involves behavioral therapies and the prescription of stimulant medications, which paradoxically cause |
a calming effect in these patients. Figure 26.33 Many people with ADHD have one or more other neurological disorders. (credit “chart design and illustration”: modification of work by Leigh Coriale; credit “data”: Drs. Biederman and Faraone, Massachusetts General Hospital). Neurologist Neurologists are physicians who specialize in disorders of the nervous system. They diagnose and treat disorders such as epilepsy, stroke, dementia, nervous system injuries, Parkinson’s disease, sleep disorders, and multiple sclerosis. Neurologists are medical doctors who have attended college, medical school, and completed three to four years of neurology residency. When examining a new patient, a neurologist takes a full medical history and performs a complete physical exam. The physical exam contains specific tasks that are used to determine what areas of the brain, spinal cord, or peripheral nervous system may be damaged. For example, to check whether the hypoglossal nerve is functioning correctly, the neurologist will ask the patient to move his or her tongue in different ways. If the patient does not have full control over tongue movements, then the hypoglossal nerve may be damaged or there may be a lesion in the brainstem where the cell bodies of these neurons reside (or there could be damage to the tongue muscle itself). If the patient has had a seizure, Neurologists have other tools besides a physical exam they can use to diagnose particular problems the neurologist can use in the nervous system. electroencephalography (EEG), which involves taping electrodes to the scalp to record brain activity, to try to determine which brain regions are involved in the seizure. In suspected stroke patients, a neurologist can use a computerized tomography (CT) scan, which is a type of X-ray, to look for bleeding in the brain or other health conditions. To treat patients with neurological problems, neurologists can prescribe medications or refer the patient to a neurosurgeon for surgery. for example, 1148 Chapter 26 | The Nervous System This website (http://openstaxcollege.org/l/neurologic_exam) allows you to see the different tests a neurologist might use to see what regions of the nervous system may be damaged in a patient. What exam might a neurologist perform if a patient had impaired sensory functions? a. placing a 128 Hz tuning fork over a bone b. tapping a muscle tendon with a reflex hammer c. asking a patient to follow commands d. asking |
a patient to follow a target through different positions Mental Illnesses Mental illnesses are nervous system disorders that result in problems with thinking, mood, or relating with other people. These disorders are severe enough to affect a person’s quality of life and often make it difficult for people to perform the routine tasks of daily living. Debilitating mental disorders plague approximately 12.5 million Americans (about 1 in 17 people) at an annual cost of more than $300 billion. There are several types of mental disorders including schizophrenia, major depression, bipolar disorder, anxiety disorders and phobias, post-traumatic stress disorders, and obsessive-compulsive disorder (OCD), among others. The American Psychiatric Association publishes the Diagnostic and Statistical Manual of Mental Disorders (or DSM), which describes the symptoms required for a patient to be diagnosed with a particular mental disorder. Each newly released version of the DSM contains different symptoms and classifications as scientists learn more about these disorders, their causes, and how they relate to each other. A more detailed discussion of two mental illnesses—schizophrenia and major depression—is given below. Schizophrenia Schizophrenia is a serious and often debilitating mental illness affecting one percent of people in the United States. Symptoms of the disease include the inability to differentiate between reality and imagination, inappropriate and unregulated emotional responses, difficulty thinking, and problems with social situations. People with schizophrenia can suffer from hallucinations and hear voices; they may also suffer from delusions. Patients also have so-called “negative” symptoms like a flattened emotional state, loss of pleasure, and loss of basic drives. Many schizophrenic patients are diagnosed in their late adolescence or early 20s. The development of schizophrenia is thought to involve malfunctioning dopaminergic neurons and may also involve problems with glutamate signaling. Treatment for the disease usually requires antipsychotic medications that work by blocking dopamine receptors and decreasing dopamine neurotransmission in the brain. This decrease in dopamine can cause Parkinson’s disease-like symptoms in some patients. While some classes of antipsychotics can be quite effective at treating the disease, they are not a cure, and most patients must remain medicated for the rest of their lives. Depression Major depression affects approximately 6.7 percent of the adults in the United States each year and is one of the most common mental disorders. To be diagnosed with major depressive disorder, a person must have experienced a severely depressed mood lasting longer than two weeks along with other symptoms including a loss of enjoyment in activities that were previously enjoyed, changes in appetite and sleep schedules, |
difficulty concentrating, feelings of worthlessness, and suicidal thoughts. The exact causes of major depression are unknown and likely include both genetic and environmental risk factors. Some research supports the “classic monoamine hypothesis,” which suggests that depression is caused by a decrease in norepinephrine and serotonin neurotransmission. One argument against this hypothesis is the fact that some antidepressant medications cause an increase in norepinephrine and serotonin release within a few hours of beginning treatment—but clinical results of these medications are not seen until weeks later. This has led to alternative hypotheses: for example, dopamine may also be decreased in depressed patients, or it may actually be an increase in norepinephrine and serotonin that causes the disease, and antidepressants force a feedback loop that decreases this release. Treatments for This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1149 depression include psychotherapy, electroconvulsive therapy, deep-brain stimulation, and prescription medications. There are several classes of antidepressant medications that work through different mechanisms. For example, monoamine oxidase inhibitors (MAO inhibitors) block the enzyme that degrades many neurotransmitters (including dopamine, serotonin, norepinephrine), resulting in increased neurotransmitter in the synaptic cleft. Selective serotonin reuptake inhibitors (SSRIs) block the reuptake of serotonin into the presynaptic neuron. This blockage results in an increase in serotonin in the synaptic cleft. Other types of drugs such as norepinephrine-dopamine reuptake inhibitors and norepinephrine-serotonin reuptake inhibitors are also used to treat depression. Other Neurological Disorders There are several other neurological disorders that cannot be easily placed in the above categories. These include chronic pain conditions, cancers of the nervous system, epilepsy disorders, and stroke. Epilepsy and stroke are discussed below. Epilepsy Estimates suggest that up to three percent of people in the United States will be diagnosed with epilepsy in their lifetime. While there are several different types of epilepsy, all are characterized by recurrent seizures. Epilepsy itself can be a symptom of a brain injury, disease, or other illness. For example, people who have intellectual disability or ASD can experience seizures, presumably because the developmental wiring malfunctions that caused their disorders also put them at risk for epilepsy. For many patients, however, the cause of their epilepsy is never identified and is likely to be a combination |
of genetic and environmental factors. Often, seizures can be controlled with anticonvulsant medications. However, for very severe cases, patients may undergo brain surgery to remove the brain area where seizures originate. Stroke A stroke results when blood fails to reach a portion of the brain for a long enough time to cause damage. Without the oxygen supplied by blood flow, neurons in this brain region die. This neuronal death can cause many different symptoms—depending on the brain area affected— including headache, muscle weakness or paralysis, speech disturbances, sensory problems, memory loss, and confusion. Stroke is often caused by blood clots and can also be caused by the bursting of a weak blood vessel. Strokes are extremely common and are the third most common cause of death in the United States. On average one person experiences a stroke every 40 seconds in the United States. Approximately 75 percent of strokes occur in people older than 65. Risk factors for stroke include high blood pressure, diabetes, high cholesterol, and a family history of stroke. Because a stroke is a medical emergency, patients with symptoms of a stroke should immediately go to the emergency room, where they can receive drugs that will dissolve any clot that may have formed. These drugs will not work if the stroke was caused by a burst blood vessel or if the stroke occurred more than three hours before arriving at the hospital. Treatment following a stroke can include blood pressure medication (to prevent future strokes) and (sometimes intense) physical therapy. 1150 Chapter 26 | The Nervous System KEY TERMS acetylcholine neurotransmitter released by neurons in the central nervous system and peripheral nervous system action potential self-propagating momentary change in the electrical potential of a neuron (or muscle) membrane Alzheimer’s disease neurodegenerative disorder characterized by problems with memory and thinking amygdala structure within the limbic system that processes fear arachnoid mater spiderweb-like middle layer of the meninges that cover the central nervous system astrocyte glial cell in the central nervous system that provide nutrients, extracellular buffering, and structural support for neurons; also makes up the blood-brain barrier attention deficit hyperactivity disorder (ADHD) neurodevelopmental disorder characterized by difficulty maintaining attention and controlling impulses autism spectrum disorder (ASD) communication abilities neurodevelopmental disorder characterized by impaired social interaction and autonomic nervous system part of the peripheral nervous system that controls bodily functions axon tube-like structure that propagates a signal from a neuron’s cell body to ax |
on terminals axon hillock electrically sensitive structure on the cell body of a neuron that integrates signals from multiple neuronal connections axon terminal structure on the end of an axon that can form a synapse with another neuron basal ganglia interconnected collections of cells in the brain that are involved in movement and motivation; also known as basal nuclei basal nuclei see basal ganglia brainstem portion of the brain that connects with the spinal cord; controls basic nervous system functions like breathing, heart rate, and swallowing cerebellum brain structure involved in posture, motor coordination, and learning new motor actions cerebral cortex outermost sheet of brain tissue; involved in many higher-order functions cerebrospinal fluid (CSF) clear liquid that surrounds the brain and spinal cord and fills the ventricles and central canal; acts as a shock absorber and circulates material throughout the brain and spinal cord. choroid plexus spongy tissue within ventricles that produces cerebrospinal fluid cingulate gyrus experiences helps regulate emotions and pain; thought to directly drive the body’s conscious response to unpleasant corpus callosum thick fiber bundle that connects the cerebral hemispheres cranial nerve sensory and/or motor nerve that emanates from the brain dendrite structure that extends away from the cell body to receive messages from other neurons depolarization change in the membrane potential to a less negative value dura mater tough outermost layer that covers the central nervous system ependymal cell that lines fluid-filled ventricles of the brain and the central canal of the spinal cord; involved in production of cerebrospinal fluid This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1151 epilepsy neurological disorder characterized by recurrent seizures excitatory postsynaptic potential (EPSP) molecules released from a presynaptic cell depolarization of a postsynaptic membrane caused by neurotransmitter frontal lobe part of the cerebral cortex that contains the motor cortex and areas involved in planning, attention, and language glia (also, glial cells) cells that provide support functions for neurons gyrus (plural: gyri) ridged protrusions in the cortex hippocampus brain structure in the temporal lobe involved in processing memories hyperpolarization change in the membrane potential to a more negative value hypothalamus brain structure that controls hormone release and body homeostasis inhibitory postsynaptic potential (IPSP) |
