In the human  enteric nervous system , the  slow-wave threshold  is the slow-wave potential which must be reached before a slow wave can be propagated in gut wall  smooth muscle . Slow waves themselves seldom cause any smooth muscle contraction (Except for, probably in the stomach). When the amplitude of slow waves in smooth muscle cells reaches the slow-wave threshold — the L-type Ca 2+  channels are activated, resulting in calcium influx and initiation of motility. [ 3 ]
Slow waves are generated at unique intrinsic frequencies by the interstitial cells of Cajal, even within the same organ.  Entrainment  of these different intrinsic frequencies through electrical coupling allows these unique intrinsic frequencies to occur at a single frequency within the stomach and segments of the small intestine. Electron microscopic and dye coupling studies to date have confirmed gap junctions as the major coupling mechanisms between interstitial cells of Cajal. [ 4 ] [ 5 ]


Coupling between ICC and smooth muscle cells is uncertain. Gap junctions have been demonstrated in rare circumstances as one coupling mechanism between ICC and smooth muscle cells. [ 6 ]  Another potential coupling mechanism is the "Peg and Socket" theory which demonstrates that the membranes of smooth muscle cells have the ability either form physical narrow "sockets" or "pegs" to lock onto other smooth muscle cells and/or interstitial cells of Cajal. [ 7 ]


Gastric slow waves occur at around 3 cycles-per-minute in humans and exhibit significance variances in both amplitudes and propagation velocities in the stomach [ 8 ] [ 9 ] [ 10 ]  due to the existence of a gradient of resting membrane potential gradient, [ 11 ]  interstitial cells of Cajal distributions, and gastric wall thickness. Gastric slow waves frequency, propagation velocity, and amplitude demonstrate significant inter-species differences. Extracellular bioelectrical recording studies have demonstrated that gastric slow waves originate from a pacemaker region located on the greater curvature of the stomach. [ 8 ] [ 9 ] [ 10 ]  Human gastric slow waves propagate slower in the corpus than in the pacemaker region and antrum of the stomach. [ 8 ]  Up to four simultaneous slow wave wavefronts can occur in the human stomach.


Intestinal slow waves occur at around 12 cycles-per-minute in the duodenum, and decreases in frequency towards the colon. [ 12 ] [ 13 ]  Entrainment of intestinal slow waves forms "frequency plateaus" in a piece-wise manner along the intestine. Similar to the stomach, intestinal slow waves frequency, propagation velocity, and amplitude also demonstrate significant inter-species differences.


In  uterine smooth muscle , slow waves have not been consistently observed. Uterine muscle seems to generate action potentials spontaneously. [ 14 ]


In gastrointestinal smooth muscle, the slow-wave threshold can be altered by input from endogenous and exogenous innervation, as well as excitatory ( acetylcholine  and  Substance P ) and inhibitory ( vasoactive intestinal peptide  and  nitric oxide ) compounds. [ 15 ]


Textbook of Medical Physiology - Gyton and Hall (12th edition) [ page needed ]


Smooth   muscle  is one of the three major types of  vertebrate   muscle tissue , the others being  skeletal  and  cardiac muscle . It can also be found in invertebrates and is controlled by the  autonomic nervous system . It is non- striated , so-called because it has no  sarcomeres  and therefore no striations ( bands  or  stripes ). [ 1 ] [ 2 ]  It can be divided into two subgroups,  single-unit  and  multi-unit  smooth muscle. Within single-unit muscle, the whole bundle or sheet of  smooth muscle cells   contracts  as a  syncytium .


Smooth muscle is found in the walls of  hollow organs , including the  stomach ,  intestines ,  bladder  and  uterus . In the walls of  blood vessels , and  lymph vessels , (excluding blood and lymph capillaries) it is known as  vascular smooth muscle . There is smooth muscle in the tracts of the  respiratory ,  urinary , and  reproductive   systems . In the  eyes , the  ciliary muscles ,  iris dilator muscle , and  iris sphincter muscle  are types of smooth muscles. The iris dilator and sphincter muscles are contained in the iris and contract in order to  dilate  or  constrict  the pupils. The ciliary muscles change the shape of the  lens  to focus on objects in  accommodation . In the  skin , smooth muscle cells such as those of the  arrector pili  cause  hair  to stand erect in response to  cold  temperature and  fear . [ 1 ]


Smooth muscle is grouped into two types:  single-unit smooth muscle , also known as visceral smooth muscle, and  multiunit smooth muscle . Most smooth muscle is of the single-unit type, and is found in the walls of most   internal organs  (viscera); and lines blood vessels (except large elastic arteries), the  urinary tract , and the  digestive tract . It is not found in the heart which has cardiac muscle. In single-unit smooth muscle a single  cell  in a bundle is innervated by an autonomic nerve fiber (myogenic). An action potential can be propagated through neighbouring muscle cells due to the presence of many gap junctions between the cells. Due to this property, single-unit bundles form a syncytium that contracts in a coordinated fashion making the whole muscle contract or relax. (such as the uterine muscles during childbirth). [ 3 ]


Single-unit visceral smooth muscle is myogenic; it can contract regularly without input from a motor neuron (as opposed to multiunit smooth muscle, which is neurogenic - that is, its contraction must be initiated by an autonomic nervous system neuron). A few of the cells in a given single unit may behave as pacemaker cells, generating rhythmic action potentials due to their intrinsic electrical activity. Because of its myogenic nature, single-unit smooth muscle is usually active, even when it is not receiving any neural stimulation. Multiunit smooth muscle is found in the  trachea , in the iris of the eye, and lining the large elastic arteries.


However, the terms single- and multi-unit smooth muscle represent an  oversimplification . This is due to the fact that smooth muscles for the most part are controlled and influenced by a combination of different neural elements. In addition, it has been observed that most of the time there will be some cell-to-cell communication and activators/inhibitors produced locally. This leads to a somewhat coordinated response even in multiunit smooth muscle. [ 4 ]


Smooth muscle differs from  skeletal muscle  and  cardiac muscle  in terms of structure, function, regulation of contraction, and  excitation-contraction coupling . However, smooth muscle tissue tends to demonstrate greater elasticity and function within a larger length-tension curve than  striated muscle . This ability to stretch and still maintain contractility is important in organs like the intestines and urinary bladder. Smooth muscle in the  gastrointestinal tract  is activated by a composite of  smooth muscle cells  (SMCs),   interstitial cells of Cajal  (ICCs), and  platelet-derived growth factor receptor alpha  (PDGFRα) that are electrically coupled and work together as an  SIP functional syncytium . [ 5 ] [ 6 ]


A smooth-muscle cell is a spindle-shaped  myocyte  with a wide middle and tapering ends, and a single nucleus. Like striated muscle, smooth muscle can  tense and relax .  In the relaxed state, each cell is 30–200 micrometers in length, some thousands of times shorter than a  skeletal muscle cell . [ 1 ]  There are no  myofibrils  present, but much of the cytoplasm is taken up by the proteins,  myosin  and  actin , which together have the capability to contract. [ 7 ]


Myosin is primarily  class II  in smooth muscle. [ 8 ]


Different combinations of heavy and light chains allow for up to hundreds of different types of myosin structures, but it is unlikely that more than a few such combinations are actually used or permitted within a specific smooth muscle bed. [ 8 ]  In the uterus, a shift in myosin expression has been hypothesized to avail for changes in the directions of  uterine contractions  that are seen during the menstrual cycle. [ 8 ]


The thin filaments that are part of the contractile machinery are predominantly composed of  alpha-actin  and  gamma-actin . [ 8 ]  Smooth muscle alpha-actin is the predominant isoform within smooth muscle. There is also a lot of actin (mainly  beta-actin ) that does not take part in contraction, but that polymerizes just below the plasma membrane in the presence of a contractile stimulant and may thereby assist in mechanical tension. [ 8 ]  Alpha-actin is also expressed as distinct genetic isoforms such as smooth muscle, cardiac muscle and skeletal muscle specific isoforms of alpha-actin. [ 10 ]


The ratio of  actin  to  myosin  is between 2:1 [ 8 ]  and 10:1 [ 8 ]  in smooth muscle. Conversely, from a mass ratio standpoint (as opposed to a molar ratio), myosin is the dominant protein in striated skeletal muscle with the actin to myosin ratio falling in the 1:2 to 1:3 range. A typical value for healthy young adults is 1:2.2. [ 11 ] [ 12 ] [ 13 ] [ 14 ]


Smooth muscle does not contain the protein  troponin ; instead  calmodulin  (which takes on the regulatory role in smooth muscle),  caldesmon  and  calponin  are significant proteins expressed within smooth muscle.


