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Malaria kills more residents; 9% of the population have reported infection, It causes three times as many deaths as AIDS. In 2008, fewer than half of children younger than five slept under antimalaria nets or had access to antimalarial drugs.
Despite lowering rates in surrounding countries, cholera rates were reported in November 2012 to be on the rise, with 1,500 cases reported and nine deaths. A 2008 cholera epidemic in Guinea-Bissau affected 14,222 people and killed 225.
The 2010 maternal mortality rate per 100,000 births for Guinea Bissau was 1000. This compares with 804.3 in 2008 and 966 in 1990. The under 5 mortality rate, per 1,000 births, was 195 and the neonatal mortality as a percentage of under 5's mortality was 24. The number of midwives per 1,000 live births was 3; one out of eighteen pregnant women die as a result of pregnancy. According to a 2013 UNICEF report, 50% of women in Guinea Bissau had undergone female genital mutilation. In 2010, Guinea Bissau had the 7th highest maternal mortality rate in the world.
Education is compulsory from the age of 7 to 13. The enrollment of boys is higher than that of girls. In 1998, the gross primary enrollment rate was 53.5%, with higher enrollment ratio for males (67.7%) compared to females (40%).
Guinea-Bissau has several secondary schools (general as well as technical) and a number of universities, to which an institutionally autonomous Faculty of Law as well as a Faculty of Medicine have been added.
The music of Guinea-Bissau is usually associated with the polyrhythmic gumbe genre, the country's primary musical export. However, civil unrest and other factors have combined over the years to keep gumbe, and other genres, out of mainstream audiences, even in generally syncretist African countries.
The calabash is the primary musical instrument of Guinea-Bissau, and is used in extremely swift and rhythmically complex dance music. Lyrics are almost always in Guinea-Bissau Creole, a Portuguese-based creole language, and are often humorous and topical, revolving around current events and controversies, especially AIDS.
The word gumbe is sometimes used generically, to refer to any music of the country, although it most specifically refers to a unique style that fuses about ten of the country's folk music traditions. Tina and tinga are other popular genres, while extent folk traditions include ceremonial music used in funerals, initiations and other rituals, as well as Balanta brosca and kussundé, Mandinga djambadon, and the kundere sound of the Bissagos Islands.
Rice is a staple in the diet of residents near the coast and millet a staple in the interior. Fish, shellfish, fruits and vegetables are commonly eaten along with cereal grains, milk, curd and whey. The Portuguese encouraged peanut production. Vigna subterranea (Bambara groundnut) and Macrotyloma geocarpum (Hausa groundnut) are also grown. Black-eyed peas are also part of the diet. Palm oil is harvested.
Common dishes include soups and stews. Common ingredients include yams, sweet potato, cassava, onion, tomato and plantain. Spices, peppers and chilis are used in cooking, including Aframomum melegueta seeds (Guinea pepper).
Flora Gomes is an internationally renowned film director; his most famous film is Nha Fala (English: My Voice). Gomes's Mortu Nega (Death Denied) (1988) was the first fiction film and the second feature film ever made in Guinea-Bissau. (The first feature film was N’tturudu, by director Umban u’Kest in 1987.) At FESPACO 1989, Mortu Nega won the prestigious Oumarou Ganda Prize. Mortu Nega is in Creole with English subtitles. In 1992, Gomes directed Udju Azul di Yonta, which was screened in the Un Certain Regard section at the 1992 Cannes Film Festival. Gomes has also served on the boards of many Africa-centric film festivals.
This article covers numbered east-west streets in Manhattan, New York City. Major streets have their own linked articles; minor streets are discussed here. The streets do not run exactly east–west, because the grid plan is aligned with the Hudson River rather than with the cardinal directions. "West" is approximately 29 degrees north of true west.
The numbered streets carry crosstown traffic. In general, even-numbered streets are one-way eastbound and odd-numbered streets are one-way west. Several exceptions reverse this. Most wider streets carry two-way traffic, as do a few of the narrow ones.
Streets' names change from West to East (for instance, East 10th Street to West 10th Street) at Broadway below 8th Street, and at Fifth Avenue from 8th Street and above.
Although the numbered streets begin just north of East Houston Street in the East Village, they generally do not extend west into Greenwich Village, which already had streets when the grid plan was laid out by the Commissioners' Plan of 1811. Streets that do continue farther west change direction before reaching the Hudson River. The grid covers the length of the island from 14th Street north.
220th Street is the highest numbered street on Manhattan Island. Marble Hill is also within the borough of Manhattan, so the highest street number in the borough is 228th Street. However, the numbering continues in the Bronx up to 263rd Street. The lowest number is East First Street—which runs in Alphabet City near East Houston Street—as well as First Place in Battery Park City.