hyperpolarization of a postsynaptic membrane caused by neurotransmitter molecules released from a presynaptic cell limbic system connected brain areas that process emotion and motivation long-term depression (LTD) prolonged decrease in synaptic coupling between a pre- and postsynaptic cell long-term potentiation (LTP) prolonged increase in synaptic coupling between a pre-and postsynaptic cell major depression mental illness characterized by prolonged periods of sadness membrane potential difference in electrical potential between the inside and outside of a cell meninge membrane that covers and protects the central nervous system microglia glia that scavenge and degrade dead cells and protect the brain from invading microorganisms myelin fatty substance produced by glia that insulates axons neurodegenerative disorder nervous system disorder characterized by the progressive loss of neurological functioning, usually caused by neuron death neuron specialized cell that can receive and transmit electrical and chemical signals nodes of Ranvier gaps in the myelin sheath where the signal is recharged norepinephrine neurotransmitter and hormone released by activation of the sympathetic nervous system occipital lobe part of the cerebral cortex that contains visual cortex and processes visual stimuli oligodendrocyte glial cell that myelinates central nervous system neuron axons parasympathetic nervous system division of autonomic nervous system that regulates visceral functions during rest and digestion parietal lobe part of the cerebral cortex involved in processing touch and the sense of the body in space Parkinson’s disease neurodegenerative disorder that affects the control of movement pia mater thin membrane layer directly covering the brain and spinal cord proprioception sense about how parts of the body are oriented in space radial glia glia that serve as scaffolds for developing neurons as they migrate to their final destinations refractory period period after an action potential when it is more difficult or impossible for an action potential to be 1152 Chapter 26 | The Nervous System fired; caused by inactivation of sodium channels and activation of additional potassium channels of the membrane saltatory conduction “jumping” of an action potential along an axon from one node of Ranvier to the next satellite glia glial cell that provides nutrients and structural support for neurons in the peripheral nervous system schizophrenia mental disorder characterized by the inability to accurately perceive reality; patients often have difficulty thinking clearly and can suffer from delusions Schwann cell glial cell that creates myelin sheath around a peripheral nervous system neuron axon sensory-somatic nervous system system of sensory and motor nerves somatosensation sense of touch spinal cord |
thick fiber bundle that connects the brain with peripheral nerves; transmits sensory and motor information; contains neurons that control motor reflexes spinal nerve nerve projecting between skin or muscle and spinal cord sulcus (plural: sulci) indents or “valleys” in the cortex summation process of multiple presynaptic inputs creating EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential sympathetic nervous system division of autonomic nervous system activated during stressful “fight or flight” situations synapse junction between two neurons where neuronal signals are communicated synaptic cleft space between the presynaptic and postsynaptic membranes synaptic vesicle spherical structure that contains a neurotransmitter temporal lobe part of the cerebral cortex that processes auditory input; parts of the temporal lobe are involved in speech, memory, and emotion processing thalamus brain area that relays sensory information to the cortex threshold of excitation level of depolarization needed for an action potential to fire ventricle cavity within brain that contains cerebrospinal fluid CHAPTER SUMMARY 26.1 Neurons and Glial Cells The nervous system is made up of neurons and glia. Neurons are specialized cells that are capable of sending electrical as well as chemical signals. Most neurons contain dendrites, which receive these signals, and axons that send signals to other neurons or tissues. There are four main types of neurons: unipolar, bipolar, multipolar, and pseudounipolar neurons. Glia are non-neuronal cells in the nervous system that support neuronal development and signaling. There are several types of glia that serve different functions. 26.2 How Neurons Communicate Neurons have charged membranes because there are different concentrations of ions inside and outside of the cell. Voltagegated ion channels control the movement of ions into and out of a neuron. When a neuronal membrane is depolarized to at least the threshold of excitation, an action potential is fired. The action potential is then propagated along a myelinated axon to the axon terminals. In a chemical synapse, the action potential causes release of neurotransmitter molecules into the synaptic cleft. Through binding to postsynaptic receptors, the neurotransmitter can cause excitatory or inhibitory postsynaptic potentials by depolarizing or hyperpolarizing, respectively, the postsynaptic membrane. In electrical synapses, the action potential is directly communicated to the postsynaptic cell through gap junctions |
—large channel This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1153 proteins that connect the pre-and postsynaptic membranes. Synapses are not static structures and can be strengthened and weakened. Two mechanisms of synaptic plasticity are long-term potentiation and long-term depression. 26.3 The Central Nervous System The vertebrate central nervous system contains the brain and the spinal cord, which are covered and protected by three meninges. The brain contains structurally and functionally defined regions. In mammals, these include the cortex (which can be broken down into four primary functional lobes: frontal, temporal, occipital, and parietal), basal ganglia, thalamus, hypothalamus, limbic system, cerebellum, and brainstem—although structures in some of these designations overlap. While functions may be primarily localized to one structure in the brain, most complex functions, like language and sleep, involve neurons in multiple brain regions. The spinal cord is the information superhighway that connects the brain with the rest of the body through its connections with peripheral nerves. It transmits sensory and motor input and also controls motor reflexes. 26.4 The Peripheral Nervous System The peripheral nervous system contains both the autonomic and sensory-somatic nervous systems. The autonomic nervous system provides unconscious control over visceral functions and has two divisions: the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is activated in stressful situations to prepare the animal for a “fight or flight” response. The parasympathetic nervous system is active during restful periods. The sensory-somatic nervous system is made of cranial and spinal nerves that transmit sensory information from skin and muscle to the CNS and motor commands from the CNS to the muscles. 26.5 Nervous System Disorders Some general themes emerge from the sampling of nervous system disorders presented above. The causes for most disorders are not fully understood—at least not for all patients—and likely involve a combination of nature (genetic mutations that become risk factors) and nurture (emotional trauma, stress, hazardous chemical exposure). Because the causes have yet to be fully determined, treatment options are often lacking and only address symptoms. REVIEW QUESTIONS 1. Where are parasympathetic preganglionic cell bodies located? a. cerebellum b. brainstem c. dorsal root gang |
lia d. spinal cord 2. Which of the following statements about the parasympathetic nervous system is true? a. controls “fight or flight” response b. can reset organ function to the normal range c. transmits information from the skin to the central nervous system d. stimulates glycogen breakdown 3. Proper nervous system function involves various types of organic molecules. In particular, what is released by motor nerve endings onto muscle cells or tissue? a. acetylcholine b. norepinephrine c. dopamine d. serotonin 4. If the sensory-somatic nervous system of an animal is damaged, what might happen? a. enhanced processing of environmental information b. decreased digestion ability c. perpetually low heart rate d. impaired control of motor movements 5. The nervous system regulates proper processing of information and behavior control. The parasympathetic and sympathetic nervous systems are part of the _____ nervous system. a. autonomic b. sensory-somatic c. central d. “fight or flight” 6. Medications can be used to treat certain neurodevelopmental disorders. For example, which medications are often used to treat patients with ADHD? a. tranquilizers b. blood pressure medications c. stimulants d. anti-convulsant medications 1154 Chapter 26 | The Nervous System 7. If a child appears to have impaired social skills, such as difficulty reading social cues or making eye contact, what might they be tested for? a. major depression b. attention deficit hyperactivity disorder (ADHD) c. schizophrenia d. autism spectrum disorder 8. Parkinson’s disease is a neurodegenerative disease that CRITICAL THINKING QUESTIONS 9. When you stick your hand in a bucket of ice, it grows numb after a while. Based on what you know regarding neuronal signaling, explain how the sensation of touch is blocked from signaling to the brain. 10. Lidocaine is a local anesthetic that works by blocking voltage-gated sodium channels. Explain how blocking voltage-gated sodium channels would cause numbness and pain. 11. What are the main differences between the sympathetic and parasympathetic nervous systems? a. The sympathetic nervous system is activated by stressful situations, whereas the parasympathetic nervous system resets organ function of sympathetic reactions and allows animals to “rest and digest.” b. The parasympathetic nervous system is activated by stressful situations, whereas the sympathetic nervous system resets organ function of sympathetic reactions and |
allows animals to “rest and digest.” c. The sympathetic nervous system is involved in unconscious body function control, whereas the parasympathetic nervous system is involved in conscious body function control. d. The parasympathetic nervous system is involved in unconscious body function control, whereas the sympathetic nervous system is involved in conscious body function control. 12. How is the sensory-somatic nervous system involved in sensing information and motor function? can produce symptoms such as tremors, slowed movement, speech changes, balance and posture problems, and rigid muscles. Parkinson’s disease is caused by the degeneration of neurons that release ____. a. serotonin b. dopamine c. glutamate d. norepinephrine a. The sensory-somatic nervous system transmits information from the skin, muscles, and sensory organs to the peripheral nervous system. Motor information is sent to and from the central nervous system and the muscles. b. The sensory-somatic nervous system transmits information from the skin, muscles, and sensory organs to the central nervous system. Motor information is sent to and from the central nervous system and the muscles. c. The sensory-somatic nervous system transmits information from the skin, muscles, and sensory organs to the central nervous system. Motor information is sent to and from the peripheral nervous system and the muscles. d. The sensory-somatic nervous system transmits information from the skin, muscles, and sensory organs to the peripheral nervous system. Motor information is sent to and from the peripheral nervous system and the muscles. 13. Public speaking can be very stressful. How can anticipating giving a public speech stimulate the sympathetic nervous system? a. During stress, multiple preganglionic neurons can synapse on one postganglionic neuron, and the adrenal gland releases adrenaline. b. During stress, one preganglionic neuron can synapse on multiple postganglionic neurons, and the thymus gland releases norepinephrine. c. During stress, one postganglionic neuron can synapse on multiple preganglionic neurons, and the adrenal gland releases norepinephrine. d. During stress, one preganglionic neuron can synapse on multiple postganglionic neurons, and the adrenal gland releases norepinephrine. 14. What might make you suspect that an individual has Alzheimer’s disease? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter |
26 | The Nervous System 1155 a. disruptive memory loss, confusion about time or place, difficulty with planning and executing tasks, poor judgment, and/or personality changes a. blood pressure medication, deep-brain stimulation, taking monoamine oxidase inhibitors, psychotherapy, and physical therapy b. psychotherapy, electroconvulsive therapy, deepbrain stimulation, taking monoamine oxidase inhibitors, and/or taking selective melatonin reuptake inhibitors c. psychotherapy, electroconvulsive therapy, deepbrain stimulation, taking monoamine oxidase inhibitors, and/or taking selective serotonin reuptake inhibitors d. blood pressure medication, classes of antipsychotics, psychotherapy, electroconvulsive therapy, deep-brain stimulation, and/or taking selective serotonin reuptake inhibitors a. This neuron would not be able to receive signals. b. This neuron would not be able to recharge the signal. c. This neuron would not be able to integrate information from numerous synapses. d. This neuron would not be able to send signals. b. c. slowed movements, balance and posture problems, rigid muscles, speech changes, and/or psychological symptoms such as dementia impaired social skills, repetitive motor behaviors, strict adherence to certain rituals, and preoccupation with specific subjects d. balance and posture problems, repetitive motor behaviors, difficulty with planning and executing tasks, poor judgment, and/or personality changes 15. What treatment options are available for an individual diagnosed with major depression? TEST PREP FOR AP® COURSES 16. If a neuron has damaged synapses, what would be impaired? a. Integration of signals from several synapses b. Speed of signal transduction c. Receiving signals from other neurons d. Ability to recharge electrical signals 17. Signal transmission from one neuron to another requires a series of processes pertaining to different components of each neuron. What happens at the axon terminals to facilitate signal transmission to another neuron? a. Chemicals released at the axon terminals transmit signals through synapses into other neurons via the second neuron’s dendrites. b. Chemicals released at the axon terminals transmit signals through synapses into other neurons via the second neuron’s axons. c. Chemicals released at the dendrites transmit signals through synapses into other neurons via the second neuron’s axon terminal. 19. d. Chemicals released at the axon terminals transmit signals directly into other neurons via the second neuron’s axons. 18. This figure shows |