Also, all three of these proteins may have a role in inhibiting the  ATPase  activity of the myosin complex that otherwise provides energy to fuel muscle contraction. [ 8 ]


The actin filaments are attached to dense bodies, which are analogous to the Z-discs in striated muscle sarcomeres. Dense bodies are rich in  alpha-actinin  (α-actinin), [ 8 ]  and also attach  intermediate filaments  (consisting largely of  vimentin  and  desmin ), and thereby appear to serve as anchors from which the thin filaments can exert force. [ 8 ]  Dense bodies also are associated with  beta-actin , which is the type found in the cytoskeleton, suggesting that dense bodies may coordinate tensions from both the contractile machinery and the cytoskeleton. [ 8 ]  Dense bodies appear darker under an electron microscope, and so they are sometimes described as electron dense. [ 15 ]


The intermediate filaments are connected to other intermediate filaments via dense bodies, which eventually are attached to  adherens junctions  (also called focal adhesions) in the  cell membrane  of the smooth muscle cell, called the  sarcolemma . The adherens junctions consist of large number of proteins including alpha-actinin (α-actinin), vinculin and cytoskeletal actin. [ 8 ]  The adherens junctions are scattered around  dense bands  that are circumfering the smooth muscle cell in a rib-like pattern. [ 7 ]  The dense band (or dense plaques) areas alternate with regions of membrane containing numerous  caveolae . When complexes of actin and myosin contract, force is transduced to the sarcolemma through intermediate filaments attaching to such dense bands.


During contraction, there is a spatial reorganization of the contractile machinery to optimize force development. [ 8 ]  part of this reorganization consists of vimentin being phosphorylated at  Ser 56  by a  p21 activated kinase , resulting in some disassembly of vimentin polymers. [ 8 ]


Also, the number of myosin filaments is dynamic between the relaxed and contracted state in some tissues as the ratio of actin to myosin changes, and the length and number of myosin filaments change.


Isolated single smooth muscle cells have been observed contracting in a spiral corkscrew fashion, and isolated permeabilized smooth muscle cells adhered to glass (so contractile proteins allowed to internally contract) demonstrate zones of contractile protein interactions along the long axis as the cell contracts.


Smooth muscle-containing tissue needs to be stretched often, so elasticity is an important attribute of smooth muscle. Smooth muscle cells may secrete a complex extracellular matrix containing  collagen  (predominantly types I and III),  elastin ,  glycoproteins , and  proteoglycans . Smooth muscle also has specific elastin and collagen receptors to interact with these proteins of the extracellular matrix. These fibers with their extracellular matrices contribute to the  viscoelasticity  of these tissues. For example, the great arteries are viscolelastic vessels that act like a  Windkessel , propagating ventricular contraction and smoothing out the pulsatile flow, and the smooth muscle within the  tunica media  contributes to this property.


The sarcolemma also contains  caveolae , which are microdomains of  lipid rafts  specialized to  cell signaling  events and  ion channels . These invaginations in the sarcoplasm contain a host of  receptors  ( prostacyclin ,  endothelin ,  serotonin ,  muscarinic receptors ,  adrenergic receptors ),  second messenger  generators ( adenylate cyclase ,  phospholipase C ),  G proteins  (RhoA, G alpha), kinases ( rho kinase -ROCK,  protein kinase C ,  protein Kinase A ), ion channels (L type  calcium channels , ATP sensitive potassium channels, calcium sensitive  potassium channels ) in close proximity. The caveolae are often close to sarcoplasmic reticulum or mitochondria, and have been proposed to organize signaling molecules in the membrane.


A smooth muscle is excited by external stimuli, which causes contraction. Each step is further detailed below.


Smooth muscle may contract spontaneously (via  ionic channel  dynamics) or as in the gut special pacemakers cells  interstitial cells of Cajal  produce rhythmic contractions. Also, contraction, as well as relaxation, can be induced by a number of physiochemical agents (e.g., hormones, drugs, neurotransmitters – particularly from the  autonomic nervous system ).


Smooth muscle in various regions of the vascular tree, the airway and lungs, kidneys and vagina is different in their expression of ionic channels, hormone receptors, cell-signaling pathways, and other proteins that determine function.


For instance, blood vessels in skin, gastrointestinal system, kidney and brain respond to  norepinephrine  and  epinephrine  (from  sympathetic  stimulation or the adrenal medulla) by producing vasoconstriction (this response is mediated through  alpha-1 adrenergic receptors ). However, blood vessels within skeletal muscle and cardiac muscle respond to these  catecholamines  producing vasodilation because they possess beta- adrenergic receptors . So there is a difference in the distribution of the various adrenergic receptors that explains the difference in why blood vessels from different areas respond to the same agent norepinephrine/epinephrine differently as well as differences due to varying amounts of these catecholamines that are released and sensitivities of various receptors to concentrations.


Generally, arterial smooth muscle responds to carbon dioxide by producing vasodilation, and responds to oxygen by producing vasoconstriction. Pulmonary blood vessels within the lung are unique as they vasodilate to high oxygen tension and vasoconstrict when it falls. Bronchiole, smooth muscle that line the airways of the lung, respond to high carbon dioxide producing vasodilation and vasoconstrict when carbon dioxide is low. These responses to carbon dioxide and oxygen by pulmonary blood vessels and bronchiole airway smooth muscle aid in matching perfusion and ventilation within the lungs. Further different smooth muscle tissues display extremes of abundant to little sarcoplasmic reticulum so excitation-contraction coupling varies with its dependence on intracellular or extracellular calcium. [ citation needed ]


Recent research indicates that  sphingosine-1-phosphate  (S1P) signaling is an important regulator of  vascular smooth muscle  contraction. When  transmural pressure  increases,  sphingosine kinase 1  phosphorylates sphingosine to S1P, which binds to the S1P2 receptor in plasma membrane of cells. This leads to a transient increase in intracellular calcium, and activates Rac and Rhoa signaling pathways. Collectively, these serve to increase  MLCK  activity and decrease MLCP activity, promoting muscle contraction. This allows arterioles to increase resistance in response to increased blood pressure and thus maintain constant blood flow. The Rhoa and Rac portion of the signaling pathway provides a calcium-independent way to regulate  resistance artery  tone. [ 16 ]


To maintain organ dimensions against force, cells are fastened to one another by  adherens junctions . As a consequence, cells are mechanically coupled to one another such that contraction of one cell invokes some degree of contraction in an adjoining cell.  Gap junctions  couple adjacent cells chemically and electrically, facilitating the spread of chemicals (e.g., calcium) or action potentials between smooth muscle cells. Single unit smooth muscle displays numerous gap junctions and these tissues often organize into sheets or bundles which contract in bulk.


Smooth muscle contraction is caused by the sliding of  myosin  and  actin  filaments (a  sliding filament mechanism ) over each other. The energy for this to happen is provided by the  hydrolysis  of  ATP . Myosin functions as an ATPase utilizing ATP to produce a molecular conformational change of part of the myosin and produces movement. Movement of the filaments over each other happens when the globular heads protruding from myosin filaments attach and interact with actin filaments to form crossbridges. The myosin heads tilt and drag along the actin filament a small distance (10–12 nm). The heads then release the actin filament and then changes angle to relocate to another site on the actin filament a further distance (10–12 nm) away. They can then re-bind to the actin molecule and drag it along further. This process is called crossbridge cycling and is the same for all muscles (see  muscle contraction ). Unlike cardiac and skeletal muscle, smooth muscle does not contain the calcium-binding protein troponin. Contraction is initiated by a calcium-regulated phosphorylation of myosin, rather than a calcium-activated troponin system.


Crossbridge cycling causes contraction of myosin and actin complexes, in turn causing increased tension along the entire chains of tensile structures, ultimately resulting in contraction of the entire smooth muscle tissue.


Smooth muscle may contract phasically with rapid contraction and relaxation, or tonically with slow and sustained contraction. The reproductive, digestive, respiratory, and urinary tracts, skin, eye, and vasculature all contain this tonic muscle type. This type of smooth muscle can maintain force for prolonged time with only little energy utilization. There are differences in the myosin heavy and light chains that also correlate with these differences in contractile patterns and kinetics of contraction between tonic and phasic smooth muscle.


Crossbridge cycling cannot occur until the  myosin heads  have been activated to allow crossbridges to form. When the light chains are phosphorylated, they become active and will allow contraction to occur. The enzyme that phosphorylates the light chains is called  myosin light-chain kinase  (MLCK), also called MLC 20  kinase. [ 8 ]  In order to control contraction, MLCK will work only when the muscle is stimulated to contract. Stimulation will increase the intracellular concentration of calcium ions. These bind to a molecule called  calmodulin , and form a calcium-calmodulin complex. It is this complex that will bind to MLCK to activate it, allowing the chain of reactions for contraction to occur. [ 1 ]


Activation consists of phosphorylation of a  serine  on position 19 (Ser19) on the MLC 20  light chain, which causes a conformational change that increases the angle in the neck domain of the myosin heavy chain, [ 8 ]  which corresponds to the part of the cross-bridge cycle where the myosin head is unattached to the actin filament and relocates to another site on it. After attachment of the myosin head to the actin filament, this serine phosphorylation also activates the ATPase activity of the myosin head region to provide the energy to fuel the subsequent contraction. [ 8 ]  Phosphorylation of a threonine on position 18 (Thr18) on MLC20 is also possible and may further increase the ATPase activity of the myosin complex. [ 8 ]


Phosphorylation of the MLC 20  myosin light chains correlates well with the shortening velocity of smooth muscle. During this period there is a rapid burst of energy utilization as measured by oxygen consumption. Within a few minutes of initiation the calcium level markedly decrease, MLC 20  myosin light chains phosphorylation decreases, and energy utilization decreases and the muscle can relax. Still, smooth muscle has the ability of sustained maintenance of force in this situation as well. This sustained phase has been attributed to certain myosin crossbridges, termed latch-bridges, that are cycling very slowly, notably slowing the progression to the cycle stage whereby dephosphorylated myosin detaches from the actin, thereby maintaining the force at low energy costs. [ 8 ]  This phenomenon is of great value especially for tonically active smooth muscle. [ 8 ]


Isolated preparations of vascular and visceral smooth muscle contract with depolarizing high potassium balanced saline generating a certain amount of contractile force. The same preparation stimulated in normal balanced saline with an agonist such as endothelin or serotonin will generate more contractile force. This increase in force is termed calcium sensitization. The myosin light chain phosphatase is inhibited to increase the gain or sensitivity of myosin light chain kinase to calcium. There are a number of cell signalling pathways believed to regulate this decrease in myosin light chain phosphatase: a RhoA-Rock kinase pathway, a Protein kinase C-Protein kinase C potentiation  inhibitor protein  17 (CPI-17) pathway, telokin, and a Zip kinase pathway. Further Rock kinase and Zip kinase have been implicated to directly phosphorylate the 20kd myosin light chains.