East 1st Street begins just North of East Houston Street at Avenue A and continues to Bowery. Peretz Square, a small triangular sliver park where Houston Street, First Street and First Avenue meet marks the spot where the grid takes hold.
East 2nd Street begins just North of East Houston Street at Avenue C and also continues to Bowery. The East end of East 3rd, 4th, 5th, and 7th Streets is Avenue D, with East 6th Street continuing further Eastward and connecting to FDR Drive.
The west end of these streets is Bowery and Third Avenue, except for 3rd Street (formerly Amity Place; to Sixth Avenue) and 4th Street (to 13th Street), which extend west and north, respectively, into Greenwich Village. Great Jones Street connects East 3rd to West 3rd.
East 5th Street goes west to Cooper Square, but is interrupted between Avenues B and C by The Earth School, Public School 364, and between First Avenue and Avenue A by the Village View Apartments.
8th and 9th Streets run parallel to each other, beginning at Avenue D, interrupted by Tompkins Square Park at Avenue B, resuming at Avenue A and continuing to Sixth Avenue. West 8th Street is an important local shopping street. 8th Street between Avenue A and Third Avenue is called St Mark's Place, but it is counted in the length below.
10th Street (40°44′03″N 74°00′11″W / 40.7342580°N 74.0029670°W / 40.7342580; -74.0029670) begins at the FDR Drive and Avenue C. West of Sixth Avenue, it turns southward about 40 degrees to join the Greenwich Village street grid and continue to West Street on the Hudson River. Because West 4th Street turns northward at Sixth Avenue, it intersects 10th, 11th and 12th and 13th Streets in the West Village. The M8 bus operates on 10th Street in both directions between Avenue D and Avenue A, and eastbound between West Street and Sixth Avenue. 10th Street has an eastbound bike lane from West Street to the East River. In 2009, the two-way section of 10th Street between Avenue A and the East River had bicycle markings and sharrows installed, but it still has no dedicated bike lane. West 10th Street was previously named Amos Street for Richard Amos. The end of West 10th Street toward the Hudson River was once the home of Newgate Prison, New York City's first prison and the United States' second.
11th Street is in two parts. It is interrupted by the block containing Grace Church between Broadway and Fourth Avenue. East 11th streets runs from Fourth Avenue to Avenue C and runs past Webster Hall. West 11th Street runs from Broadway to West Street. 11th Street and 6th Avenue was the location of the Old Grapevine tavern from the 1700s to its demolition in the early 20th century.
13th Street is in three parts. The first is a dead end from Avenue C. The second starts at a dead end, just before Avenue B, and runs to Greenwich Avenue, and the third part is from Eighth Avenue to Tenth Avenue.
14th Street is a main numbered street in Manhattan. It begins at Avenue C and ends at West Street. Its length is 3.4 km (2.1 mi). It has six subway stations:
15th Street starts at FDR Drive, and 16th Street starts at a dead end half way between FDR Drive and Avenue C. They are both stopped at Avenue C and continue from First Avenue to West Street, stopped again at Union Square, and 16th Street also pauses at Stuyvesant Square.
On 17th Street (40°44′08″N 73°59′12″W / 40.735532°N 73.986575°W / 40.735532; -73.986575), traffic runs one way along the street, from east to west excepting the stretch between Broadway and Park Avenue South, where traffic runs in both directions. It forms the northern borders of both Union Square (between Broadway and Park Avenue South) and Stuyvesant Square. Composer Antonín Dvořák's New York home was located at 327 East 17th Street, near Perlman Place. The house was razed by Beth Israel Medical Center after it received approval of a 1991 application to demolish the house and replace it with an AIDS hospice. Time Magazine was started at 141 East 17th Street.
18th Street has a local subway station at the crossing with Seventh Avenue, served by the 1 2 trains on the IRT Broadway – Seventh Avenue Line. There used to be an 18th Street station on the IRT Lexington Avenue Line at the crossing with Park Avenue South.
20th Street starts at Avenue C, and 21st and 22nd Streets begin at First Avenue. They all end at Eleventh Avenue. Travel on the last block of the 20th, 21st and 22nd Streets, between Tenth and Eleventh Avenues, is in the opposite direction than it is on the rest of the respective street. 20th Street is very wide from the Avenue C to First Avenue.
Between Second and Third Avenues, 21st Street is alternatively known as Police Officer Anthony Sanchez Way. Along the northern perimeter of Gramercy Park, between Gramercy Park East and Gramercy Park West, 21st Street is known as Gramercy Park North.
23rd Street is another main numbered street in Manhattan. It begins at FDR Drive and ends at Eleventh Avenue. Its length is 3.1 km/1.9m. It has two-way travel. On 23rd Street there are five local subway stations:
24th Street is in two parts. 24th Street starts at First Avenue and it ends at Madison Avenue, because of Madison Square Park. 25th Street, which is in three parts, starts at FDR Drive, is a pedestrian plaza between Third Avenue and Lexington Avenue, and ends at Madison. Then West 24th and 25th Streets continue from Fifth Avenue to Eleventh Avenue (25th) or Twelfth Avenue (24th).