a malformed neuron. Why would this neuron be nonfunctional? 1156 Chapter 26 | The Nervous System a. A signal is released from an axon, passes through the axon terminal, and synapses with dendrites. Dendrites receive the signal, which passes through the soma. Multiple signals from a single synapse are integrated at the axon hillock, which then passes the signal into the axon, where the signal is transferred to another cell. b. A signal is released from axon terminal, passes through the axon, and synapse with dendrites. Dendrites receive the signal, which passes through the soma. Multiple signals from multiple synapses are integrated at the axon hillock, which then passes the signal into the axon, where the signal is transferred to another cell. c. A signal is released from an axon and passes through the axon terminal, which synapses with dendrites. Dendrites receive the signal as it passes through the soma. Multiple signals from multiple synapses are integrated at the axon hillock, which then passes the signal into the axon, where the signal is transferred to another cell. d. A signal is released from the axon terminal, passes through the axon, and synapse with dendrites. Dendrites receive the signal as it passes through the soma. Multiple signals from a single synapse are integrated at the axon hillock, which then passes the signal into the axon, where the signal is transferred to another cell. 20. Transmission of signals between two neurons requires proper communication between neurons. Dendrites are a component of many neurons that facilitate signal reception. Which of the following is true of dendrites? a. All neurons have several dendrites for signal reception. b. Dendritic spines decrease possible synaptic connections. c. Dendrites carry the signal to the soma. d. Chemical release at dendrites allows signal communication to other cells. 21. This figure shows the transmission of a signal among a network of neurons. How is a signal transferred from one neuron to another? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1157 Resting membrane potential has a negative charge. Which ions correspond to each row of data in the chart? a. b. c. d. Ion 1: Cl-, |
Ion 2: Na+, Ion 3: K+ Ion 1: Na+, Ion 2: K+, Ion 3: Cl- Ion 1: K+, Ion 2: Na+, Ion 3: Cl- Ion 1:Cl-, Ion 2: K+, Ion 3: Na+ 22. Voltage-gated ion channels are essential for producing an action potential and returning a neuron to its resting state. Why would it be impossible to trigger an action potential without voltage-gated ion channels? a. The cell would not undergo depolarization, which is necessary to fire an action potential and then return the cell to the resting state. b. The cell would not undergo repolarization, which is necessary to fire an action potential and then return the cell to the resting state. c. The cell would not undergo depolarization, repolarization, and hyperpolarization, which are necessary to fire an action potential and then return the cell to the resting state. d. The cell would not undergo depolarization and hyperpolarization, which are necessary to fire an action potential and then return the cell to the resting state. When an action potential is fired, what happens immediately after the peak action potential occurs? a. Na+ channels open. b. K+ channels open. c. K+ channels close. d. Na+/K+ transporter restores resting potential. 24. Potassium channel blockers, such as amiodarone and procainamide, which are used to treat abnormal electrical activity in the heart, impede the movement of K+through voltage-gated K+channels. Which part of the action potential would potassium channels affect, and why? a. Depolarization after peak action potential would be affected because that is the point when K+ begins to leave the cell. b. Repolarization after peak action potential would be affected because that is the point when K+ begins to leave the cell. c. Repolarization after peak action potential would be affected because that is the point when K+ begins to enter the cell. d. Polarization after peak action potential would be affected because that is the point when K+ begins to enter the cell. 23. 25. 1158 Chapter 26 | The Nervous System This figure shows the transfer of an action potential through a neuron. What is occurring in panel 3? a. Depolarization occurs closest to the cell body. b. The first part of the neuron cannot fire another action potential. c. The first part of the neuron can fire |
another action potential. d. Sodium channels have closed. 26. This figure depicts an essential component of signal formation and transmission in neurons. What is happening in this figure? a. A nerve impulse opens the Na+ channel, which makes Na+ enter the cell and depolarizes the membrane. b. A nerve impulse opens the Ca+2 channel, which makes Ca+2 enter the cell and depolarizes the membrane. c. A nerve impulse opens the Na+ channel, which makes Na+ enter the cell and repolarizes the membrane. d. A nerve impulse opens the K+ channel, which makes K+ enter the cell and polarizes the membrane. 27. Chemical and electrical synapse are two mechanisms by which signals can be transferred between neurons. Which of the following occurs during chemical synapse? a. Repolarization at the presynaptic membrane b. Calcium influx causes synaptic vesicles to fuse to the membrane c. Neurotransmitters diffuse out of gap junctions d. Neurotransmitters bind to synaptic vesicles 28. Chemical synapse is a multiple-step process in which neurotransmitters undergo transfer and binding to different parts of the cell. What happens when a neurotransmitter binds to ligand-gated ion channels? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1159 a. The ligand-gated ion channels open. b. The presynaptic neuron reuptakes the neurotransmitter. c. The neurotransmitter diffuses away from the synapse. d. The neurotransmitter is enzymatically degraded. 29. Different components of the brain control different parts of the body. One important part of the brain is the occipital lobe. What might happen if an individual’s occipital lobe was damaged? a. The individual would not feel hot or cold. b. The individual would be unable to form new memories. c. The individual would be unable to recognize certain objects. d. The individual would have no sense of smell. 30. Both cerebral hemispheres are essential for proper body function. However, the left cerebral hemisphere controls the right side of the body, whereas the right cerebral hemisphere controls the left side of the body. Why is this the case? a. The descending neural connections are not switched in the brainstem, which means that the neural connections of the left hemisphere are transmitted to the right side of |
the body and vice versa. b. The ascending neural connections are not switched in the brainstem, which means that the neural connections of the left hemisphere are transmitted to the right side of the body and vice versa. c. The descending neural connections are switched in the brainstem, which means that the neural connections of the left hemisphere are transmitted to the right side of the body and vice versa. d. The ascending neural connections are switched in the brainstem, which means that the neural connections of the left hemisphere are transmitted to the right side of the body and vice versa. 31. If an increased number of folds in the cortical sheets of the brain is associated with increased social complexity, which of the following animals has the greatest social complexity? a. Rat b. Dolphin c. Chimpanzee d. Cat 32. 1160 Chapter 26 | The Nervous System This image shows a cross section of the spinal column. How does gray matter facilitate communication along the spinal column? 34. a. All myelin sheaths are located in the gray matter, which transmit signals along the brain and spinal cord through the gray matter. b. All synapses are located in the gray matter, which transmit signals along the brain and spinal cord through the gray matter. c. All synapses are located in the gray matter, which transmit signals along the spinal cord through the gray matter. d. All dendrites are located in the gray matter, which transmit signals along the spinal cord through the gray matter. 33. This figure represents a split-brain individual processing information. What has happened to the brain of this individual? Why does the processing of information occur as depicted? a. The parietal lobe has been cut, which severs the ability of the left hemisphere from communicating but increases the ability of the right hemisphere. b. The corpus callosum has been cut, which severs the ability of the left hemisphere from communicating but increases the ability of the right hemisphere. c. The frontal lobe has been cut, which severs the ability of the left and right hemispheres to communicate. d. The corpus callosum has been cut, which severs the ability of the left and right hemispheres to communicate. 35. The thalamus is part of the brain that is involved in various functions in the human body. What might result from the damage of an individual’s thalamus? a. Insomnia b. Lack of interest in everything c. Lack of fear d. Inability to learn new motor tasks This |
figure depicts the parts of the body that are controlled by different parts of the motor cortex. What can be inferred about the organization of the motor cortex relative to the organization of muscles in the body? a. The motor cortex is found throughout the body. b. Motor cortex neurons are generally located near neurons that control nearby body parts. c. Motor cortex neurons control speaking and processing what an individual reads. d. The motor cortex controls involuntary muscle movements. SCIENCE PRACTICE CHALLENGE QUESTIONS 36. A neurotransmitter provides a chemical signal between neurons to inhibit or excite an action potential. A. Describe a model of this signaling and in this description include the roles played by synapse, receptors, post and pre-synaptic neurons, exocytosis, endocytosis, ligand-gated ion channel and the electric potential of the This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 26 | The Nervous System 1161 membrane. B. Explain the stimulatory or inhibitory effect of key ionic elements, Na+ and Cl-, on the electric potential of the postsynaptic membrane. C. Modify the diagram to create a representation of the effect explained above. Select from the following list to fill in the blanks: the motion of potassium ions is driven by the concentration gradient • with a labeled arrow indicate the direction in which the motion of sodium ions is driven by the concentration gradient • give a brief statement of the roles of potassium and sodium ion pumps in maintaining the rest electric potential • with a labeled arrow indicate the relative sign of the electric potential difference (voltage) between intracellular and extracellular spaces at the rest electric potential Figure 26.35 When an excitatory neurotransmitter receptor is activated the electric potential difference of membrane of a neuron is lowered inducing a change in the configuration of sodium pump proteins. B. Justify the effect on the flux of sodium ions across the membrane as a positive feedback in a situation in which the electric potential difference falls below a threshold voltage and an action potential is created. The action potential is transmitted along the neuron as a voltage wave. One cycle of the wave is shown below the diagram at the instant at which the maximum of the electric potential of the membrane has been reached. Figure 26.34 • Na+ • Cl- • • stimulatory inhibitory D. In the 1960s Burnstock and co-workers provided evidence that ATP is a neurotrans |
mitter. This was received skeptically and largely rejected until 1984 when a modified form of ATP that was known to block the intracellular function of ATP was shown to effect extracellular signal transmission. Based on the central role played by ATP in biological systems justify the resistance within the scientific community to accept a role for ATP as a neurotransmitter. Based on the fact that ATP has been conserved throughout evolution of life on Earth justify such a role for ATP. Based on these two perspectives analyze the role of cooperative interactions in the positive selection of ATP as a neurotransmitter. 37. Neurons and muscle cells maintain a high concentration gradient of potassium ions across the plasma membrane. The extracellular space has a high concentration of sodium ions. At the rest electric potential the cell membrane is polarized. A. Construct a representation of the cell membrane with annotation of the diagram below that includes the following: • with a labeled arrow indicate the direction in which Figure 26.36 1162 Chapter 26 | The Nervous System C. Construct a representation of the key elements of the signal propagation with annotation of the diagram that includes the following: Consider the interaction of these three cell types that integrate information to produce a response to external cues: • a labeled arrow that indicates the direction in which the motion of potassium ions is driven by the concentration gradient • a labeled arrow that indicates the direction in which the motion of potassium ions is driven by the electric potential difference across the membrane • give a brief statement of the roles of potassium and sodium ion pumps in terminating the action potential Most neurons must transmit a signal quickly. The sarcolemmas (muscle cell membranes) of the cardiac muscles receive signals that integrate information from both the sympathetic (quick response with shorter time scale) and parasympathetic (steady response with longer time scale) divisions of the autonomic nervous system. The action potential that induces periodic contractions of the cardiac muscle (see figure below) is broadened at the maximum by the release of Ca+2 from the smooth endoplasmic reticulum, referred to as calcium-induced calcium release (CICR). Figure 26.37 D. In terms of the function of the heart in the supply of oxygen and nutrients during “fight or flight” or restful conditions, justify the claim that this broadening demonstrates that the coordination of events must be regulated. E. To stop a beating heart during open-heart surgery a solution of KCl is injected into the cardiac muscle. Explain the effect of a large dose of ext |