Other cell signaling pathways and protein kinases ( Protein kinase C ,  Rho kinase , Zip kinase, Focal adhesion kinases) have been implicated as well and actin polymerization dynamics plays a role in force maintenance. While myosin light chain phosphorylation correlates well with shortening velocity, other cell signaling pathways have been implicated in the development of force and maintenance of force. Notably the phosphorylation of specific tyrosine residues on the focal adhesion adapter protein-paxillin by specific tyrosine kinases has been demonstrated to be essential to force development and maintenance. For example, cyclic nucleotides can relax arterial smooth muscle without reductions in crossbridge phosphorylation, a process termed force suppression. This process is mediated by the phosphorylation of the small heat shock protein,  hsp20 , and may prevent phosphorylated myosin heads from interacting with actin.


The phosphorylation of the light chains by MLCK is countered by a  myosin light-chain phosphatase , which dephosphorylates the MLC 20  myosin light chains and thereby inhibits contraction. [ 8 ]  Other signaling pathways have also been implicated in the regulation actin and myosin dynamics. In general, the relaxation of smooth muscle is by cell-signaling pathways that increase the myosin phosphatase activity, decrease the intracellular calcium levels, hyperpolarize the smooth muscle, and/or regulate actin and myosin muscle can be mediated by the endothelium-derived relaxing factor-nitric oxide, endothelial derived hyperpolarizing factor (either an endogenous cannabinoid, cytochrome P450 metabolite, or hydrogen peroxide), or  prostacyclin  (PGI2).  Nitric oxide  and PGI2 stimulate soluble  guanylate cyclase  and membrane bound adenylate cyclase, respectively. The cyclic nucleotides (cGMP and cAMP) produced by these cyclases activate Protein Kinase G and Protein Kinase A and phosphorylate a number of proteins. The phosphorylation events lead to a decrease in intracellular calcium (inhibit L type Calcium channels, inhibits  IP3 receptor  channels, stimulates  sarcoplasmic reticulum Calcium pump ATPase ), a decrease in the 20kd myosin light chain phosphorylation by altering calcium sensitization and increasing myosin light chain phosphatase activity, a stimulation of calcium sensitive potassium channels which hyperpolarize the cell, and the phosphorylation of amino acid residue serine 16 on the small heat shock protein (hsp20)by Protein Kinases A and G. The phosphorylation of hsp20 appears to alter actin and focal adhesion dynamics and actin-myosin interaction, and recent evidence indicates that hsp20 binding to 14-3-3 protein is involved in this process. An alternative hypothesis is that phosphorylated  Hsp20  may also alter the affinity of phosphorylated myosin with actin and inhibit contractility by interfering with crossbridge formation. The endothelium derived hyperpolarizing factor stimulates calcium sensitive potassium channels and/or ATP sensitive potassium channels and stimulate potassium efflux which hyperpolarizes the cell and produces relaxation.


In invertebrate smooth muscle, contraction is initiated with the binding of calcium directly to myosin and then rapidly cycling cross-bridges, generating force. Similar to the mechanism of vertebrate smooth muscle, there is a low calcium and low energy utilization catch phase. This sustained phase or catch phase has been attributed to a catch protein that has similarities to myosin light-chain kinase and the elastic protein-titin called twitchin. Clams and other bivalve mollusks use this catch phase of smooth muscle to keep their shell closed for prolonged periods with little energy usage.


Although the structure and function is basically the same in smooth muscle cells in different organs, their specific effects or end-functions differ.


The contractile function of vascular smooth muscle regulates the lumenal diameter of the small arteries-arterioles called  resistance arteries , thereby contributing significantly to setting the level of blood pressure and blood flow to vascular beds. Smooth muscle contracts slowly and may maintain the contraction (tonically) for prolonged periods in blood vessels, bronchioles, and some sphincters. Activating arteriole smooth muscle can decrease the lumenal diameter 1/3 of resting so it drastically alters blood flow and resistance. Activation of aortic smooth muscle doesn't significantly alter the lumenal diameter but serves to increase the viscoelasticity of the vascular wall.


In the digestive tract, smooth muscle contracts in a rhythmic  peristaltic  fashion, rhythmically forcing foodstuffs through the digestive tract as the result of phasic contraction.


A non-contractile function is seen in specialized smooth muscle within the afferent arteriole of the juxtaglomerular apparatus, which secretes  renin  in response to osmotic and pressure changes, and also it is believed to secrete ATP in tubuloglomerular regulation of glomerular filtration rate. Renin in turn activates the  renin–angiotensin system  to regulate blood pressure.


The mechanism in which external factors stimulate growth and rearrangement is not yet fully understood. A number of growth factors and neurohumoral agents influence smooth muscle growth and differentiation. The Notch receptor and cell-signaling pathway have been demonstrated to be essential to vasculogenesis and the formation of arteries and veins. The proliferation is implicated in the pathogenesis of atherosclerosis and is inhibited by nitric oxide.


The embryological origin of smooth muscle is usually of mesodermal origin, after the creation of  muscle cells  in a process known as  myogenesis . However, the smooth muscle within the Aorta and Pulmonary arteries (the Great Arteries of the heart) is derived from  ectomesenchyme  of  neural crest  origin, although coronary artery smooth muscle is of mesodermal origin.


Multisystemic smooth muscle dysfunction syndrome  is a genetic condition in which the body of a developing embryo does not create enough smooth muscle for the  gastrointestinal system . This condition is fatal.


Anti-smooth muscle antibodies  (ASMA) can be a symptom of an  auto-immune  disorder, such as  hepatitis ,  cirrhosis , or  lupus .


Smooth muscle tumors  are most commonly benign, and are then called  leiomyomas . They can occur in any organ, but they usually occur in the  uterus ,  small bowel , and  esophagus . Malignant smooth muscle tumors are called  leiomyosarcomas . Leiomyosarcomas are one of the more common types of  soft-tissue sarcomas . Vascular smooth muscle tumors are very rare. They can be  malignant or benign , and morbidity can be significant with either type.  Intravascular leiomyomatosis  is a  benign neoplasm  that extends through the  veins ;  angioleiomyoma  is a benign neoplasm of the extremities; vascular leiomyosarcomas is a  malignant neoplasm  that can be found in the  inferior vena cava ,  pulmonary arteries  and  veins , and other  peripheral vessels .
See  Atherosclerosis .


A  sphincter  is a circular  muscle  that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the  human body , [ citation needed ]  some microscopically small, in particular the millions of  precapillary sphincters . [ 1 ]  Sphincters relax at  death , often releasing  fluids  and  faeces . [ 2 ]


Each sphincter is associated with the lumen (opening) it surrounds. As long as the sphincter muscle is contracted, its length is shortened and the lumen is constricted (closed). Relaxation of the muscle causes it to lengthen, opening the lumen and allowing the passage of liquids, solids, or gases.


This is evident, for example, in the  blowholes  of numerous  marine mammals .


Many sphincters are used every day in the normal course of  digestion . For example, the lower  oesophageal  sphincter (or cardiac sphincter), which resides at the top of the  stomach , is closed most of the time, keeping  acids  and other stomach contents from pushing up and into the oesophagus, but opens to let swallowed food pass into the stomach.


Sphincters can be further classified into functional and anatomical sphincters: [ citation needed ]


Sphincters can also be voluntarily or involuntarily controlled:


Striated muscle tissue  is a  muscle tissue  that features repeating functional units called  sarcomeres . The presence of sarcomeres manifests as a series of bands visible along the muscle fibers, which is responsible for the striated appearance observed in microscopic images of this tissue. There are two types of striated muscle:


Striated muscle tissue contains  T-tubules  which enables the release of calcium ions from the  sarcoplasmic reticulum . [ 1 ]


Skeletal muscle includes  skeletal muscle fibers , blood vessels, nerve fibers, and connective tissue. Skeletal muscle is wrapped in  epimysium , allowing structural integrity of the muscle despite contractions. The  perimysium  organizes the muscle fibers, which are encased in collagen and  endomysium , into  fascicles . Each muscle fiber contains  sarcolemma ,  sarcoplasm , and  sarcoplasmic reticulum . The functional unit of a muscle fiber is called a  sarcomere . [ 2 ]
Each muscle cell contains  myofibrils  composed of actin and myosin  myofilaments  repeated as a sarcomere. [ 3 ]   Many nuclei  are present in each muscle cell placed at regular intervals beneath the sarcolemma.