27th Street is a one-way street runs from Second Avenue to the West Side Highway with an interruption between Eighth Avenue and Tenth Avenue. It is most noted for its strip between Tenth and Eleventh Avenues, known as Club Row because it features numerous nightclubs and lounges.
In recent years, the nightclubs on West 27th Street have succumbed to stiff competition from Manhattan's Meatpacking District about fifteen blocks south, and other venues in downtown Manhattan.
Heading east, 27th Street passes through Chelsea Park between Tenth and Ninth Avenues, with the Fashion Institute of Technology (FIT) on the corner of Eighth. On Madison Avenue between 26th and 27th streets, on the site of the old Madison Square Garden, is the New York Life Building, built in 1928 and designed by Cass Gilbert, with a square tower topped by a striking gilded pyramid. Twenty-Seventh Street passes one block north of Madison Square Park and culminates at Bellevue Hospital Center on First Avenue.
31st Street begins on the West Side at the West Side Yard, while 32nd Street, which includes a segment officially known as Korea Way between Fifth Avenue and Broadway in Manhattan's Koreatown, begins at the entrance to Penn Station and Madison Square Garden. On the East Side, both streets end at Second Avenue at Kips Bay Towers and NYU Medical Center which occupy the area between 30th and 34th Streets. The Catholic church of St. Francis of Assisi is situated at 135–139 West 31st Street. At 210 West is the Capuchin Monastery of St. John the Baptist, part of St. John the Baptist Church on 30th Street. At the corner of Broadway and West 31st Street is the Grand Hotel. The former Hotel Pierrepont was located at 43 West 32nd Street, The Continental NYC tower is at the corner of Sixth Avenue and 32nd Street. 29 East 32nd Street was the location of the first building owned by the Grolier Club between 1890 and 1917.
35th Street runs from FDR Drive to Eleventh Avenue. Notable locations include East River Ferry, LaptopMD headquarters, Mercy College Manhattan Campus, and Jacob K. Javits Convention Center.
A section of East 58th Street 40°45′40.3″N 73°57′56.9″W / 40.761194°N 73.965806°W / 40.761194; -73.965806 between Lexington and Second Avenues is known as Designers' Way and features a number of high end interior design and decoration establishments, including
90th Street is split into two segments. The first segment, West 90th Street begins at Riverside Drive and ends at Central Park West or West Drive, when it is open, in Central Park on the Upper West Side. The second segment of East 90th Street begins at East Drive, at Engineers Gate of Central Park. When East Drive is closed, East 90th Street begins at Fifth Avenue on the Upper East Side and curves to the right at the FDR Drive becoming East End Avenue. Our Lady of Good Counsel Church, is located on East 90th Street between Third Avenue and Second Avenue, across the street from Ruppert Towers (1601 and 1619 Third Avenue) and Ruppert Park. Asphalt Green, which is located on East 90th Street between York Avenue and East End Avenue.
112th Street starts in Morningside Heights and runs from Riverside Drive to Amsterdam Avenue, where it meets the steps of the Cathedral of Saint John the Divine. The street resumes at the eastern edge of Morningside Park and extends through Harlem before ending at First Avenue adjacent Thomas Jefferson Park in East Harlem. Notable locations include:
114th Street marks the southern boundary of Columbia University’s Morningside Heights Campus and is the location of Butler Library, which is the University’s largest.
Above 114th Street between Amsterdam Avenue and Morningside Drive, there is a private indoor pedestrian bridge connecting two buildings on the campus of St. Luke's–Roosevelt Hospital Center.
40°48′27″N 73°57′18″W / 40.8076°N 73.9549°W / 40.8076; -73.9549 120th Street traverses the neighborhoods of Morningside Heights, Harlem, and Spanish Harlem. It begins on Riverside Drive at the Interchurch Center. It then runs east between the campuses of Barnard College and the Union Theological Seminary, then crosses Broadway and runs between the campuses of Columbia University and Teacher's College. The street is interrupted by Morningside Park. It then continues east, eventually running along the southern edge of Marcus Garvey Park, passing by 58 West, the former residence of Maya Angelou. It then continues through Spanish Harlem; when it crosses Pleasant Avenue it becomes a two‑way street and continues nearly to the East River, where for automobiles, it turns north and becomes Paladino Avenue, and for pedestrians, continues as a bridge across FDR Drive.
40°48′32″N 73°57′14″W / 40.8088°N 73.9540°W / 40.8088; -73.9540 122nd Street is divided into three noncontiguous segments, E 122nd Street, W 122nd Street, and W 122nd Street Seminary Row, by Marcus Garvey Memorial Park and Morningside Park.