racellular K+ on the transmission of the action potential in the sarcolemma. 38. The brain integrates new information through the formation of memories and by learning. Alternative explanations of the ability of the brain to remodel in response to experience, called plasticity, are given. This item explores those explanations. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Figure 26.38 A. Use the figure to construct a representation of the direction of information flow. The central body of a neuron is elaborated by tree-like structures called dendrites. These allow the neuron to integrate information from multiple sensory receptors. B. Describe what refinement of the basic stimulusresponse system in the diagram is needed to achieve even the simplest response: “move away.” Awareness of orientation and motion within a body is called proprioception. Describe how multiple neurons are required to acquire proprioception. The generation of neurons occurs during development. However, adults continue to form memories and learn. Rearrangement of connections between neurons is a possible explanation and in several studies steroid hormones have been shown to produce dendritic plasticity. The hippocampus is active during memory formation and learning and significant variations in the number of dendrites were observed in the hippocampus (Wooley et al., Journal of Neuroscience, 10, 1990) were correlated with variations in estrogen during the estrus cycle. More recently variation in these structures is implicated in a collection of behaviors known as chronic unpredictable mild stress (CUMS). In rats CUMS can be induced by environmental factors such as electric shock, immobilization, or isolation (Qiao et al., Neural Plasticity, 2016). C. Pose two scientific questions that can be investigated to connect the dynamic homeostasis and survival advantage of the individual to dendritic plasticity. An alternative explanation of the manner in which the brain integrates new information is through synaptic plasticity. This has been demonstrated by Nabavi and coworkers (Nature, 511, 2014). An associated memory was created in a rat by pairing two stimuli: an audio tone and a foot shock. The animal had previously been trained to avoid pressing a lever that delivers a reward by associating the lever press with a shock. After conditioning the animal responded to the tone as if it was a shock and avoided Chapter 26 | The Nervous System 1163 reward. The ratio of stimulatory to inhibitory receptors at the synaptic membrane was shown to increase with |
the experience. A miniaturized device, optically activated and controlled by flashing light, was inserted in the nuclei of neurons transmitting the tone to the rat’s brain. When the experimenters used light with 1 flash per second (1 Hz) the device caused the expression by the cell of one type of protein and when a light with 100 flashes per second (100 Hz) was used the device caused expression of another type of protein. Each of the graphs describe the response of the rat to environmental cues. One day elapsed between each data collection represented by one graph. Figure 26.39 D. Analyze these data in terms of the evidence provided for synaptic plasticity. A third explanation for the formation of memory and learning is found in the lab of David Glanzman (Elife, 2014). The sea slug (Aplysia) can be trained to withdraw its siphon tube. Sensory and motor neurons can be grown in tissue culture. The addition of serotonin to the tissue culture increases the number of synaptic connections and the training can be induced in vitro. Cells that had acquired the stimulus-response behavior were treated with an agent that destroys the synaptic receptors. Yet the trained response was retained and there were indications that the information was retrieved from the neuron nucleus. E. Suppose that this work concerning the location of memory is confirmed. Create a representation of information flow in which a fourth box labeled “neuron nucleus” is added to the diagram in part A between the stimulus and the neuron. Annotate the representation to indicate the flow of information. Explain Pose questions regarding the ethical or social consequences of this technology. • how this form of plasticity is more dynamic than theories in which memory resides in synaptic or dendritic structures, and • how it might lead to a treatments for disorders, such as post-traumatic stress syndrome, in which recollection creates a disability. 39. Autism is a collection of communication and socialization behaviors. Evidence of inheritance of genes predisposing the individual during early development is indicated by pedigrees such as the following (after AllenBrady, Molecular Psychiatry, 14, 2009). Males (squares) and females (circles) are affected when the symbol is filled, are struck through when deceased and the genome cannot be determined, and are dashed when living and the genome was not determined. Figure 26.40 A. Other evidence indicates that autism is not x-linked. Give an alternative explanation that can account for these data. B. Stem cells taken from fathers who do |
not present characteristic of autism and from their sons were induced to form tissue cultures of neurons. Compared to the father those taken from the son showed accelerated growth with a higher number of synapses. Describe possible consequences for the integration of information if this in vitro growth also occurs in vivo. The variety of phenotypes and large number of genes that have been implicated in this disorder have led researchers to refer to the characteristics as autisms described by a spectrum of disorders, ASD. One of the gene implicated is bola2. While humans and other primates have genomes that are reported to have only a 2% deviation the particular form of bola2 that occurs in 99% of human genomes that have been mapped does not occur in other primates. And bola2 is not present in the Neanderthal genome. Even more interesting is that single nucleotide variations in human bola2 are significantly less frequent than genes 1164 Chapter 26 | The Nervous System associated with other brain disorders such as schizophrenia. C. Evaluate the selection pressure and direction (positive or negative) indicated by this observation. D. Several hundred genes have been implicated in ASD and many others probably will eventually be discovered. Expression in a gene networks can depend on factors that are both genetic and environmental. Given the complexity of ASD what questions should be researched by the physician of children or their parents when genetic screening is considered? 40. Describe how neurons transmit information. 41. You are probably acquainted with the effects of local anesthetics. While the injection of lidocaine at the dentist is unpleasant no injection would be more so. Lidocaine is a sodium channel blocker. A. Explain the absence of pain in terms of the effect of lidocaine on signal reception and transduction. The pain of the dentist’s drill is caused by trauma at the cellular level. Chemical messengers such as cytokines, serotonin, and prostaglandins are released by broken cells. The receptors for these messages of trauma are called nociceptors whose activation is transmitted to the central nervous system by specialized cells called the A and C fibres. B. The nervous system is a network of cells and tissues that is activated by these chemical messengers. Identify another system that should be activated by these messengers and support your claim by applying the idea that dynamic homeostasis is maintained by timing and coordination of regulated events. C. Chronic pain often persists after damaged tissue has healed. This pain is often accompanied by sterile inflammation with components of the innate immune system such |
as macrophages. Refine the model of coordinated response identified in part B to describe how chemical messengers associated with the immune response can cause chronic pain. Unlike local anesthetics general anesthetics block signal transduction of the entire central nervous system and the brain. However, while the patient is unconscious the peripheral nervous system continues to support signaling to other systems such as heart and lungs. An explanation might be that the signal in the central and peripheral nervous systems are segregated and that the latter functions without cognitive integration (thought) as the name “autonomic” implies. The respiratory center that provides autonomic control of breathing is part of the medulla oblongata. D. Create a visual representation of system composed only of the cortex, the medulla oblongata, the heart and the lungs. Using arrows describe the flow of information. Consider “holding your breath” in creating your representation. Consider why you always stop holding your breath eventually. Consider “holding your heart.” Experimental data on the voluntary control of heart rate by people who practice yoga have been reported (Raghavendra et al., International Journal of Yoga, 6, 2013; Telles et al., Integrative Physiological and Behavioral Science, 39, 2004). E. Analyze the data provided in the following sketch of blood flow, a process controlled by the autonomic nervous system, in the two ears of a rabbit (after Blessing, Trends in Neuroscience, 20, 1997) in terms of cognitive integration of the response to the stimulus provided by touching the rabbit. Figure 26.41 This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1165 27 | SENSORY SYSTEMS Figure 27.1 This shark uses its senses of sight, vibration (lateral-line system), and smell to hunt, but it also relies on its ability to sense the electric fields of prey, a sense not present in most land animals. (credit: modification of work by Hermanus Backpackers Hostel, South Africa) Chapter Outline 27.1: Sensory Processes 27.2: Somatosensation 27.3: Taste and Smell 27.4: Hearing and Vestibular Sensation 27.5: Vision Introduction All bilaterally symmetric animals have a sensory system, the development of which has been driven by natural selection. Thus, sensory systems differ among species according to the demands of their environments. Animals� |
� senses are constantly at work, making them aware of stimuli, such as light, or sound, or the presence of a chemical substance in the external environment. They also monitor information about the organism’s internal environment. The shark pictured above has the ability to perceive natural electrical stimuli produced by other animals in its environment, a sense called electroreception. This enhanced ability to sense prey gives the shark an evolutionary advantage over other fish. While it is helpful to this underwater predator, electroreception is a sense not found in most land animals. You can read more about electroreception in sharks at the Sharks info website (http://openstax.org/l/32sharks). Connection for AP® Courses The content in this chapter is not within the scope of AP® other than to recognize the link between the sensory organs and the nervous system. The ability to detect and respond to information is critical to an organism’s survival and fitness. If time permits, you might explore the evolution of one type of sensory receptor (photoreceptors, chemoreceptor, thermoreceptor, or proprioceptor) in several different animal species, with special consideration of the features that allow it to convert a stimulus to a nerve impulse. (This task is an application of information in Big Idea 1 of the AP® Curriculum Framework—that the evolution of a structure such as a sensory receptor supports the concept that evolution continues to occur.) 1166 Chapter 27 | Sensory Systems 27.1 | Sensory Processes In this section, you will explore the following questions: • What are the general and special senses in humans? • What are three important steps in sensory perception? • What is the concept of just-noticeable difference in sensory perception? Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration. Vestibular sensation, which is an organism’s sense of spatial orientation and balance, proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track kinesthesia (limb movement) are part of somatosensation. Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (such as light, or |
sound, or the position of the body) into an electrical signal in the nervous system. This process is called sensory transduction. There are two broad types of cellular systems that perform sensory transduction. In one, a neuron works with a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus. Stimulation of the sensory receptor activates the associated afferent neuron, which carries information about the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment: this neuron constitutes the sensory receptor. Free nerve endings can be stimulated by several different stimuli, thus showing little receptor specificity. For example, pain receptors in your gums and teeth may be stimulated by temperature changes, chemical stimulation, or pressure. Reception The first step in sensation is reception, which is the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor’s receptive field. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with body. For the sense of hearing, a stimulus can be a moderate distance away (some baleen whale sounds can propagate for many kilometers). For vision, a stimulus can be very far away; for example, the visual system perceives light from stars at enormous distances. Transduction The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor, and the change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? In this example, a type of receptor called a mechanoreceptor (as shown in Figure 27.2) possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. Recall that in the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor |
) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1167 Figure 27.2 (a) Mechanosensitive ion channels are gated ion channels that respond to mechanical deformation of the plasma membrane. A mechanosensitive channel is connected to the plasma membrane and the cytoskeleton by hairlike tethers. When pressure causes the extracellular matrix to move, the channel opens, allowing ions to enter or exit the cell. (b) Stereocilia in the human ear are connected to mechanosensitive ion channels. When a sound causes the stereocilia to move, mechanosensitive ion channels transduce the signal to the cochlear nerve. Sensory receptors for different senses are very different from each other, and they are specialized according to the type of stimulus they sense: they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound; they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste. Encoding and Transmission of Sensory Information Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor’s afferent axons encode one type of stimulus, and this segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system, and electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus—a sound. The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials, and reducing the stimulus will likewise slow the rate of production of action potentials |
. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the CNS, and the brain will further process incoming signals. Perception Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory receptors, 1168 Chapter 27 | Sensory Systems perception happens not at the level of the sensory receptor, but at higher levels in the nervous system, in the brain. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that particular stimulus and synapse with particular neurons in the brain or spinal cord. All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex. Recall that the thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex (Figure 27.3) dedicated to processing that particular sense. How are neural signals interpreted? Interpretation of sensory signals between individuals of the same species is largely similar, owing to the inherited similarity of their nervous systems; however, there are some individual differences. A good example of this is individual tolerances to a painful stimulus, such as dental pain, which certainly differ. Figure 27.3 In humans, with the exception of olfaction, all sensory signals are routed from the (a) thalamus to (b) final processing regions in the cortex of the brain. (credit b: modification of work by Polina Tishina) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1169 Just-Noticeable Difference It is easy to differentiate between a one-pound bag of rice and a two-pound bag of rice. There is a onepound difference, and one bag is twice as heavy as the other. However, would it be as easy to differentiate between a 20- and a 21-pound bag? Question: What is the smallest detectible weight difference between a one-pound bag of rice and a |
larger bag? What is the smallest detectible difference between a 20-pound bag and a larger bag? In both cases, at what weights are the differences detected? This smallest detectible difference in stimuli is known as the just-noticeable difference (JND). Background: Research background literature on JND and on Weber’s Law, a description of a proposed mathematical relationship between the overall magnitude of the stimulus and the JND. You will be testing JND of different weights of rice in bags. Choose a convenient increment that is to be stepped through while testing. For example, you could choose 10 percent increments between one and two pounds (1.1, 1.2, 1.3, 1.4, and so on) or 20 percent increments (1.2, 1.4, 1.6, and 1.8). Hypothesis: Develop a hypothesis about JND in terms of percentage of the whole weight being tested (such as “the JND between the two small bags and between the two large bags is proportionally the same,” or “... is not proportionally the same.”) So, for the first hypothesis, if the JND between the one-pound bag and a larger bag is 0.2 pounds (that is, 20 percent; 1.0 pound feels the same as 1.1 pounds, but 1.0 pound feels less than 1.2 pounds), then the JND between the 20-pound bag and a larger bag will also be 20 percent. (So, 20 pounds feels the same as 22 pounds or 23 pounds, but 20 pounds feels less than 24 pounds.) Test the hypothesis: Enlist 24 participants, and split them into two groups of 12. To set up the demonstration, assuming a 10 percent increment was selected, have the first group be the one-pound group. As a counter-balancing measure against a systematic error, however, six of the first group will compare one pound to two pounds, and step down in weight (1.0 to 2.0, 1.0 to 1.9, and so on.), while the other six will step up (1.0 to 1.1, 1.0 to 1.2, and so on). Apply the same principle to the 20-pound group (20 to 40, 20 to 38, and so on, and 20 to 22, 20 to 24, and so on). Given the large difference between 20 and 40 pounds, you may wish |
to use 30 pounds as your larger weight. In any case, use two weights that are easily detectable as different. Record the observations: Record the data in a table similar to the table below. For the one-pound and 20-pound groups (base weights) record a plus sign (+) for each participant that detects a difference between the base weight and the step weight. Record a minus sign (-) for each participant that finds no difference. If one-tenth steps were not used, then replace the steps in the “Step Weight” columns with the step you are using. Results of JND Testing (+ = difference; – = no difference) Step Weight One pound 20 pounds Step Weight 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 22 24 26 28 30 32 34 36 1170 Chapter 27 | Sensory Systems Results of JND Testing (+ = difference; – = no difference) Step Weight One pound 20 pounds Step Weight 1.9 2.0 Table 27.1 38 40 Analyze the data/report the results: What step weight did all participants find to be equal with one-pound base weight? What about the 20-pound group? Draw a conclusion: Did the data support the hypothesis? Are the final weights proportionally the same? If not, why not? Do the findings adhere to Weber’s Law? Weber’s Law states that the concept that a justnoticeable difference in a stimulus is proportional to the magnitude of the original stimulus. 27.2 | Somatosensation In this section, you will explore the following questions: • What are four important mechanoreceptors in human skin? • What is the topographical distribution of somatosensory receptors between glabrous and hairy skin? • Why is the perception of pain subjective? Somatosensation is a mixed sensory category and includes all sensation received from the skin and mucous membranes, as well from as the limbs and joints. Somatosensation is also known as tactile sense, or more familiarly, as the sense of touch. Somatosensation occurs all over the exterior of the body and at some interior locations as well. A variety of receptor types—embedded in the skin, mucous membranes, muscles, joints, internal organs, and cardiovascular system—play a role. Recall that the epidermis is the outermost layer of skin in mammals. It is relatively thin, is composed of keratin-filled cells, and has no blood supply |
. The epidermis serves as a barrier to water and to invasion by pathogens. Below this, the much thicker dermis contains blood vessels, sweat glands, hair follicles, lymph vessels, and lipid-secreting sebaceous glands (Figure 27.4). Below the epidermis and dermis is the subcutaneous tissue, or hypodermis, the fatty layer that contains blood vessels, connective tissue, and the axons of sensory neurons. The hypodermis, which holds about 50 percent of the body’s fat, attaches the dermis to the bone and muscle, and supplies nerves and blood vessels to the dermis. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1171 Figure 27.4 Mammalian skin has three layers: an epidermis, a dermis, and a hypodermis. (credit: modification of work by Don Bliss, National Cancer Institute) Somatosensory Receptors Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, pain receptors, and chemoreceptors. These categories are based on the nature of stimuli each receptor class transduces. What is commonly referred to as “touch” involves more than one kind of stimulus and more than one kind of receptor. Mechanoreceptors in the skin are described as encapsulated (that is, surrounded by a capsule) or unencapsulated (a group that includes free nerve endings). A free nerve ending, as its name implies, is an unencapsulated dendrite of a sensory neuron. Free nerve endings are the most common nerve endings in skin, and they extend into the middle of the epidermis. Free nerve endings are sensitive to painful stimuli, to hot and cold, and to light touch. They are slow to adjust to a stimulus and so are less sensitive to abrupt changes in stimulation. There are three classes of mechanoreceptors: tactile, proprioceptors, and baroreceptors. Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound.” There are four primary tactile mechanoreceptors in human skin: Merkel’s disks, Meissner’s corpuscles, Ruffini |
endings, and Pacinian corpuscle; two are located toward the surface of the skin and two are located deeper. A fifth type of mechanoreceptor, Krause end bulbs, are found only in specialized regions. Merkel’s disks (shown in Figure 27.5) are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on glabrous skin, that is, the hairless skin found on the palms and fingers, the soles of the feet, and the lips of humans and other primates. Merkel’s disks are densely distributed in the fingertips and lips. They are slow-adapting, encapsulated nerve endings, and they respond to light touch. Light touch, also known as discriminative touch, is a light pressure that allows the location of a stimulus to be pinpointed. The receptive fields of Merkel’s disks are small with welldefined borders. That makes them finely sensitive to edges and they come into use in tasks such as typing on a keyboard. 1172 Chapter 27 | Sensory Systems Figure 27.5 Four of the primary mechanoreceptors in human skin are shown. Merkel’s disks, which are unencapsulated, respond to light touch. Meissner’s corpuscles, Ruffini endings, Pacinian corpuscles, and Krause end bulbs are all encapsulated. Meissner’s corpuscles respond to touch and low-frequency vibration. Ruffini endings detect stretch, deformation within joints, and warmth. Pacinian corpuscles detect transient pressure and high-frequency vibration. Krause end bulbs detect cold. Which of the following statements about mechanoreceptors is true? a. Meissner’s corpuscles extend far into the epidermis. b. Ruffini endings are the only encapsulated mechanoreceptors. c. Light touch is detected by Pacini corpuscles. d. Merkel’s disks are abundant on the fingertips and lips. Meissner’s corpuscles, (shown in Figure 27.6) also known as tactile corpuscles, are found in the upper dermis, but they project into the epidermis. They, too, are found primarily in the glabrous skin on the fingertips and eyelids. They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter. They are rapidly adapting, fluid-filled, encapsulated neurons with small, well-defined borders and |
are responsive to fine details. Like Merkel’s disks, Meissner’s corpuscles are not as plentiful in the palms as they are in the fingertips. Figure 27.6 Meissner corpuscles in the fingertips, such as the one viewed here using bright field light microscopy, allow for touch discrimination of fine detail. (credit: modification of work by "Wbensmith"/Wikimedia Commons; scalebar data from Matt Russell) Deeper in the epidermis, near the base, are Ruffini endings, which are also known as bulbous corpuscles. They are found in both glabrous and hairy skin. These are slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints, so they provide valuable feedback for gripping objects and controlling finger position and movement. Thus, they also contribute to proprioception and kinesthesia. Ruffini endings also detect warmth. Note that these This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1173 warmth detectors are situated deeper in the skin than are the cold detectors. It is not surprising, then, that humans detect cold stimuli before they detect warm stimuli. Pacinian corpuscles (seen in Figure 27.7) are located deep in the dermis of both glabrous and hairy skin and are structurally similar to Meissner’s corpuscles; they are found in the bone periosteum, joint capsules, pancreas and other viscera, breast, and genitals. They are rapidly adapting mechanoreceptors that sense deep transient (but not prolonged) pressure and high-frequency vibration. Pacinian receptors detect pressure and vibration by being compressed, stimulating their internal dendrites. There are fewer Pacinian corpuscles and Ruffini endings in skin than there are Merkel’s disks and Meissner’s corpuscles. Figure 27.7 Pacinian corpuscles, such as these visualized using bright field light microscopy, detect pressure (touch) and high-frequency vibration. (credit: modification of work by Ed Uthman; scale-bar data from Matt Russell) In proprioception, proprioceptive and kinesthetic signals travel through myelinated afferent neurons running from the spinal cord to the medulla. Neurons are not physically connected, but communicate via neurotransmitters secreted into synapses or “gaps |