Based on their contractile and metabolic phenotypes, skeletal muscle can be classified as slow-oxidative (Type I) or fast-oxidative (Type II). [ 1 ]


Cardiac muscle lies between the  epicardium  and the  endocardium  in the heart. [ 4 ]   Cardiac muscle cells  generally only contain one nucleus, located in the central region. They contain many mitochondria and myoglobin. [ 5 ]  Unlike skeletal muscle, cardiac muscle cells are unicellular. [ 4 ]  These cells are connected to each other by  intercalated disks , which contain  gap junctions  and  desmosomes . [ 5 ]


Unlike skeletal and cardiac muscle tissue,  smooth muscle tissue  is not striated since there are no sarcomeres present. Skeletal  muscles are attached to some component of the  skeleton , and smooth muscle is found in hollow structures such as the walls of intestines or blood vessels. The fibres of striated muscle have a cylindrical shape with blunt ends, whereas those in smooth muscle are spindle-like with tapered ends. Striated muscle tissue has more  mitochondria  than smooth muscle. Both smooth muscle cells and cardiac muscle cells have a single  nucleus , and skeletal muscle cells have many nuclei. [ 6 ]


The main function of striated muscle tissue is to create force and contract. These  contractions  in cardiac muscle will pump blood throughout the body. In skeletal muscle the contractions enable  breathing , movement, and  posture maintenance . [ 1 ]


Contractions in cardiac muscle tissue are due to a  myogenic response  of the  heart 's  pacemaker cells . These cells respond to signals from the autonomic nervous system to either increase or decrease the heart rate. Pacemaker cells have  autorhythmicity . The set intervals at which they depolarize to threshold and fire action potentials is what determines the heart rate. Because of the gap junctions, the pacemaker cells transfer the depolarization to other cardiac muscle fibers, in order to contract in unison. [ 5 ]


Signals from  motor neurons  cause skeletal muscle fibers to depolarize and therefore release calcium ions from the sarcoplasmic reticulum. The calcium drives the movement of myosin and actin filaments. The sarcomere then shortens which causes the muscle to contract. [ 3 ]  In the skeletal muscles connected to tendons that pull on bones, the mysia fuses to the  periosteum  that coats the bone. Contraction of the muscle will transfer to the mysia, then the tendon and the periosteum before causing the bone to move. The mysia also may bind to an  aponeurosis  or to  fascia . [ 2 ]


Adult humans cannot regenerate cardiac muscle tissue after an injury, which can lead to scarring and thus heart failure. Mammals have the ability to complete small amounts of cardiac regeneration during development. Other vertebrates can regenerate cardiac muscle tissue throughout their entire life span. [ 7 ]


Skeletal muscle is able to regenerate far better than cardiac muscle due to  satellite cells , which are dormant in all healthy skeletal muscle tissue. [ 8 ]  There are three phases to the regeneration process. These phases include the inflammatory response, the activation, differentiation, and fusion of satellite cells, and the maturation and remodeling of newly formed myofibrils. This process begins with the necrosis of damaged muscle fibers, which in turn induces the inflammatory response. Macrophages induce phagocytosis of the cell debris. They will eventually secrete anti-inflammatory cytokines, which results in the termination of inflammation. These macrophages can also facilitate the proliferation and differentiation of satellite cells. [ 3 ]  The satellite cells re-enter the cell cycle to multiply. They then leave the cell cycle to self-renew or differentiate as  myoblasts . [ 8 ]


Superhuman strength  is a  superpower  commonly invoked in  fiction  and other literary works, such as  mythology . A fictionalized representation of the phenomenon of  hysterical strength , it is the power to exert  force  and lift weights beyond what is  physically possible  for an ordinary  human being . Alternate terms of superhuman strength have included  enhanced strength ,  super-strength  and  increased strength . Superhuman strength is an amorphous ability, varying in potency depending on the writer or the context of the story in which it is depicted.


Characters and  deities  with superhuman strength have been found in multiple ancient  mythological accounts  and  religions .


Superhuman strength is a common trope in  fantasy  and  science fiction . This is generally by means of mechanisms such as  cybernetic  body parts,  genetic modification ,  telekinetic fields  in science fiction, or  magical / supernatural  sources within fantasy. A plethora of comic book  superheroes  and  super-villains  display some degree of super strength. Some films invoke a fictional substance or drug that gives the superpower. The level of superhuman strength portrayed can vary greatly, from just outside the "normal" or "natural" human range of the strongest  strongmen ,  powerlifters , and  weightlifters  (e.g. unarmored  Master Chief ,  Captain America , or  Deathstroke ), to nearly unlimited strength (e.g.,  Hulk ,  Juggernaut ,  Superman ,  Supergirl ,  Wonder Woman ,  Thor ,  Hercules , or  Goku ).


Humans are actively trying to achieve superhuman strength via technology and scientific experimentation. Athletes have turned to various methods to improve performance, such as  blood doping  or taking  anabolic steroids . Other technologies being researched are robotic  exoskeletons  to be worn by humans to enhance movement and strength.


Superhuman strength is a common ability of many  gods  and  demigods  in ancient  mythology , such as  Hercules / Heracles  (Roman/Greek),  Beowulf  (Anglo-Saxon),  Samson  (the Bible),  Bhima  (Hindu),  Kintarō  (Japanese),  Māui  (Polynesian), and  Achilles  (Greek). Attempts to modify the human body in order to gain extraordinary strength is common throughout history, as seen in fiction through characters such as  Terminator ,  Robocop ,  Iron Man , and  Cyborg .


Humans have tried to enhance their strength through  the use of substances . Accordingly, "In Ancient Rome, gladiators would drink herbal infusions to strengthen them before chariot races." Currently, drugs including  stimulants ,  anabolic steroids ,  diuretics , and  β-blockers  are ingested to enhance strength and other attributes. [ 1 ]


Humans have tried to use external devices to enhance their strength. The earliest device that was patented for this specific purpose can be credited to Nicholas Yagn, who filed the patent in 1890. The device was described to be an "apparatus for facilitating walking, running, and jumping" through the use of bags of compressed air. [ 2 ]  The  United States Department of Defense  is considering a variety of technologies to create an  exoskeleton  intended for military use to enhance soldier performance. [ 3 ]


In the real world, extraordinary strength can occur via science. A person can become stronger, tougher, and more physically powerful than would seem humanly possible when using enhancements such as  doping ,  substances  and  training .


Records describe instances of people going beyond "normal" strength in specific circumstances without taking any specific measures, as in the case of Tom Boyle, who was able to lift the front of a car in order to rescue a person trapped beneath it. [ 4 ]  Penn State professor of  kinesiology  Vladimir Zatsiorsky stated that extraordinary strength can occur when a person engages their muscles through the conscious "exertion of will". [ 5 ]  Zatsiorsky claims that trained athletes can improve their strength under specific conditions of competition. [ 5 ]  Fear can also cause a person to exhibit enhanced human strength. (see  hysterical strength )


The term appears in weight-lifting [ 6 ]  and 'protein formula' [ 7 ]  commercials without proof of their efficacy.


Weight lifters and other athletes routinely perform feats that appear to be superhuman to others. For examples, in 2016 Blaine Sumner achieved the heaviest single  bench press  of 401.05 kilograms (884 lb). At the same competition, he squat-lifted 500 kilograms (1,102 lb). [ 8 ]  In 2020,  Hafþór Júlíus Björnsson  of  Iceland  broke the world record for the heaviest  deadlift  at 501 kilograms (1,105 lb). [ 9 ]


Many fictional works involve superhuman strength, rooted in religious texts or in scientific form. [ 10 ]  The depiction of superhuman strength dates as far back as the earliest recorded work of writing, with the Sumarian hero  Gilgamesh . Early legends portray characters gaining their superhuman strength from the gods and exhibiting characteristics of both heroic humans and gods.


More recently, superhuman strength is employed by characters called superheroes in comic books, which dates back to the 1930s. Characters such as  Mr. Incredible ,  The Incredible Hulk ,  Superman , and  Wonder Woman  possess the strength to perform physical feats impossible for the human body. [ 11 ]  These characters and their powers draw from earlier myths. Wonder Woman ostensibly descended from the  Amazons , a group of women possessing superhuman strength. [ 12 ]  In many of these fictional works, the dilemma and solution lies in the character's superhuman abilities. Having these powers alienates them from society but also aids them in their quest. Recurring adaptations of well-known characters are often employed, which continue to perpetuate the use of superhuman strength in fiction.