E 122nd Street runs four blocks (2,250 feet (690 m)) west from the intersection of Second Avenue and terminates at the intersection of Madison Avenue at Marcus Garvey Memorial Park. This segment runs in East Harlem and crosses portions of Third Avenue, Lexington, and Park (Fourth Avenue).
W 122nd Street runs six blocks (3,280 feet (1,000 m)) west from the intersection of Mount Morris Park West at Marcus Garvey Memorial Park and terminates at the intersection of Morningside Avenue at Morningside Park. This segment runs in the Mount Morris Historical District and crosses portions of Lenox Avenue (Sixth Avenue), Seventh Avenue, Frederick Douglass Boulevard (Eighth Avenue), and Manhattan Avenue.
W 122nd Street Seminary Row runs three blocks (1,500 feet (460 m)) west from the intersection of Amsterdam Avenue (Tenth Avenue) and terminates at the intersection of Riverside Drive. East of Amsterdam, Seminary Row bends south along Morningside Park and is resigned as Morningside Drive (Ninth Avenue). Seminary row runs in Morningside Heights, the district surrounding Columbia University, and crosses portions of Broadway and Claremont Avenue.
Seminary Row is named for the Union Theological Seminary and the Jewish Theological Seminary which it touches. Seminary Row also runs by the Manhattan School of Music, Riverside Church, Sakura Park, Grant's Tomb, and Morningside Park.
122nd Street is mentioned in the movie Taxi Driver by main character Travis Bickle as the location where a fellow cab driver is assaulted with a knife. The street and the surrounding neighborhood of Harlem is then referred to as "Mau Mau Land" by another character named Wizard, slang indicating it is a majority black area.
40°48′47″N 73°57′27″W / 40.813°N 73.9575°W / 40.813; -73.9575 La Salle Street is a street in West Harlem that runs just two blocks between Amsterdam Avenue and Claremont Avenue. West of Convent Avenue, 125th Street was re-routed onto the old Manhattan Avenue. The original 125th Street west of Convent Avenue was swallowed up to make the super-blocks where the low income housing projects now exist. La Salle Street is the only vestige of the original routing.
40°48′52″N 73°56′53″W / 40.814583°N 73.947944°W / 40.814583; -73.947944 132nd Street runs east-west above Central Park and is located in Harlem just south of Hamilton Heights. The main portion of 132nd Street runs eastbound from Frederick Douglass Boulevard to northern end of Park Avenue where there is a southbound exit from/entrance to the Harlem River Drive. After an interruption from St. Nicholas Park and City College, there is another small stretch of West 132nd Street between Broadway and Twelfth Avenue
The 132nd Street Community Garden is located on 132nd Street between Adam Clayton Powell Jr. Boulevard and Malcolm X Boulevard. In 1997, the lot received a garden makeover; the Borough President's office funded the installation of a $100,000 water distribution system that keeps the wide variety of trees green. The garden also holds a goldfish pond and several benches. The spirit of the neighborhood lives in gardens like this one, planted and tended by local residents.
The Manhattanville Bus Depot (formerly known as the 132nd Street Bus Depot) is located on West 132nd and 133rd Street between Broadway and Riverside Drive in the Manhattanville neighborhood.
155th Street is a major crosstown street considered to form the boundary between Harlem and Washington Heights. It is the northernmost of the 155 crosstown streets mapped out in the Commissioner's Plan of 1811 that established the numbered street grid in Manhattan.
155th Street starts on the West Side at Riverside Drive, crossing Broadway, Amsterdam Avenue and St. Nicholas Avenue. At St. Nicholas Place, the terrain drops off steeply, and 155th Street is carried on a 1,600-foot (490 m) long viaduct, a City Landmark constructed in 1893, that slopes down towards the Harlem River, continuing onto the Macombs Dam Bridge, crossing over (but not intersecting with) the Harlem River Drive. A separate, unconnected section of 155th Street runs under the viaduct, connecting Bradhurst Avenue and the Harlem River Drive.
181st Street is a major thoroughfare running through the Washington Heights neighborhood. It runs from the Washington Bridge in the east, to the Henry Hudson Parkway in the west, near the George Washington Bridge and the Hudson River. The west end is called Plaza Lafayette.
West of Fort Washington Avenue, 181st Street is largely residential, bordering Hudson Heights and having a few shops to serve the local residents. East of Fort Washington Avenue, the street becomes increasingly commercial, becoming dominated entirely by retail stores where the street reaches Broadway and continues as such until reaching the Harlem River. It is the area's major shopping district.