” between communicating neurons. Once in the medulla, the neurons continue carrying the signals to the thalamus. Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. Related to these are Golgi tendon organs, which are tension receptors that detect the force of muscle contraction. Proprioceptive and kinesthetic signals come from limbs. Unconscious proprioceptive signals run from the spinal cord to the cerebellum, the brain region that coordinates muscle contraction, rather than to the thalamus, like most other sensory information. Baroreceptors detect pressure changes in an organ. They are found in the walls of the carotid artery and the aorta where they monitor blood pressure, and in the lungs where they detect the degree of lung expansion. Stretch receptors are found at various sites in the digestive and urinary systems. In addition to these two types of deeper receptors, there are also rapidly adapting hair receptors, which are found on nerve endings that wrap around the base of hair follicles. There are a few types of hair receptors that detect slow and rapid hair movement, and they differ in their sensitivity to movement. Some hair receptors also detect skin deflection, and certain rapidly adapting hair receptors allow detection of stimuli that have not yet touched the skin. Integration of Signals from Mechanoreceptors The configuration of the different types of receptors working in concert in human skin results in a very refined sense of touch. The nociceptive receptors—those that detect pain—are located near the surface. Small, finely calibrated mechanoreceptors—Merkel’s disks and Meissner’s corpuscles—are located in the upper layers and can precisely localize even gentle touch. The large mechanoreceptors—Pacinian corpuscles and Ruffini endings—are located in the lower layers and respond to deeper touch. (Consider that the deep pressure that reaches those deeper receptors would not need to be finely localized.) Both the upper and lower layers of the skin hold rapidly and slowly adapting receptors. Both primary somatosensory cortex and secondary cortical areas are responsible for processing the complex picture of stimuli transmitted from the interplay of mechanoreceptors. Density of Mechanoreceptors The distribution of touch receptors in human skin is not consistent over the body. In humans, touch receptors are less dense in skin covered with any type of hair, such as the arms, legs, torso, and face. Touch receptors are denser in glabrous skin 1174 Chapter |
27 | Sensory Systems (the type found on human fingertips and lips, for example), which is typically more sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm). How is receptor density estimated in a human subject? The relative density of pressure receptors in different locations on the body can be demonstrated experimentally using a two-point discrimination test. In this demonstration, two sharp points, such as two thumbtacks, are brought into contact with the subject’s skin (though not hard enough to cause pain or break the skin). The subject reports if he or she feels one point or two points. If the two points are felt as one point, it can be inferred that the two points are both in the receptive field of a single sensory receptor. If two points are felt as two separate points, each is in the receptive field of two separate sensory receptors. The points could then be moved closer and re-tested until the subject reports feeling only one point, and the size of the receptive field of a single receptor could be estimated from that distance. Thermoreception In addition to Krause end bulbs that detect cold and Ruffini endings that detect warmth, there are different types of cold receptors on some free nerve endings: thermoreceptors, located in the dermis, skeletal muscles, liver, and hypothalamus, that are activated by different temperatures. Their pathways into the brain run from the spinal cord through the thalamus to the primary somatosensory cortex. Warmth and cold information from the face travels through one of the cranial nerves to the brain. You know from experience that a tolerably cold or hot stimulus can quickly progress to a much more intense stimulus that is no longer tolerable. Any stimulus that is too intense can be perceived as pain because temperature sensations are conducted along the same pathways that carry pain sensations Pain Pain is the name given to nociception, which is the neural processing of injurious stimuli in response to tissue damage. Pain is caused by true sources of injury, such as contact with a heat source that causes a thermal burn or contact with a corrosive chemical. But pain also can be caused by harmless stimuli that mimic the action of damaging stimuli, such as contact with capsaicins, the compounds that cause peppers to taste hot and which are used in self-defense pepper sprays and certain topical medications. Peppers taste “hot” because the protein receptors that bind capsaicin open the same calcium channels that are activated by warm receptors. Noc |
iception starts at the sensory receptors, but pain, in-as-much as it is the perception of nociception, does not start until it is communicated to the brain. There are several nociceptive pathways to and through the brain. Most axons carrying nociceptive information into the brain from the spinal cord project to the thalamus (as do other sensory neurons) and the neural signal undergoes final processing in the primary somatosensory cortex. Interestingly, one nociceptive pathway projects not to the thalamus but directly to the hypothalamus in the forebrain, which modulates the cardiovascular and neuroendocrine functions of the autonomic nervous system. Recall that threatening—or painful—stimuli stimulate the sympathetic branch of the visceral sensory system, readying a fight-or-flight response. View this video (http://openstaxcollege.org/l/nociceptive) that animates the five phases of nociceptive pain. Select the stimulus that can activate the nociceptive system. a. a pleasant melody on a harp b. a delicious apple c. the aroma of freshly baked cookies d. burning your hand on a stove This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1175 27.3 | Taste and Smell In this section, you will explore the following questions: • In what way do smell and taste stimuli differ from other sensory stimuli? • What are the five primary tastes that can be distinguished by humans? • In anatomical terms why is a dog’s sense of smell more acute than a human’s? Taste, also called gustation, and smell, also called olfaction, are the most interconnected senses in that both involve molecules of the stimulus entering the body and bonding to receptors. Smell lets an animal sense the presence of food or other animals—whether potential mates, predators, or prey—or other chemicals in the environment that can impact their survival. Similarly, the sense of taste allows animals to discriminate between types of foods. While the value of a sense of smell is obvious, what is the value of a sense of taste? Different tasting foods have different attributes, both helpful and harmful. For example, sweet-tasting substances tend to be highly caloric, which could be necessary for survival in lean times. Bitterness is associated with toxicity, and sourness is associated with spoiled |
food. Salty foods are valuable in maintaining homeostasis by helping the body retain water and by providing ions necessary for cells to function. Tastes and Odors Both taste and odor stimuli are molecules taken in from the environment. The primary tastes detected by humans are sweet, sour, bitter, salty and umami. The first four tastes need little explanation. The identification of umami as a fundamental taste occurred fairly recently—it was identified in 1908 by Japanese scientist Kikunae Ikeda while he worked with seaweed broth, but it was not widely accepted as a taste that could be physiologically distinguished until many years later. The taste of umami, also known as savoriness, is attributable to the taste of the amino acid L-glutamate. In fact, monosodium glutamate, or MSG, is often used in cooking to enhance the savory taste of certain foods. What is the adaptive value of being able to distinguish umami? Savory substances tend to be high in protein. All odors that we perceive are molecules in the air we breathe. If a substance does not release molecules into the air from its surface, it has no smell. And if a human or other animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell. Humans have about 350 olfactory receptor subtypes that work in various combinations to allow us to sense about 10,000 different odors. Compare that to mice, for example, which have about 1,300 olfactory receptor types, and therefore probably sense more odors. Both odors and tastes involve molecules that stimulate specific chemoreceptors. Although humans commonly distinguish taste as one sense and smell as another, they work together to create the perception of flavor. A person’s perception of flavor is reduced if he or she has congested nasal passages. Reception and Transduction Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of the nasal cavity (as illustrated in Figure 27.8). The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity that spans an area about 5 cm2 in humans. Recall that sensory cells are neurons. An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million o |
lfactory receptors, distributed among hundreds of different receptor types that respond to different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about 100 million, most dogs have about 1 billion, and bloodhounds—dogs selectively bred for their sense of smell—have about 4 billion. The overall size of the olfactory epithelium also differs between species, with that of bloodhounds, for example, being many times larger than that of humans. Olfactory neurons are bipolar neurons (neurons with two processes from the cell body). Each neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins, and it is the variations in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia, and the receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus. 1176 Chapter 27 | Sensory Systems Figure 27.8 In the human olfactory system, (a) bipolar olfactory neurons extend from (b) the olfactory epithelium, where olfactory receptors are located, to the olfactory bulb. (credit: modification of work by Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1177 Pheromones A pheromone is a chemical released by an animal that affects the behavior or physiology of animals of the same species. Pheromonal signals can have profound effects on animals that inhale them, but pheromones apparently are not consciously perceived in the same way as other odors. There are several different types of pheromones, which are released in urine or as glandular secretions. Certain pheromones are attractants to potential mates, others are repellants to potential competitors of the same sex, and |
still others play roles in mother-infant attachment. Some pheromones can also influence the timing of puberty, modify reproductive cycles, and even prevent embryonic implantation. While the roles of pheromones in many nonhuman species are important, pheromones have become less important in human behavior over evolutionary time compared to their importance to organisms with more limited behavioral repertoires. The vomeronasal organ (VNO, or Jacobson’s organ) is a tubular, fluid-filled, olfactory organ present in many vertebrate animals that sits adjacent to the nasal cavity. It is very sensitive to pheromones and is connected to the nasal cavity by a duct. When molecules dissolve in the mucosa of the nasal cavity, they then enter the VNO where the pheromone molecules among them bind with specialized pheromone receptors. Upon exposure to pheromones from their own species or others, many animals, including cats, may display the flehmen response (shown in Figure 27.9), a curling of the upper lip that helps pheromone molecules enter the VNO. Pheromonal signals are sent, not to the main olfactory bulb, but to a different neural structure that projects directly to the amygdala (recall that the amygdala is a brain center important in emotional reactions, such as fear). The pheromonal signal then continues to areas of the hypothalamus that are key to reproductive physiology and behavior. While some scientists assert that the VNO is apparently functionally vestigial in humans, even though there is a similar structure located near human nasal cavities, others are researching it as a possible functional system that may, for example, contribute to synchronization of menstrual cycles in women living in close proximity. Figure 27.9 The flehmen response in this tiger results in the curling of the upper lip and helps airborne pheromone molecules enter the vomeronasal organ. (credit: modification of work by "chadh"/Flickr) Describe how a male snake can physiologically detect the presence of a female that is trying to attract a mate. a. The pheromones secreted by a female dissolve and enter the vomeronasal organ. The dissolved molecules bind to receptors, which send a signal to the hypothalamus, which in turn sends the signal to the amygdala. b. The pheromones secreted by a female dissolve and enter the vomeronasal organ. |
The dissolved molecules bind to receptors, which send a signal to the amygdala, which in turn sends the signal to the hypothalamus. c. The pheromones secreted by a female dissolve and enter the amygdala. The dissolved molecules bind to receptors, which send a signal to the vomeronasal organ, which in turn sends the signal to the hypothalamus. d. The pheromones secreted by a female dissolve and enter the amygdala. The dissolved molecules bind to receptors, which send a signal to the hypothalamus, which in turn sends the signal to the vomeronasal organ. 1178 Taste Chapter 27 | Sensory Systems Detecting a taste (gustation) is fairly similar to detecting an odor (olfaction), given that both taste and smell rely on chemical receptors being stimulated by certain molecules. The primary organ of taste is the taste bud. A taste bud is a cluster of gustatory receptors (taste cells) that are located within the bumps on the tongue called papillae (singular: papilla) (illustrated in Figure 27.10). There are several structurally distinct papillae. Filiform papillae, which are located across the tongue, are tactile, providing friction that helps the tongue move substances, and contain no taste cells. In contrast, fungiform papillae, which are located mainly on the anterior two-thirds of the tongue, each contain one to eight taste buds and also have receptors for pressure and temperature. The large circumvallate papillae contain up to 100 taste buds and form a V near the posterior margin of the tongue. Figure 27.10 (a) Foliate, circumvallate, and fungiform papillae are located on different regions of the tongue. (b) Foliate papillae are prominent protrusions on this light micrograph. (credit a: modification of work by NCI; scale-bar data from Matt Russell) In addition to those two types of chemically and mechanically sensitive papillae are foliate papillae—leaf-like papillae located in parallel folds along the edges and toward the back of the tongue, as seen in the Figure 27.10 micrograph. Foliate papillae contain about 1,300 taste buds within their folds. Finally, there are circumvallate papillae, which are wall-like papillae in the shape of an inverted “V” at the back of the tongue. Each of these |