T-tubules  ( transverse tubules ) are extensions of the  cell membrane  that penetrate into the center of  skeletal  and  cardiac muscle cells .   With membranes that contain large concentrations of  ion channels , transporters, and pumps, T-tubules permit rapid transmission of the  action potential  into the cell, and also play an important role in regulating cellular calcium concentration.


Through these mechanisms, T-tubules allow heart muscle cells to contract more forcefully by synchronising calcium release from the  sarcoplasmic reticulum  throughout the cell. [ 1 ]   T-tubule structure and function are affected beat-by-beat by cardiomyocyte contraction, [ 2 ]  as well as by diseases, potentially contributing to  heart failure  and  arrhythmias . Although these structures were first seen in 1897, research into T-tubule biology is ongoing.


T-tubules are tubules formed from the same  phospholipid bilayer  as the surface membrane or  sarcolemma  of skeletal or cardiac muscle cells. [ 1 ]   They connect directly with the sarcolemma at one end before travelling deep within the cell, forming a network of tubules with sections running both perpendicular (transverse) to and parallel (axially) to the sarcolemma. [ 1 ]  Due to this complex orientation, some refer to T-tubules as the transverse-axial tubular system. [ 3 ]   The inside or lumen of the T-tubule is open at the cell surface, meaning that the T-tubule is filled with fluid containing the same constituents as the solution that surrounds the cell (the extracellular fluid).  Rather than being just a passive connecting tube, the membrane that forms T-tubules is highly active, being studded with proteins including  L-type calcium channels ,  sodium-calcium exchangers ,  calcium ATPases  and  Beta adrenoceptors . [ 1 ]


T-tubules are found in both  atrial  and  ventricular  cardiac muscle cells ( cardiomyocytes ), in which they develop in the first few weeks of life. [ 4 ]   They are found in ventricular muscle cells in most species, and in atrial muscle cells from large mammals. [ 5 ]   In cardiac muscle cells, across different species, T-tubules are between 20 and 450 nanometers in diameter and are usually located in regions called  Z-discs  where the  actin myofilaments  anchor within the cell. [ 1 ]  T-tubules within the heart are closely associated with the intracellular calcium store known as the  sarcoplasmic reticulum  in specific regions referred to as terminal cisternae.  The association of the T-tubule with a  terminal cisterna  is known as a  diad . [ 6 ]


In skeletal muscle cells, T-tubules are three to four times narrower than those in cardiac muscle cells, and are between 20 and 40 nm in diameter. [ 1 ]  They are typically located at either side of the myosin strip, at the junction of overlap (A-I junction) between the A and I bands. [ 7 ]  T-tubules in skeletal muscle are associated with two  terminal cisternae , known as a  triad . [ 1 ] [ 8 ]


The shape of the T-tubule system is produced and maintained by a variety of proteins. The protein  amphiphysin-2  is encoded by the gene  BIN1  and is responsible for forming the structure of the T-tubule and ensuring that the appropriate proteins (in particular L-type calcium channels) are located within the T-tubule membrane. [ 9 ]  Junctophilin-2 is encoded by the gene  JPH2  and helps to form a junction between the T-tubule membrane and the sarcoplasmic reticulum, vital for  excitation-contraction coupling . [ 6 ]   Titin capping protein known as  telethonin  is encoded by the  TCAP gene  and helps with T-tubule development and is potentially responsible for the increasing number of T-tubules seen as muscles grow. [ 6 ]


T-tubules are an important link in the chain from electrical excitation of a cell to its subsequent contraction (excitation-contraction coupling).  When contraction of a muscle is needed, stimulation from a nerve or an adjacent muscle cell causes a characteristic flow of  charged particles  across the cell membrane known as an  action potential .  At rest, there are fewer positively charged particles on the inner side of the membrane compared to the outer side, and the membrane is described as being polarised. During an action potential, positively charged particles (predominantly sodium and calcium ions) flow across the membrane from the outside to the inside.  This reverses the normal imbalance of charged particles and is referred to as  depolarization .  One region of membrane depolarizes adjacent regions, and the resulting wave of depolarization then spreads along the cell membrane. [ 10 ]   The polarization of the membrane is restored as potassium ions flow back across the membrane from the inside to the outside of the cell.


In cardiac muscle cells, as the action potential passes down the T-tubules it activates L-type calcium channels in the T-tubular membrane.  Activation of the L-type calcium channel  allows calcium to pass into the cell.  T-tubules contain a higher concentration of L-type calcium channels than the rest of the sarcolemma and therefore the majority of the calcium that enters the cell occurs via T-tubules. [ 11 ]  This calcium binds to and activates a receptor, known as a  ryanodine receptor , located on the cell's own internal calcium store, the sarcoplasmic reticulum. Activation of the ryanodine receptor causes calcium to be released from the sarcoplasmic reticulum, causing the muscle cell to contract. [ 12 ]  In  skeletal muscle  cells, however, the L-type calcium channel is directly attached to the ryanodine receptor on the sarcoplasmic reticulum allowing activation of the ryanodine receptor directly without the need for an influx of calcium. [ 13 ]


The importance of T-tubules is not solely due to their concentration of L-type calcium channels, but lies also within their ability to synchronise calcium release within the cell.  The rapid spread of the action potential along the T-tubule network activates all of the L-type calcium channels near-simultaneously.  As T-tubules bring the sarcolemma very close to the sarcoplasmic reticulum at all regions throughout the cell,  calcium can then be released from the sarcoplasmic reticulum across the whole cell at the same time.  This synchronisation of calcium release allows muscle cells to contract more forcefully. [ 14 ]   In cells lacking T-tubules such as  smooth muscle cells , diseased cardiomyocytes, or muscle cells in which T-tubules have been artificially removed, the calcium that enters at the sarcolemma has to diffuse gradually throughout the cell, activating the ryanodine receptors much more slowly as a wave of calcium leading to less forceful contraction. [ 14 ]


As the T-tubules are the primary location for excitation-contraction coupling, the ion channels and proteins involved in this process are concentrated here - there are 3 times as many L-type calcium channels located within the T-tubule membrane compared to the rest of the sarcolemma. Furthermore, beta adrenoceptors are also highly concentrated in the T-tubular membrane, [ 15 ]  and their stimulation increases calcium release from the sarcoplasmic reticulum. [ 16 ]


As the space within the lumen of the T-tubule is continuous with the space that surrounds the cell (the extracellular space), ion concentrations between the two are very similar. However, due to the importance of the ions within the T-tubules (particularly calcium in cardiac muscle), it is very important that these concentrations remain relatively constant. As the T-tubules are very thin, they essentially trap the ions. This is important as, regardless of the ion concentrations elsewhere in the cell, T-tubules still have enough calcium ions to permit muscle contraction. Therefore, even if the concentration of calcium outside the cell falls ( hypocalcaemia ), the concentration of calcium within the  T-tubule remains relatively constant, allowing cardiac contraction to continue. [ 6 ]


As well as T-tubules being a site for calcium entry into the cell, they are also a site for calcium removal. This is important as it means that calcium levels within the cell can be tightly controlled in a small area (i.e. between the T-tubule and sarcoplasmic reticulum, known as local control). [ 17 ]  Proteins such as the sodium-calcium exchanger and the sarcolemmal ATPase are located mainly in the T-tubule membrane. [ 6 ]  The sodium-calcium exchanger passively removes one calcium ion from the cell in exchange for three sodium ions.  As a passive process it can therefore allow calcium to flow into or out of the cell depending on the combination of the relative concentrations of these ions and the voltage across the cell membrane (the  electrochemical gradient ). [ 10 ]   The calcium ATPase removes calcium from the cell actively, using energy derived from   adenosine triphosphate  (ATP). [ 10 ]


In order to study T-tubule function, T-tubules can be artificially uncoupled from the surface membrane using a technique known as  detubulation .  Chemicals such as  glycerol [ 18 ]   or  formamide [ 14 ]  (for skeletal and cardiac muscle respectively) can be added to the extracellular solution that surrounds the cells. These agents increase the osmolarity of the extracellular solution, causing the cells to shrink. When these agents are withdrawn, the cells rapidly expand and return to their normal size.  This shrinkage and re-expansion of the cell causes T-tubules to detach from the surface membrane. [ 19 ]   Alternatively, the osmolarity of the extracellular solution can be decreased, using for example hypotonic saline, causing a transient cell swelling.  Returning the extracellular solution to a normal osmolarity allows the cells to return to their previous size, again leading to detubulation. [ 20 ]


The idea of a cellular structure that later became known as a T-tubule was first proposed in 1881. The very brief time lag between stimulating a striated muscle cell and its subsequent contraction was too short to have been caused by a signalling chemical travelling the distance between the sarcolemma and the sarcoplasmic reticulum. It was therefore suggested that pouches of membrane reaching into the cell might explain the very rapid onset of contraction that had been observed. [ 21 ] [ 22 ]  It took until 1897 before the first T-tubules were seen, using  light microscopy  to study cardiac muscle injected with  India ink .  Imaging technology advanced, and with the advent of  transmission electron microscopy  the structure of T-tubules became more apparent [ 23 ]  leading to the description of the longitudinal component of the T-tubule network in 1971. [ 24 ]  In the 1990s and 2000s  confocal microscopy  enabled three-dimensional reconstruction of the T-tubule network and quantification of T-tubule size and distribution, [ 25 ]  and the important relationships between T-tubules and  calcium release  began to be unravelled with the discovery of  calcium sparks . [ 26 ]  While early work focussed on ventricular cardiac muscle and skeletal muscle, in 2009 an extensive T-tubule network in atrial cardiac muscle cells was observed. [ 27 ]  Ongoing research focusses on the regulation of T-tubule structure and how T-tubules are affected by and contribute to cardiovascular diseases. [ 28 ]