181st Street is served by two New York City Subway lines; there is a 181st Street station at Fort Washington Avenue on the IND Eighth Avenue Line (A trains) and a 181st Street station at St. Nicholas Avenue on the IRT Broadway – Seventh Avenue Line (1 trains). The stations are about 500 metres (550 yd) from each other and are not connected. The George Washington Bridge Bus Terminal is a couple of blocks south on Fort Washington Avenue. 181st Street is also the last south/west exit in New York on the Trans-Manhattan Expressway (I-95), just before crossing the George Washington Bridge to New Jersey.
187th Street crosses Washington Heights and running from Laurel Hill Terrace in the east to Chittenden Avenue in the west near the George Washington Bridge and Hudson River. The street is interrupted by a long set of stairs east of Fort Washington Avenue leading to the Broadway valley. West of there, it is mostly lined with store fronts and serves as a main shopping district for the Hudson Heights neighborhood.
187th Street intersects with, from East to West, Laurel Hill Terrace, Amsterdam Avenue, Audubon Avenue, St. Nicholas Avenue, Wadsworth Avenue, Broadway, Bennett Avenue, Overlook Terrace, Fort Washington Avenue, Pinehurst Avenue, Cabrini Boulevard and Chittenden Avenue.
The many institutions on 187th Street include Mount Sinai Jewish Center, the Dombrov Shtiebel, and the uptown campus of Yeshiva University. The local public elementary school P.S. 187 is located on Cabrini Boulevard, just north of the eponymous 187th Street
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. Only a few invertebrates such as sponges, jellyfish, adult sea squirts and starfish do not have a brain; diffuse or localised nerve nets are present instead. The brain is located in the head, usually close to the primary sensory organs for such senses as vision, hearing, balance, taste, and smell. The brain is the most complex organ in a vertebrate's body. In a typical human, the cerebral cortex (the largest part) is estimated to contain 15–33 billion neurons, each connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.
Physiologically, the function of the brain is to exert centralized control over the other organs of the body. The brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment. Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain.
The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer, very different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, and processes it in a variety of ways, analogous to the central processing unit (CPU) in a computer.
This article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar as it shares the properties of other brains. The ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context. The most important is brain disease and the effects of brain damage, covered in the human brain article because the most common diseases of the human brain either do not show up in other species, or else manifest themselves in different ways.
The shape and size of the brain varies greatly in different species, and identifying common features is often difficult. Nevertheless, there are a number of principles of brain architecture that apply across a wide range of species. Some aspects of brain structure are common to almost the entire range of animal species; others distinguish "advanced" brains from more primitive ones, or distinguish vertebrates from invertebrates.
The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, and then sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations. It is also possible to examine the microstructure of brain tissue using a microscope, and to trace the pattern of connections from one brain area to another.
The brains of all species are composed primarily of two broad classes of cells: neurons and glial cells. Glial cells (also known as glia or neuroglia) come in several types, and perform a number of critical functions, including structural support, metabolic support, insulation, and guidance of development. Neurons, however, are usually considered the most important cells in the brain. The property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, which is a thin protoplasmic fiber that extends from the cell body and projects, usually with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body. The length of an axon can be extraordinary: for example, if a pyramidal cell, (an excitatory neuron) of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon, equally magnified, would become a cable a few centimeters in diameter, extending more than a kilometer. These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials constantly, at rates of 10–100 per second, usually in irregular patterns; other neurons are quiet most of the time, but occasionally emit a burst of action potentials.
Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells. When an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell.
Synapses are the key functional elements of the brain. The essential function of the brain is cell-to-cell communication, and synapses are the points at which communication occurs. The human brain has been estimated to contain approximately 100 trillion synapses; even the brain of a fruit fly contains several million. The functions of these synapses are very diverse: some are excitatory (exciting the target cell); others are inhibitory; others work by activating second messenger systems that change the internal chemistry of their target cells in complex ways. A large number of synapses are dynamically modifiable; that is, they are capable of changing strength in a way that is controlled by the patterns of signals that pass through them. It is widely believed that activity-dependent modification of synapses is the brain's primary mechanism for learning and memory.
Most of the space in the brain is taken up by axons, which are often bundled together in what are called nerve fiber tracts. A myelinated axon is wrapped in a fatty insulating sheath of myelin, which serves to greatly increase the speed of signal propagation. (There are also unmyelinated axons). Myelin is white, making parts of the brain filled exclusively with nerve fibers appear as light-colored white matter, in contrast to the darker-colored grey matter that marks areas with high densities of neuron cell bodies.
Except for a few primitive organisms such as sponges (which have no nervous system) and cnidarians (which have a nervous system consisting of a diffuse nerve net), all living multicellular animals are bilaterians, meaning animals with a bilaterally symmetric body shape (that is, left and right sides that are approximate mirror images of each other). All bilaterians are thought to have descended from a common ancestor that appeared early in the Cambrian period, 485-540 million years ago, and it has been hypothesized that this common ancestor had the shape of a simple tubeworm with a segmented body. At a schematic level, that basic worm-shape continues to be reflected in the body and nervous system architecture of all modern bilaterians, including vertebrates. The fundamental bilateral body form is a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (a ganglion) for each body segment, with an especially large ganglion at the front, called the brain. The brain is small and simple in some species, such as nematode worms; in other species, including vertebrates, it is the most complex organ in the body. Some types of worms, such as leeches, also have an enlarged ganglion at the back end of the nerve cord, known as a "tail brain".