papillae is surrounded by a groove and contains about 250 taste buds. Each taste bud’s taste cells are replaced every 10 to 14 days. These are elongated cells with hair-like processes called microvilli at the tips that extend into the taste bud pore (illustrate in Figure 27.11). Food molecules (tastants) are dissolved in saliva, and they bind with and stimulate the receptors on the microvilli. The receptors for tastants are located across the outer portion and front of the tongue, outside of the middle area where the filiform papillae are most prominent. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1179 Figure 27.11 Pores in the tongue allow tastants to enter taste pores in the tongue. (credit: modification of work by Vincenzo Rizzo) In humans, there are five primary tastes, and each taste has only one corresponding type of receptor. Thus, like olfaction, each receptor is specific to its stimulus (tastant). Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant. A salty tastant (containing NaCl) provides the sodium ions (Na+) that enter the taste neurons and excite them directly. Sour tastants are acids and belong to the thermoreceptor protein family. Binding of an acid or other sour-tasting molecule triggers a change in the ion channel and these increase hydrogen ion (H+) concentrations in the taste neurons, thus depolarizing them. Sweet, bitter, and umami tastants require a G-protein coupled receptor. These tastants bind to their respective receptors, thereby exciting the specialized neurons associated with them. Both tasting abilities and sense of smell change with age. In humans, the senses decline dramatically by age 50 and continue to decline. A child may find a food to be too spicy, whereas an elderly person may find the same food to be bland and unappetizing. View this animation (http://openstaxcollege.org/l/taste) that shows how the sense of taste works. Which of the following is true about human taste? a. Taste buds are covered in papillae. b. Saliva contains taste receptor cells. c. Papillae stimulate the hair-like endings of taste buds. d. The hair-like endings of taste buds generate nerve impulses to the |
brain. 1180 Chapter 27 | Sensory Systems Smell and Taste in the Brain Olfactory neurons project from the olfactory epithelium to the olfactory bulb as thin, unmyelinated axons. The olfactory bulb is composed of neural clusters called glomeruli, and each glomerulus receives signals from one type of olfactory receptor, so each glomerulus is specific to one odorant. From glomeruli, olfactory signals travel directly to the olfactory cortex and then to the frontal cortex and the thalamus. Recall that this is a different path from most other sensory information, which is sent directly to the thalamus before ending up in the cortex. Olfactory signals also travel directly to the amygdala, thereafter reaching the hypothalamus, thalamus, and frontal cortex. The last structure that olfactory signals directly travel to is a cortical center in the temporal lobe structure important in spatial, autobiographical, declarative, and episodic memories. Olfaction is finally processed by areas of the brain that deal with memory, emotions, reproduction, and thought. Taste neurons project from taste cells in the tongue, esophagus, and palate to the medulla, in the brainstem. From the medulla, taste signals travel to the thalamus and then to the primary gustatory cortex. Information from different regions of the tongue is segregated in the medulla, thalamus, and cortex. 27.4 | Hearing and Vestibular Sensation In this section, you will explore the following questions: • What is the relationship of amplitude and frequency of a sound wave to the attributes of sound? • What path does sound travel within the auditory system to the site of transduction of sound? • What are the structures of the vestibular system that respond to gravity? Audition, or hearing, is important to humans and to other animals for many different interactions. It enables an organism to detect and receive information about danger, such as an approaching predator, and to participate in communal exchanges like those concerning territories or mating. On the other hand, although it is physically linked to the auditory system, the vestibular system is not involved in hearing. Instead, an animal’s vestibular system detects its own movement, both linear and angular acceleration and deceleration, and balance. Sound Auditory stimuli are sound waves, which are mechanical, pressure waves that move through a medium, such as air or water. There are no sound |
waves in a vacuum since there are no air molecules to move in waves. The speed of sound waves differs, based on altitude, temperature, and medium, but at sea level and a temperature of 20º C (68º F), sound waves travel in the air at about 343 meters per second. As is true for all waves, there are four main characteristics of a sound wave: frequency, wavelength, period, and amplitude. Frequency is the number of waves per unit of time, and in sound is heard as pitch. High-frequency (≥15.000Hz) sounds are higher-pitched (short wavelength) than low-frequency (long wavelengths; ≤100Hz) sounds. Frequency is measured in cycles per second, and for sound, the most commonly used unit is hertz (Hz), or cycles per second. Most humans can perceive sounds with frequencies between 30 and 20,000 Hz. Women are typically better at hearing high frequencies, but everyone’s ability to hear high frequencies decreases with age. Dogs detect up to about 40,000 Hz; cats, 60,000 Hz; bats, 100,000 Hz; and dolphins 150,000 Hz, and American shad (Alosa sapidissima), a fish, can hear 180,000 Hz. Those frequencies above the human range are called ultrasound. Amplitude, or the dimension of a wave from peak to trough, in sound is heard as volume and is illustrated in Figure 27.12. The sound waves of louder sounds have greater amplitude than those of softer sounds. For sound, volume is measured in decibels (dB). The softest sound that a human can hear is the zero point. Humans speak normally at 60 decibels. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1181 Figure 27.12 For sound waves, wavelength corresponds to pitch. Amplitude of the wave corresponds to volume. The sound wave shown with a dashed line is softer in volume than the sound wave shown with a solid line. (credit: NIH) Reception of Sound In mammals, sound waves are collected by the external, cartilaginous part of the ear called the pinna, then travel through the auditory canal and cause vibration of the thin diaphragm called the tympanum or ear drum, the innermost part of the outer ear (illustrated in Figure 27.13). Interior to the tympan |
um is the middle ear. The middle ear holds three small bones called the ossicles, which transfer energy from the moving tympanum to the inner ear. The three ossicles are the malleus (also known as the hammer), the incus (the anvil), and stapes (the stirrup). The aptly named stapes looks very much like a stirrup. The three ossicles are unique to mammals, and each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic membrane. The incus attaches the malleus to the stapes. In humans, the stapes is not long enough to reach the tympanum. If we did not have the malleus and the incus, then the vibrations of the tympanum would never reach the inner ear. These bones also function to collect force and amplify sounds. The ear ossicles are homologous to bones in a fish mouth: the bones that support gills in fish are thought to be adapted for use in the vertebrate ear over evolutionary time. Many animals (frogs, reptiles, and birds, for example) use the stapes of the middle ear to transmit vibrations to the middle ear. Figure 27.13 Sound travels through the outer ear to the middle ear, which is bounded on its exterior by the tympanic membrane. The middle ear contains three bones called ossicles that transfer the sound wave to the oval window, the exterior boundary of the inner ear. The organ of Corti, which is the organ of sound transduction, lies inside the cochlea. 1182 Chapter 27 | Sensory Systems Transduction of Sound Vibrating objects, such as vocal cords, create sound waves or pressure waves in the air. When these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse (electrical signal) that the brain perceives as sound. The pressure waves strike the tympanum, causing it to vibrate. The mechanical energy from the moving tympanum transmits the vibrations to the three bones of the middle ear. The stapes transmits the vibrations to a thin diaphragm called the oval window, which is the outermost structure of the inner ear. The structures of the inner ear are found in the labyrinth, a bony, hollow structure that is the most interior portion of the ear. Here, the energy from the |
sound wave is transferred from the stapes through the flexible oval window and to the fluid of the cochlea. The vibrations of the oval window create pressure waves in the fluid (perilymph) inside the cochlea. The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal (as illustrated in Figure 27.14). Inside the cochlea, the basilar membrane is a mechanical analyzer that runs the length of the cochlea, curling toward the cochlea’s center. The mechanical properties of the basilar membrane change along its length, such that it is thicker, tauter, and narrower at the outside of the whorl (where the cochlea is largest), and thinner, floppier, and broader toward the apex, or center, of the whorl (where the cochlea is smallest). Different regions of the basilar membrane vibrate according to the frequency of the sound wave conducted through the fluid in the cochlea. For these reasons, the fluid-filled cochlea detects different wave frequencies (pitches) at different regions of the membrane. When the sound waves in the cochlear fluid contact the basilar membrane, it flexes back and forth in a wave-like fashion. Above the basilar membrane is the tectorial membrane. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1183 Figure 27.14 In the human ear, sound waves cause the stapes to press against the oval window. Vibrations travel up the fluid-filled interior of the cochlea. The basilar membrane that lines the cochlea gets continuously thinner toward the apex of the cochlea. Different thicknesses of membrane vibrate in response to different frequencies of sound. Sound waves then exit through the round window. In the cross section of the cochlea (top right figure), note that in addition to the upper canal and lower canal, the cochlea also has a middle canal. The organ of Corti (bottom image) is the site of sound transduction. Movement of stereocilia on hair cells results in an action potential that travels along the auditory nerve. Cochlear implants can restore hearing in people who have a nonfunctional |
cochlea. The implant consists of a microphone that picks up sound. A speech processor selects sounds in the range of human speech, and a transmitter converts these sounds to electrical impulses, which are then sent to the auditory nerve. Which of the following types of hearing loss would not be restored by a cochlear implant? 1. Hearing loss resulting from absence or loss of hair cells in the organ of Corti. 2. Hearing loss resulting from an abnormal auditory nerve. 3. Hearing loss resulting from fracture of the cochlea. 4. Hearing loss resulting from damage to bones of the middle ear. Cochlear implants can restore hearing in people who have a nonfunctional cochlea. The implant consists of a microphone that picks up sound. A speech processor selects sounds in the range of human speech, and a transmitter converts these sounds to electrical impulses, which are then sent to the auditory nerve. Which of the following types of hearing loss would not be restored by a cochlear implant? 1184 Chapter 27 | Sensory Systems a. Hearing loss resulting from absence or loss of hair cells in the organ of Corti. b. Hearing loss resulting from an abnormal auditory nerve. c. Hearing loss resulting from fracture of the cochlea. d. Hearing loss resulting from damage to bones of the middle ear. The site of transduction is in the organ of Corti (spiral organ). It is composed of hair cells held in place above the basilar membrane like flowers projecting up from soil, with their exposed short, hair-like stereocilia contacting or embedded in the tectorial membrane above them. The inner hair cells are the primary auditory receptors and exist in a single row, numbering approximately 3,500. The stereocilia from inner hair cells extend into small dimples on the tectorial membrane’s lower surface. The outer hair cells are arranged in three or four rows. They number approximately 12,000, and they function to fine tune incoming sound waves. The longer stereocilia that project from the outer hair cells actually attach to the tectorial membrane. All of the stereocilia are mechanoreceptors, and when bent by vibrations they respond by opening a gated ion channel (refer to Figure 27.2). As a result, the hair cell membrane is depolarized, and a signal is transmitted to the chochlear nerve. Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated. The hair cells |