The structure of T-tubules can be altered by disease, which in the heart may contribute to weakness of the heart muscle or abnormal heart rhythms.  The alterations seen in disease range from a complete loss of T-tubules to more subtle changes in their orientation or branching patterns. [ 29 ]    T-tubules may be lost or disrupted following a  myocardial infarction , [ 29 ]  and are also disrupted in the ventricles of patients with  heart failure , contributing to reduced force of contraction and potentially decreasing the chances of recovery. [ 30 ]  Heart failure can also cause the near-complete loss of T-tubules from atrial cardiomyocytes, reducing atrial contractility and potentially contributing to  atrial fibrillation . [ 27 ]


Structural changes in T-tubules can lead to the L-type calcium channels moving away from the ryanodine receptors. This can increase the time taken for calcium levels within the cell to rise leading to weaker contractions and  arrhythmias . [ 6 ] [ 27 ]  However, disordered T-tubule structure may not be permanent, as some suggest that T-tubule remodelling might be reversed through the use of  interval training . [ 6 ]


Tachyaerobic  is a term used in  biology  to describe the  muscles  of large animals and birds that are able to maintain high levels or physical activity because their hearts make up at least 0.5-0.6 percent of their body mass and maintain high blood pressures. [ 1 ]   A reptile displaying equal size to a tachyaerobic mammal does not have the same capabilities.  Tachyaerobic animals' hearts beat more quickly, produce more oxygen, and distribute blood at a quicker rate than reptiles.


The use of tachyaerobic muscles is important to animals such as  giraffes  that need blood circulated through a large body size quickly.


Tensiomyography   (TMG)  is a measuring method for detection of  skeletal muscles ' contractile properties. [ 1 ]  Tensiomyography assesses  muscle  mechanical response based on radial muscle belly displacement induced by a single  electrical stimulus . [ 2 ]  It is performed using the TMG S2 system. A tensiomyography  measurement instrument  includes an electrical stimulator and  data acquisition  subunit (1), a digital  sensor  (2), a  tripod  with manipulating hand (3), and muscle  electrodes  (4) that work with an essential software interface installed on a PC. [ 1 ]


Tensiomyography is a  non-invasive , evidence-based measurement method that precisely measures the speed of muscle contraction under isometric conditions. It is used in sports performance and rehabilitation, and in sports medicine and research, for instance. Tensiomyography data can be used to determine  muscle fiber  type (e.g., by comparing displacement signal and muscle histochemistry [ 3 ] / heavy chain myosin amount20) and muscle status/condition (e.g.,fatigue, [ 4 ]  potentiation, [ 5 ]  inhibition, stress influence on the body, etc.), to diagnose functional muscular symmetry, either temporal or morphological, to evaluate muscular synchronization, and for fast detection (less than 5 minutes) of infra-clinical lesions in the muscles. [ 6 ]


A tensiomyography measurement consists of four steps:


Tensiomyography specific software on a 1 kHz frequency receives tensiomyography signals. Two supra-maximal responses are stored and then the average is calculated. The supra-maximal stimulation is regarded as corresponding to a minimal stimulation and it determines maximum amplitude of muscular deformation, recorded as Dm. [ 6 ]


TMG method output is a displacement-time signal evaluated with standard parameters. [ 7 ]


Some recent studies have also elected to examine  Contraction velocity (Vc)  as a change in Dm over time between 10% and 90% of the contraction. [ 9 ]


Tensiomyography is a simple to use selective and non-invasive method for detecting skeletal muscle contractile parameters using the linear displacement sensor. [ 10 ]  It assesses skeletal muscle thickening [ 11 ]  and low frequency lateral oscillations of active skeletal muscle fibers during twitch contractions. Future directions should be multidimensional: further validation especially with muscle force; increasing the research power of established theories; determining the trends of  physiological  processes and adaptations through longitudinal designs; characterizing muscle fatigue, and developing its application in dynamic muscle contractions. [ 12 ]
Tensiomyography was originally designed to be used by  medical professionals  but has transitioned from  medicine , [ 13 ]  through  sports medicine [ 14 ]  and is now being utilized in sports  training  programs and post injury rehabilitation quantification. [ 15 ]  Because of its non-invasive nature, tensiomyography provides rapid accurate diagnostic data without discomfort or disruption of the routine of the person whose muscles are being assessed.


Tensiomyography was originally designed for optimization of rehabilitation processes in medicine. Moving on, tensiomyography is used in military, veterinary and most widely spread in the field of elite sports, especially where explosiveness [ 16 ]  is needed.


Tensiomyography can be applied in training optimization [ 17 ]  process to prevent negative effects of muscle asymmetry and asynchrony on athletes’ maximal speed, explosiveness, endurance and flexibility. [ 18 ]  Application of tensiomyography method identifies muscle pair asymmetry and asynchronous action in the kinetic chain, which decreases movement economy and increase injury risk. Tensiomyographic assessment identifies muscle dysfunctions to adjust training accordingly (activate, strengthen or release tension in specific muscles). [ 19 ] [ 20 ]  The contractile characteristics of athletes' muscles from specific sports disciplines have been defined using tensiomyography, despite the fact that very few sports have been thoroughly researched. [ 21 ]


After an injury, tensiomyography can be used to determine injured muscle’s functional capacity by measuring individual muscle heads in isolation, providing unique and selective information. [ 22 ]  Data comparison with the uninjured contralateral muscle helps monitor early phase recovery. Measurement results help determine recovery of functional capacity so the most effective rehabilitation can be safely administered. [ 15 ]  During rehabilitation athletes can be assessed with tensiomyography for progress and rehabilitation strategy can be adjusted for faster rehabilitation.


Since its first scientific publication in 1990 more than 300 articles show tensiomyography use and purpose: in the estimation of muscle composition; [ 23 ] [ 24 ]  for evaluating  muscle atrophy ; [ 11 ]  for measuring adaptation to different  pathologies ; [ 10 ] [ 19 ] [ 25 ]  for measuring adaptation to specific training; and for measuring muscle fatigue. [ 4 ]


Tensiomyography has been used in several research areas including acute muscle diagnostics, chronic muscle change diagnostics, local muscle fatigue [ 26 ] [ 27 ]  and non-invasive determination of muscle fiber type composition. [ 1 ] [ 3 ] [ 24 ] [ 28 ]


ISOT strives to support further development in the field of tensiomyography and standardize tensiomyographic methods while connecting tensiomyography users worldwide for know-how exchange and networking. ISOT was formed at a congress held in Rome on 24 October 2014. [ 29 ]


The tensiomyography method was developed by a group of  experts  from various fields at the Faculty for  Electrical Engineering  at the  University of Ljubljana . The  Rehabilitation  Institute in Ljubljana,  Valdoltra Orthopedic Hospital  and  Slovenian Olympic Committee  were among several other institutions that cooperated and were the method's early users and implementers.


The inventor of the method, Vojko Valenčič, was a leading researcher and head of Laboratory for Biomedical Imaging and Muscle Biomechanics. He had a close working relationship with the Department of Electronics at the University of Ljubljana, which provided human and technical resources and was aided in his work by three research assistants.
The tensiomyography method was invented in the late 1980s and has since been improved through many prototypes and developed to the stage where clinical application of the method was possible. Although tensiomyography was initially intended for medical use, the method was also introduced into sports medicine and athletic training in 1996. In recent years, the development and application of the method has shifted more towards sports in cooperation with Srdjan Djordjevič, a biologist, applied physiologist and the founder of the company TMG-BMC Ltd.