There are a few types of existing bilaterians that lack a recognizable brain, including echinoderms, tunicates, and acoelomorphs (a group of primitive flatworms). It has not been definitively established whether the existence of these brainless species indicates that the earliest bilaterians lacked a brain, or whether their ancestors evolved in a way that led to the disappearance of a previously existing brain structure.
Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others), and cephalopods (octopuses, squids, and similar molluscs). The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain with three divisions and large optical lobes behind each eye for visual processing. Cephalopods such as the octopus and squid have the largest brains of any invertebrates.
There are several invertebrate species whose brains have been studied intensively because they have properties that make them convenient for experimental work:
The first vertebrates appeared over 500 million years ago (Mya), during the Cambrian period, and may have resembled the modern hagfish in form. Sharks appeared about 450 Mya, amphibians about 400 Mya, reptiles about 350 Mya, and mammals about 200 Mya. Each species has an equally long evolutionary history, but the brains of modern hagfishes, lampreys, sharks, amphibians, reptiles, and mammals show a gradient of size and complexity that roughly follows the evolutionary sequence. All of these brains contain the same set of basic anatomical components, but many are rudimentary in the hagfish, whereas in mammals the foremost part (the telencephalon) is greatly elaborated and expanded.
Brains are most simply compared in terms of their size. The relationship between brain size, body size and other variables has been studied across a wide range of vertebrate species. As a rule, brain size increases with body size, but not in a simple linear proportion. In general, smaller animals tend to have larger brains, measured as a fraction of body size. For mammals, the relationship between brain volume and body mass essentially follows a power law with an exponent of about 0.75. This formula describes the central tendency, but every family of mammals departs from it to some degree, in a way that reflects in part the complexity of their behavior. For example, primates have brains 5 to 10 times larger than the formula predicts. Predators tend to have larger brains than their prey, relative to body size.
All vertebrate brains share a common underlying form, which appears most clearly during early stages of embryonic development. In its earliest form, the brain appears as three swellings at the front end of the neural tube; these swellings eventually become the forebrain, midbrain, and hindbrain (the prosencephalon, mesencephalon, and rhombencephalon, respectively). At the earliest stages of brain development, the three areas are roughly equal in size. In many classes of vertebrates, such as fish and amphibians, the three parts remain similar in size in the adult, but in mammals the forebrain becomes much larger than the other parts, and the midbrain becomes very small.
The brains of vertebrates are made of very soft tissue. Living brain tissue is pinkish on the outside and mostly white on the inside, with subtle variations in color. Vertebrate brains are surrounded by a system of connective tissue membranes called meninges that separate the skull from the brain. Blood vessels enter the central nervous system through holes in the meningeal layers. The cells in the blood vessel walls are joined tightly to one another, forming the blood–brain barrier, which blocks the passage of many toxins and pathogens (though at the same time blocking antibodies and some drugs, thereby presenting special challenges in treatment of diseases of the brain).
Neuroanatomists usually divide the vertebrate brain into six main regions: the telencephalon (cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla oblongata. Each of these areas has a complex internal structure. Some parts, such as the cerebral cortex and the cerebellar cortex, consist of layers that are folded or convoluted to fit within the available space. Other parts, such as the thalamus and hypothalamus, consist of clusters of many small nuclei. Thousands of distinguishable areas can be identified within the vertebrate brain based on fine distinctions of neural structure, chemistry, and connectivity.
Although the same basic components are present in all vertebrate brains, some branches of vertebrate evolution have led to substantial distortions of brain geometry, especially in the forebrain area. The brain of a shark shows the basic components in a straightforward way, but in teleost fishes (the great majority of existing fish species), the forebrain has become "everted", like a sock turned inside out. In birds, there are also major changes in forebrain structure. These distortions can make it difficult to match brain components from one species with those of another species.
The most obvious difference between the brains of mammals and other vertebrates is in terms of size. On average, a mammal has a brain roughly twice as large as that of a bird of the same body size, and ten times as large as that of a reptile of the same body size.
Size, however, is not the only difference: there are also substantial differences in shape. The hindbrain and midbrain of mammals are generally similar to those of other vertebrates, but dramatic differences appear in the forebrain, which is greatly enlarged and also altered in structure. The cerebral cortex is the part of the brain that most strongly distinguishes mammals. In non-mammalian vertebrates, the surface of the cerebrum is lined with a comparatively simple three-layered structure called the pallium. In mammals, the pallium evolves into a complex six-layered structure called neocortex or isocortex. Several areas at the edge of the neocortex, including the hippocampus and amygdala, are also much more extensively developed in mammals than in other vertebrates.