are arranged on the basilar membrane in an orderly way. The basilar membrane vibrates in different regions, according to the frequency of the sound waves impinging on it. Likewise, the hair cells that lay above it are most sensitive to a specific frequency of sound waves. Hair cells can respond to a small range of similar frequencies, but they require stimulation of greater intensity to fire at frequencies outside of their optimal range. The difference in response frequency between adjacent inner hair cells is about 0.2 percent. Compare that to adjacent piano strings, which are about six percent different. Place theory, which is the model for how biologists think pitch detection works in the human ear, states that high frequency sounds selectively vibrate the basilar membrane of the inner ear near the entrance port (the oval window). Lower frequencies travel farther along the membrane before causing appreciable excitation of the membrane. The basic pitchdetermining mechanism is based on the location along the membrane where the hair cells are stimulated. The place theory is the first step toward an understanding of pitch perception. Considering the extreme pitch sensitivity of the human ear, it is thought that there must be some auditory “sharpening” mechanism to enhance the pitch resolution. When sound waves produce fluid waves inside the cochlea, the basilar membrane flexes, bending the stereocilia that attach to the tectorial membrane. Their bending results in action potentials in the hair cells, and auditory information travels along the neural endings of the bipolar neurons of the hair cells (collectively, the auditory nerve) to the brain. When the hairs bend, they release an excitatory neurotransmitter at a synapse with a sensory neuron, which then conducts action potentials to the central nervous system. The cochlear branch of the vestibulocochlear cranial nerve sends information on hearing. The auditory system is very refined, and there is some modulation or “sharpening” built in. The brain can send signals back to the cochlea, resulting in a change of length in the outer hair cells, sharpening or dampening the hair cells’ response to certain frequencies. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1185 Watch an animation (http://openstaxcollege.org/l/hearing) of sound entering the outer ear, moving through the ear structure, stimulating cochlear nerve impulses, and eventually sending |
signals to the temporal lobe. Imagine that your friend’s pet dog ran off into the woods while she was hiking. After about 10 minutes while she still hiked, your friend whistled for her dog, who came running back to her. Explain how the dog was able to use your friend’s whistling to find her, even though she had moved from the location at which the dog had last seen her. a. The dog was used to his master’s whistling. This enabled the dog to determine where the sound was coming from. b. The dog was used to the hiking place. This enabled the dog to determine where the sound was coming from. c. The ears of the dog received the sounds at the same time. This enabled the dog to use the timing of sound reception in each ear to determine where the sound was coming from. d. The two ears of the dog received the sounds at slightly different times. This enabled the dog to use the timing of sound reception in each ear to determine where the sound was coming from. Higher Processing The inner hair cells are most important for conveying auditory information to the brain. About 90 percent of the afferent neurons carry information from inner hair cells, with each hair cell synapsing with 10 or so neurons. Outer hair cells connect to only 10 percent of the afferent neurons, and each afferent neuron innervates many hair cells. The afferent, bipolar neurons that convey auditory information travel from the cochlea to the medulla, through the pons and midbrain in the brainstem, finally reaching the primary auditory cortex in the temporal lobe. Vestibular Information The stimuli associated with the vestibular system are linear acceleration (gravity) and angular acceleration and deceleration. Gravity, acceleration, and deceleration are detected by evaluating the inertia on receptive cells in the vestibular system. Gravity is detected through head position. Angular acceleration and deceleration are expressed through turning or tilting of the head. The vestibular system has some similarities with the auditory system. It utilizes hair cells just like the auditory system, but it excites them in different ways. There are five vestibular receptor organs in the inner ear: the utricle, the saccule, and three semicircular canals. Together, they make up what’s known as the vestibular labyrinth that <|endoftext|>is shown in Figure 27.15. The utricle and saccule respond |
to acceleration in a straight line, such as gravity. The roughly 30,000 hair cells in the utricle and 16,000 hair cells in the saccule lie below a gelatinous layer, with their stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate crystals—like tiny rocks. When the head is tilted, the crystals continue to be pulled straight down by gravity, but the new angle of the head causes the gelatin to shift, thereby bending the stereocilia. The bending of the stereocilia stimulates the neurons, and they signal to the brain that the head is tilted, allowing the maintenance of balance. It is the vestibular branch of the vestibulocochlear cranial nerve that deals with balance. 1186 Chapter 27 | Sensory Systems Figure 27.15 The structure of the vestibular labyrinth is shown. (credit: modification of work by NIH) The fluid-filled semicircular canals are tubular loops set at oblique angles. They are arranged in three spatial planes. The base of each canal has a swelling that contains a cluster of hair cells. The hairs project into a gelatinous cap called the cupula and monitor angular acceleration and deceleration from rotation. They would be stimulated by driving your car around a corner, turning your head, or falling forward. One canal lies horizontally, while the other two lie at about 45 degree angles to the horizontal axis, as illustrated in Figure 27.15. When the brain processes input from all three canals together, it can detect angular acceleration or deceleration in three dimensions. When the head turns, the fluid in the canals shifts, thereby bending stereocilia and sending signals to the brain. Upon cessation accelerating or decelerating—or just moving—the movement of the fluid within the canals slows or stops. For example, imagine holding a glass of water. When moving forward, water may splash backwards onto the hand, and when motion has stopped, water may splash forward onto the fingers. While in motion, the water settles in the glass and does not splash. Note that the canals are not sensitive to velocity itself, but to changes in velocity, so moving forward at 60mph with your eyes closed would not give the sensation of movement, but suddenly accelerating or braking would stimulate the receptors. Higher Processing Hair cells from the utricle, saccule, and semicircular canals also communicate through bipolar neurons to the cochlear nucleus in the medulla. Cochlear |
neurons send descending projections to the spinal cord and ascending projections to the pons, thalamus, and cerebellum. Connections to the cerebellum are important for coordinated movements. There are also projections to the temporal cortex, which account for feelings of dizziness; projections to autonomic nervous system areas in the brainstem, which account for motion sickness; and projections to the primary somatosensory cortex, which monitors subjective measurements of the external world and self-movement. People with lesions in the vestibular area of the somatosensory cortex see vertical objects in the world as being tilted. Finally, the vestibular signals project to certain optic muscles to coordinate eye and head movements. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1187 Click through this interactive tutorial (http://openstaxcollege.org/l/ear_anatomy) to review the parts of the ear and how they function to process sound. You have trouble maintaining balance. Identify the part of your ear that has been damaged. a. eustachian tube b. cochlea c. semicircular canals d. ear canal 27.5 | Vision In this section, you will explore the following questions: • How do electromagnetic waves differ from sound waves? • What path does light take as it travels through the eye to the point of the optic nerve? • What is tonic activity as it is manifested in photoreceptors in the retina? Vision is the ability to detect light patterns from the outside environment and interpret them into images. Animals are bombarded with sensory information, and the sheer volume of visual information can be problematic. Fortunately, the visual systems of species have evolved to attend to the most-important stimuli. The importance of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information. Light As with auditory stimuli, light travels in waves. The compression waves that compose sound must travel in a medium—a gas, a liquid, or a solid. In contrast, light is composed of electromagnetic waves and needs no medium; light can travel in a vacuum (Figure 27.16). The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light—a packet of electromagnetic radiation called a photon. A glance at the |
electromagnetic spectrum shows that visible light for humans is just a small slice of the entire spectrum, which includes radiation that we cannot see as light because it is below the frequency of visible red light and above the frequency of visible violet light. Certain variables are important when discussing perception of light. Wavelength (which varies inversely with frequency) manifests itself as hue. Light at the red end of the visible spectrum has longer wavelengths (and is lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength of light is expressed in nanometers (nm); one nanometer is one billionth of a meter. Humans perceive light that ranges between approximately 380 nm and 740 nm. Some other animals, though, can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers, and some non-avian reptiles sense infrared light (heat that prey gives off). 1188 Chapter 27 | Sensory Systems Figure 27.16 In the electromagnetic spectrum, visible light lies between 380 nm and 740 nm. (credit: modification of work by NASA) Wave amplitude is perceived as luminous intensity, or brightness. The standard unit of intensity of light is the candela, which is approximately the luminous intensity of a one common candle. Light waves travel 299,792 km per second in a vacuum, (and somewhat slower in various media such as air and water), and those waves arrive at the eye as long (red), medium (green), and short (blue) waves. What is termed “white light” is light that is perceived as white by the human eye. This effect is produced by light that stimulates equally the color receptors in the human eye. The apparent color of an object is the color (or colors) that the object reflects. Thus a red object reflects the red wavelengths in mixed (white) light and absorbs all other wavelengths of light. Anatomy of the Eye The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the retina (shown in Figure 27.17) on the inner surface of the back of the eye. But light does not impinge on the retina unaltered. It passes through other layers that process it so that it can be interpreted by the retina (Figure 27.17b). The cornea, the front transparent layer of the eye, and the crystalline lens, a transparent convex structure behind the cornea, both refract ( |
bend) light to focus the image on the retina. The iris, which is conspicuous as the colored part of the eye, is a circular muscular ring lying between the lens and cornea that regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | Sensory Systems 1189 Figure 27.17 (a) The human eye is shown in cross section. (b) A blowup shows the layers of the retina. Which of the following statements about the human eye is true? a. Rods detect color, whereas cones detect shades of gray. b. The pupil is the location of rods and cones. c. The iris adjusts the amount of light coming into the eye. d. The fovea is a protective layer on the front of the eye. The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing and refocusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called presbyopia results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a different type of farsightedness called hyperopia caused by an eyeball that is too short. For both defects, images in the distance are clear but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but images nearby are clear. There are two types of photoreceptors in the retina: rods and cones, named for their general appearance as illustrated in Figure 27.18. Rods are strongly photosensitive and are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina. They respond to bright |
light, and their primary role is in daytime, color vision. 1190 Chapter 27 | Sensory Systems Figure 27.18 Rods and cones are photoreceptors in the retina. Rods respond in low light and can detect only shades of gray. Cones respond in intense light and are responsible for color vision. The fovea is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object’s image falls on the fovea. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina, rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the fovea. Review the anatomical structure (http://openstaxcollege.org/l/eye_diagram) of the eye, clicking on each part to practice identification. Identify the part of your eye that could be damaged if your eyes are incapable of focusing an image on the retina. a. lens b. iris c. rods d. cones Transduction of Light The rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, rhodopsin, has two main parts Figure 27.19): an opsin, which is a membrane protein (in the form of a cluster of α-helices that span the membrane), and retinal—a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (cis) form of the molecule to its linear (trans) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na+ channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus) visual receptors become hyperpolarized and thus driven away from threshold (Figure 27.20). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 27 | |
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