Tetanic fade  refers to the diminishing muscle twitch response from an evoked potential stimulation of muscle under the effect of either a non-depolarizing neuromuscular blocking agent, or a muscle that is under a phase 2 depolarizing  neuromuscular blocking agent . [ 1 ] [ 2 ]


In order to  transduce  an excitatory signal to the muscle, an indication must transduce from the presynaptic neuron's axon terminal, travel across the synaptic cleft and be received correctly in the post synaptic muscle tissue's motor end plate to produce the desired effect, at the right intensity. The signal propagates from the presynaptic neuron using  neurotransmitter   Acetylcholine  (ACh), a molecule that is released from stored vesicles at the terminal end of the neuron. ACh travels across the space of the synaptic cleft to ACh receptors on the sarcolemma of the motor end plate. After unbinding from the receptors, ACh is broken down by the enzyme  acetylcholinesterase . [ 3 ] [ 1 ]


The  ACh receptor  is a  ligand gated channel  that goes through a conformational change when it binds to ACh, allowing the flow of  calcium ,  sodium , and  potassium  down their respective gradients. With enough movement of these ions, an electrochemical potential will form, which is called an  endplate potential  (EPP). The EPP signal will be propagated through the motor end plate and to the T-tubules to affect the muscle filaments via  sodium channels . These sodium channels have a brief activation window, after which they are inactivated until the end plate potential is restored to baseline levels. [ 3 ] [ 1 ]


Tetany  in evoked stimulus, as defined in Morgan & Mikhail's Clinical Anesthesiology as a ~5 seconds of sustained stimulus of between 50 and a 100 Hz. The reaction of  muscle tissue  to stimulus under no neuromuscular blockade should be equal in intensity throughout the stimulus: the first muscle twitch and last should be of roughly equal magnitude. Clinically it will present as equal muscle contraction throughout the duration of muscle stimulation. [ 1 ] [ 2 ]


Non-depolarizing neuromuscular blocking agents  (ie Rocuronium, Vecuronium) interact with Ach receptor without activating the channel, as well as preventing the binding of acetylcholine to it. This blocks the signal propagation from the presynaptic neuron, and the severs the transduction of the excitatory signal from the synaptic cleft. [ 4 ] [ 1 ]


Muscle tissue treated with a  non-depolarizing neuromuscular blocking agents  will produce an indicative response in the form of a tetanic fade, a diminishing response to tetanic stimulation where the initial intensity will be the highest, and the following ones will show lower and lower strength of response. [ 1 ] [ 2 ]


A some data suggest that this response is due to effect of  non-depolarizing neuromuscular blocking agents  on the presynaptic nerve, leading  to lower acetylcholine secreted to the cleft during the tetanic stimulation. In normal muscle tissue release of Ach works to stimulate more release of Ach from the presynaptic neuron's axon in a  positive feedback  manner. In tissue under a  non-depolarizing neuromuscular blocking agent  greatly diminished the levels of Ach that are released into the synaptic cleft. [ 1 ] [ 2 ]


Depolarizing neuromuscular blocking agents  (ie Succinylcholine) employ their effect by binding to the ach receptor and activating it, unlike acetylcholine though, the drug is not rapidly degraded by  acetylcholinesterase . Succinylcholine propagates in high concentration for longer duration until it is degraded in the plasma and liver by a different enzyme ( pseudocholinesterase ). This continues to activate the Ach receptor, and prevents sodium channels from recovering to their active state. This leads to muscle relaxation and termed phase 1 block, clinically it will present as a diminished twitch response during tetanic stimulation. [ 1 ]


Phase 1 block does not show fade under tetanic stimulation. With further administration of  depolarizing neuromuscular blocking agent , an effect that is similar to the response of muscle to  non-depolarizing neuromuscular blocking agents  can be observed. This response, termed phase 2 block, will demonstrate similar fade under tetanic stimulation, a diminishing response to tetanic stimulation where the initial response will be the strongest, and will produce lower and lower response intensity. [ 1 ] [ 2 ]


4UOW ,  1BPV ,  1G1C ,  1NCT ,  1NCU ,  1TIT ,  1TIU ,  1TKI ,  1TNM ,  1TNN ,  1WAA ,  1YA5 ,  2A38 ,  2BK8 ,  2F8V ,  2ILL ,  2J8H ,  2J8O ,  2NZI ,  2RQ8 ,  2WP3 ,  2WWK ,  2WWM ,  2Y9R ,  3KNB ,  3LCY ,  3LPW ,  3PUC ,  3Q5O ,  3QP3 ,  4C4K ,  4JNW ,  4O00 ,  4QEG ,  5BS0


NM_133432 NM_133437


NM_011652 NM_028004


NP_597676 NP_597681 NP_035782 NP_082280 NP_001372637


Titin [ 5 ]   / ˈ t aɪ t ɪ n /  (contraction for  Tit an prote in ) (also called  connectin ) is a  protein  that in humans is encoded by the  TTN   gene . [ 6 ] [ 7 ]  The protein, which is over 1  μm  in length, [ 8 ]  functions as a molecular  spring  that is responsible for the passive elasticity of  muscle . It comprises 244 individually folded  protein domains  connected by unstructured  peptide  sequences. [ 9 ]  These domains  unfold  when the protein is stretched and  refold  when the tension is removed. [ 10 ]


Titin is important in the contraction of  striated muscle tissues . It connects the  Z disc  to the  M line  in the  sarcomere . The protein contributes to force transmission at the Z disc and resting tension in the  I band  region. [ 11 ]  It limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle. Variations in the sequence of titin between different types of striated muscle ( cardiac  or  skeletal ) have been correlated with differences in the mechanical properties of these muscles. [ 6 ] [ 12 ]


Titin is the third most abundant protein in muscle (after  myosin  and  actin ), and an adult human contains approximately 0.5 kg of titin. [ 13 ]   With its length of ~27,000 to ~35,000  amino acids  (depending on the  splice isoform ), titin is the largest known  protein . [ 14 ]   Furthermore, the gene for titin contains the largest number of  exons  (363) discovered in any single gene, [ 15 ]  as well as the longest single exon (17,106  bp ).


In 1954, Reiji Natori proposed the existence of an elastic structure in muscle fiber to account for the return to the resting state when muscles are stretched and then released. [ 16 ]  In 1977, Koscak Maruyama and coworkers isolated an elastic protein from muscle fiber that they called connectin. [ 17 ]   Two years later,  Kuan Wang  and coworkers identified a doublet band on  electrophoresis gel  corresponding to a high molecular weight, elastic protein that they named titin. [ 5 ] [ 18 ]


In 1990, Siegfried Labeit isolated a partial  cDNA  clone of titin. [ 7 ]   Five years later, Labeit and Bernhard Kolmerer determined the cDNA sequence of human cardiac titin. [ 9 ]  In 2001, Labeit and colleagues determined the complete sequence of the human titin gene. [ 15 ] [ 19 ]


The human gene encoding for titin is located on the long arm of chromosome 2 and contains 363 exons, which together code for 38,138  amino acid   residues  (4200 kDa). [ 15 ]  Within the gene are found a large number of PEVK (proline-glutamate-valine-lysine -abundant  structural motifs ) exons 84 to 99 nucleotides in length, which code for conserved 28- to 33-residue motifs that may represent structural units of the titin PEVK spring. The number of PEVK motifs in the titin gene appears to have increased during evolution, apparently modifying the genomic region responsible for titin's spring properties. [ 20 ]


A number of titin  isoforms  are produced in different striated muscle tissues as a result of  alternative splicing . [ 21 ]   All but one of these isoforms are in the range of ~27,000 to ~36,000 amino acid residues in length. The exception is the small cardiac novex-3 isoform, which is only 5,604 amino acid residues in length. The following table lists the known titin isoforms:


Titin is the largest known protein; its human variant consists of 34,350  amino acids , with the  molecular weight  of the mature "canonical" isoform of the protein being approximately 3,816,030.05  Da . [ 22 ]  Its mouse homologue is even larger, comprising 35,213 amino acids with a molecular weight of 3,906,487.6  Da . [ 23 ]  It has a theoretical  isoelectric point  of 6.02. [ 22 ]  The protein's  empirical   chemical formula  is C 169,719 H 270,466 N 45,688 O 52,238 S 911 . [ 22 ]  It has a theoretical  instability index  (II) of 42.38, classifying the protein as unstable. [ 22 ]  The protein's  in vivo   half-life , the time it takes for half of the amount of protein in a cell to break down after its synthesis in the cell, is predicted to be approximately 30 hours (in  mammalian   reticulocytes ). [ 21 ]


The Titin protein is located between the  myosin  thick filament and the Z disk. [ 25 ]  Titin consists primarily of a linear array of two types of modules, also referred to as  protein domains  (244 copies in total): type I  fibronectin type III domain  (132 copies) and type II  immunoglobulin domain  (112 copies). [ 13 ] [ 9 ]  However, the exact number of these domains is different in different species. This linear array is further organized into two regions:


The C-terminal region also contains a serine  kinase  domain [ 27 ] [ 28 ]  that is primarily known for adapting the muscle to mechanical strain. [ 29 ]  It is “stretch-sensitive” and helps repair overstretching of the sarcomere. [ 30 ]  The N-terminal (the Z-disc end) contains a "Z repeat" that recognizes  Actinin alpha 2 . [ 31 ]


The elasticity of the PEVK region has both  entropic  and  enthalpic  contributions and is characterized by a polymer  persistence length  and a  stretch modulus . [ 32 ]  At low to moderate extensions PEVK elasticity can be modeled with a standard  worm-like chain  (WLC) model of  entropic elasticity . At high extensions PEVK stretching can be modeled with a modified WLC model that incorporates enthalpic elasticity. The difference between low-and high- stretch elasticity is due to electrostatic stiffening and  hydrophobic effects .