The elaboration of the cerebral cortex carries with it changes to other brain areas. The superior colliculus, which plays a major role in visual control of behavior in most vertebrates, shrinks to a small size in mammals, and many of its functions are taken over by visual areas of the cerebral cortex. The cerebellum of mammals contains a large portion (the neocerebellum) dedicated to supporting the cerebral cortex, which has no counterpart in other vertebrates.
The brains of humans and other primates contain the same structures as the brains of other mammals, but are generally larger in proportion to body size. The most widely accepted way of comparing brain sizes across species is the so-called encephalization quotient (EQ), which takes into account the nonlinearity of the brain-to-body relationship. Humans have an average EQ in the 7-to-8 range, while most other primates have an EQ in the 2-to-3 range. Dolphins have values higher than those of primates other than humans, but nearly all other mammals have EQ values that are substantially lower.
Most of the enlargement of the primate brain comes from a massive expansion of the cerebral cortex, especially the prefrontal cortex and the parts of the cortex involved in vision. The visual processing network of primates includes at least 30 distinguishable brain areas, with a complex web of interconnections. It has been estimated that visual processing areas occupy more than half of the total surface of the primate neocortex. The prefrontal cortex carries out functions that include planning, working memory, motivation, attention, and executive control. It takes up a much larger proportion of the brain for primates than for other species, and an especially large fraction of the human brain.
For vertebrates, the early stages of neural development are similar across all species. As the embryo transforms from a round blob of cells into a wormlike structure, a narrow strip of ectoderm running along the midline of the back is induced to become the neural plate, the precursor of the nervous system. The neural plate folds inward to form the neural groove, and then the lips that line the groove merge to enclose the neural tube, a hollow cord of cells with a fluid-filled ventricle at the center. At the front end, the ventricles and cord swell to form three vesicles that are the precursors of the forebrain, midbrain, and hindbrain. At the next stage, the forebrain splits into two vesicles called the telencephalon (which will contain the cerebral cortex, basal ganglia, and related structures) and the diencephalon (which will contain the thalamus and hypothalamus). At about the same time, the hindbrain splits into the metencephalon (which will contain the cerebellum and pons) and the myelencephalon (which will contain the medulla oblongata). Each of these areas contains proliferative zones where neurons and glial cells are generated; the resulting cells then migrate, sometimes for long distances, to their final positions.
Once a neuron is in place, it extends dendrites and an axon into the area around it. Axons, because they commonly extend a great distance from the cell body and need to reach specific targets, grow in a particularly complex way. The tip of a growing axon consists of a blob of protoplasm called a growth cone, studded with chemical receptors. These receptors sense the local environment, causing the growth cone to be attracted or repelled by various cellular elements, and thus to be pulled in a particular direction at each point along its path. The result of this pathfinding process is that the growth cone navigates through the brain until it reaches its destination area, where other chemical cues cause it to begin generating synapses. Considering the entire brain, thousands of genes create products that influence axonal pathfinding.
In humans and many other mammals, new neurons are created mainly before birth, and the infant brain contains substantially more neurons than the adult brain. There are, however, a few areas where new neurons continue to be generated throughout life. The two areas for which adult neurogenesis is well established are the olfactory bulb, which is involved in the sense of smell, and the dentate gyrus of the hippocampus, where there is evidence that the new neurons play a role in storing newly acquired memories. With these exceptions, however, the set of neurons that is present in early childhood is the set that is present for life. Glial cells are different: as with most types of cells in the body, they are generated throughout the lifespan.
The functions of the brain depend on the ability of neurons to transmit electrochemical signals to other cells, and their ability to respond appropriately to electrochemical signals received from other cells. The electrical properties of neurons are controlled by a wide variety of biochemical and metabolic processes, most notably the interactions between neurotransmitters and receptors that take place at synapses.
Neurotransmitters are chemicals that are released at synapses when an action potential activates them—neurotransmitters attach themselves to receptor molecules on the membrane of the synapse's target cell, and thereby alter the electrical or chemical properties of the receptor molecules. With few exceptions, each neuron in the brain releases the same chemical neurotransmitter, or combination of neurotransmitters, at all the synaptic connections it makes with other neurons; this rule is known as Dale's principle. Thus, a neuron can be characterized by the neurotransmitters that it releases. The great majority of psychoactive drugs exert their effects by altering specific neurotransmitter systems. This applies to drugs such as cannabinoids, nicotine, heroin, cocaine, alcohol, fluoxetine, chlorpromazine, and many others.
The two neurotransmitters that are used most widely in the vertebrate brain are glutamate, which almost always exerts excitatory effects on target neurons, and gamma-aminobutyric acid (GABA), which is almost always inhibitory. Neurons using these transmitters can be found in nearly every part of the brain. Because of their ubiquity, drugs that act on glutamate or GABA tend to have broad and powerful effects. Some general anesthetics act by reducing the effects of glutamate; most tranquilizers exert their sedative effects by enhancing the effects of GABA.
There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function. Serotonin, for example—the primary target of antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the Raphe nuclei. Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus. Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA.
As a side effect of the electrochemical processes used by neurons for signaling, brain tissue generates electric fields when it is active. When large numbers of neurons show synchronized activity, the electric fields that they generate can be large enough to detect outside the skull, using electroencephalography (EEG) or magnetoencephalography (MEG). EEG recordings, along with recordings made from electrodes implanted inside the brains of animals such as rats, show that the brain of a living animal is constantly active, even during sleep. Each part of the brain shows a mixture of rhythmic and nonrhythmic activity, which may vary according to behavioral state. In mammals, the cerebral cortex tends to show large slow delta waves during sleep, faster alpha waves when the animal is awake but inattentive, and chaotic-looking irregular activity when the animal is actively engaged in a task. During an epileptic seizure, the brain's inhibitory control mechanisms fail to function and electrical activity rises to pathological levels, producing EEG traces that show large wave and spike patterns not seen in a healthy brain. Relating these population-level patterns to the computational functions of individual neurons is a major focus of current research in neurophysiology.
All vertebrates have a blood–brain barrier that allows metabolism inside the brain to operate differently from metabolism in other parts of the body. Glial cells play a major role in brain metabolism by controlling the chemical composition of the fluid that surrounds neurons, including levels of ions and nutrients.
Brain tissue consumes a large amount of energy in proportion to its volume, so large brains place severe metabolic demands on animals. The need to limit body weight in order, for example, to fly, has apparently led to selection for a reduction of brain size in some species, such as bats. Most of the brain's energy consumption goes into sustaining the electric charge (membrane potential) of neurons. Most vertebrate species devote between 2% and 8% of basal metabolism to the brain. In primates, however, the percentage is much higher—in humans it rises to 20–25%. The energy consumption of the brain does not vary greatly over time, but active regions of the cerebral cortex consume somewhat more energy than inactive regions; this forms the basis for the functional brain imaging methods PET, fMRI, and NIRS. The brain typically gets most of its energy from oxygen-dependent metabolism of glucose (i.e., blood sugar), but ketones provide a major alternative source, together with contributions from medium chain fatty acids (caprylic and heptanoic acids), lactate, acetate, and possibly amino acids.
From an evolutionary-biological perspective, the function of the brain is to provide coherent control over the actions of an animal. A centralized brain allows groups of muscles to be co-activated in complex patterns; it also allows stimuli impinging on one part of the body to evoke responses in other parts, and it can prevent different parts of the body from acting at cross-purposes to each other.
The invention of electronic computers in the 1940s, along with the development of mathematical information theory, led to a realization that brains can potentially be understood as information processing systems. This concept formed the basis of the field of cybernetics, and eventually gave rise to the field now known as computational neuroscience. The earliest attempts at cybernetics were somewhat crude in that they treated the brain as essentially a digital computer in disguise, as for example in John von Neumann's 1958 book, The Computer and the Brain. Over the years, though, accumulating information about the electrical responses of brain cells recorded from behaving animals has steadily moved theoretical concepts in the direction of increasing realism.
The essence of the information processing approach is to try to understand brain function in terms of information flow and implementation of algorithms. One of the most influential early contributions was a 1959 paper titled What the frog's eye tells the frog's brain: the paper examined the visual responses of neurons in the retina and optic tectum of frogs, and came to the conclusion that some neurons in the tectum of the frog are wired to combine elementary responses in a way that makes them function as "bug perceivers". A few years later David Hubel and Torsten Wiesel discovered cells in the primary visual cortex of monkeys that become active when sharp edges move across specific points in the field of view—a discovery for which they won a Nobel Prize. Follow-up studies in higher-order visual areas found cells that detect binocular disparity, color, movement, and aspects of shape, with areas located at increasing distances from the primary visual cortex showing increasingly complex responses. Other investigations of brain areas unrelated to vision have revealed cells with a wide variety of response correlates, some related to memory, some to abstract types of cognition such as space.
Furthermore, even single neurons appear to be complex and capable of performing computations. So, brain models that don't reflect this are arguably too abstractive to be representative of brain operation; models that do try to capture this are very computationally expensive and arguably intractable with present computational resources. However, having said this, the Human Brain Project is trying to build a realistic, detailed computational model of the entire human brain. It remains to be seen what level of success they can achieve in the time frame of the project and the wisdom of it has been publicly contested, with high-profile scientists on both sides of the argument.

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