Embedded between the PEVK and Ig residues are N2A domains. [ 33 ]


The titin domains have evolved from a common ancestor through many gene duplication events. [ 34 ]  Domain duplication was facilitated by the fact that most domains are encoded by single exons. Other giant sarcomeric proteins made out of Fn3/Ig repeats include  obscurin  and  myomesin . Throughout evolution, titin mechanical strength appears to decrease through the loss of disulfide bonds as the organism becomes heavier. [ 35 ]


Titin A-band has homologs in invertebrates, such as twitchin (unc-22) and projectin, which also contain Ig and FNIII repeats and a protein kinase domain. [ 30 ]  The gene duplication events took place independently but were from the same ancestral Ig and FNIII domains. It is said that the protein titin was the first to diverge out of the family. [ 28 ]   Drosophila  projectin, officially known as bent ( bt ), is associated with lethality by failing to escape the egg in some mutations as well as dominant changes in wing angles. [ 36 ] [ 37 ] [ 38 ]


Drosophila  Titin, also known as Kettin or  sallimus  ( sls ), is kinase-free. It has roles in the elasticity of both muscle and chromosomes. It is homologous to vertebrate titin I-band and contains Ig PEVK domains, the many repeats being a hot target for splicing. [ 39 ]  There also exists a titin homologue,  ttn-1 , in  C. elegans . [ 40 ]  It has a kinase domain, some Ig/Fn3 repeats, and PEVT repeats that are similarly elastic. [ 41 ]


Titin is a large abundant protein of striated muscle. Titin's primary functions are to stabilize the thick filament, center it between the thin filaments, prevent overstretching of the sarcomere, and to recoil the sarcomere like a spring after it is stretched. [ 42 ]  An N-terminal Z-disc region and a C-terminal M-line region bind to the Z-line and M-line of the  sarcomere , respectively, so that a single titin molecule spans half the length of a sarcomere. Titin also contains binding sites for muscle-associated proteins so it serves as an adhesion template for the assembly of contractile machinery in muscle cells. It has also been identified as a structural protein for  chromosomes . [ 43 ] [ 44 ]  Considerable variability exists in the I-band, the M-line and the Z-disc regions of titin. Variability in the I-band region contributes to the differences in elasticity of different titin isoforms and, therefore, to the differences in elasticity of different muscle types. Of the many titin variants identified, five are described with complete transcript information available. [ 6 ] [ 7 ]


Dominant  mutation in TTN causes predisposition to  hernias . [ 45 ]


Titin interacts with many  sarcomeric  proteins including: [ 15 ]


Mutations  anywhere within the unusually long sequence of this gene can cause  premature stop codons  or other defects.  Titin mutations are associated with hereditary  myopathy  with early respiratory failure, [ 46 ] [ 47 ]  early-onset myopathy with fatal  cardiomyopathy , [ 48 ]  core myopathy with heart disease,  centronuclear myopathy ,  limb-girdle muscular dystrophy  type 2J, [ 49 ]   familial   dilated cardiomyopathy  9, [ 11 ] [ 50 ]   hypertrophic cardiomyopathy  and  tibial muscular dystrophy . [ 51 ]  Further research also suggests that no genetically linked form of any  dystrophy  or myopathy can be safely excluded from being caused by a mutation on the TTN gene. [ 49 ]   Truncating mutations in dilated cardiomyopathy patients are most commonly found in the A region; although truncations in the upstream I region might be expected to prevent translation of the A region entirely,  alternative splicing  creates some transcripts that do not encounter the premature stop codon, ameliorating its effect. [ 52 ]   mRNA splicing factors  such as RBM20 and  SLM2  ( KHDRBS3 ) were shown to mediated alternative mRNA splicing of titin mRNA contributing to the development of  heart failure  due to  cardiomyopathies . [ 53 ] [ 54 ]


Autoantibodies to titin are produced in patients with the autoimmune disease  Myasthenia gravis . [ 55 ]


Titin has been shown to  interact  with:


The name titin is derived from the Greek  Titan  (a giant deity, anything of great size). [ 5 ]


As the largest known protein, titin also has the longest  IUPAC name  of a protein.  The full chemical name  of the human canonical form of titin, which starts  methionyl ...  and ends  ... isoleucine , contains 189,819 letters and is sometimes stated to be the  longest word in the English language , or  of any language . [ 66 ]  However,  lexicographers  regard generic names of  chemical compounds  as  verbal  formulae  rather than English words. [ 67 ]


This article incorporates text from the  United States National Library of Medicine , which is in the  public domain .


In the  histology  of  skeletal muscle , a  triad  is the structure formed by a  T tubule  with a  sarcoplasmic reticulum  (SR) known as the  terminal cisterna  on either side. [ 1 ]  Each  skeletal muscle fiber  has many thousands of triads, visible in muscle fibers that have been sectioned longitudinally. (This property holds because T tubules run perpendicular to the longitudinal axis of the muscle fiber.) In mammals, triads are typically located at the  A-I junction ; [ 1 ]  that is, the junction between the  A  and  I bands  of the  sarcomere , which is the smallest unit of a muscle fiber.


Triads form the anatomical basis of  excitation-contraction coupling , whereby a stimulus excites the muscle and causes it to contract. A stimulus, in the form of positively charged current, is transmitted from the  neuromuscular junction  down the length of the  T tubules , activating  dihydropyridine receptors  (DHPRs). Their activation causes 1) a negligible influx of  calcium  and 2) a mechanical interaction with calcium-conducting  ryanodine receptors  (RyRs) on the adjacent SR membrane. Activation of RyRs causes the release of calcium from the SR, which subsequently initiates a cascade of events leading to  muscle contraction . These muscle contractions are caused by calcium's bonding to troponin and unmasking the binding sites covered by the troponin-tropomyosin complex on the actin myofilament and allowing the myosin cross-bridges to connect with the actin.


Tropomyosin  is a two-stranded  alpha-helical ,  coiled coil   protein  found in many  animal  and  fungal  cells. In animals, it is an important component of the muscular system which works in conjunction with  troponin  to regulate muscle contraction. It is present in smooth and striated muscle tissues, which can be found in various organs and body systems, including the heart, blood vessels, respiratory system, and digestive system. In fungi, tropomyosin is found in cell walls and helps maintain the structural integrity of cells.


Tropomyosin is found in other  eukaryotes  too, but not in plants. Overall, tropomyosin is an important protein that plays a vital role in the proper functioning of many different organisms.


All organisms contain organelles that provide physical integrity to their cells. These type of organelles are collectively known as the cytoskeleton, and one of the most ancient systems is based on filamentous polymers of the protein  actin . A polymer of a second protein, tropomyosin, is an integral part of most actin filaments in animals.


Tropomyosins are a large family of integral components of actin filaments that play a critical role in regulating the function of actin filaments in both muscle and nonmuscle cells. These proteins consist of rod-shaped coiled-coil hetero- or homo- dimers  that lie along the  α-helical groove  of most actin filaments. Interaction occurs along the length of the actin filament, with dimers aligning in a head-to-tail fashion.


Tropomyosins are often categorised into two groups: muscle tropomyosin  isoforms  and nonmuscle tropomyosin isoforms. Muscle tropomyosin isoforms are involved in regulating interactions between actin and  myosin  in the muscle  sarcomere  and play a pivotal role in regulated  muscle contraction . Nonmuscle tropomyosin isoforms function in both muscle and nonmuscle cells, and are involved in a range of cellular pathways that control and regulate the cell's  cytoskeleton  and other key cellular functions.


The actin filament system that is involved in regulating these cellular pathways is more complex than the actin filament systems that regulates muscle contraction. The contractile system relies upon 4 actin filament isoforms and 5 tropomyosin isoforms, [ 1 ]  whereas the actin filament system of the cytoskeleton uses two actin filament isoforms and over 40 tropomyosin isoforms. [ 1 ] [ 2 ]


In direct contrast with the 'one  gene , one  polypeptide ' rule, it is now known from a combination of  genomic sequencing , such as the  Human Genome Project  and  EST  data of expressed proteins, that many  eukaryotes  produce a range of proteins from a single gene. This plays a crucial role in the functionality of higher eukaryotes, with humans expressing more than 5 times as many different proteins (isoforms) as genes through  alternative splicing . From a mechanistic point of view, it is much easier for an organism to expand on a current gene/protein family (creating protein isoforms) than it is to create an entirely new gene.
From an evolutionary point of view, tropomyosins in higher eukaryotes are notable in retaining all 4 of the potential genes produced by the dual genomic duplication event that took place in early eukaryotic evolution.
 [ 3 ]


Within mammals, four genes are responsible for generating more than 40 different tropomyosin isoforms. In terms of structure, the genes are very similar, suggesting that they arose through  gene duplication  of an ancestral gene. In humans, these genes are no longer linked and are widely dispersed. In humans, the α1-, β-, α3-, and α4-genes are formally known as  TPM1 ,  TPM2 ,  TPM3 , and  TPM4  and are located at 15q22, [ 4 ]  9p13, [ 5 ]  1q22 [ 6 ]  and 19p13, [ 7 ]  respectively. An alternative nomenclature names the four genes (α,β,γ,δ). [ 2 ]


Isoforms are defined as highly related gene products that perform, in essence, similar biological functions, with variations existing between the isoforms in terms of biological activity, regulatory properties, temporal and spatial expression, and/or the intercellular location. Isoforms are produced by two distinct mechanisms, gene duplication and alternative splicing. The former mechanism is a process by which multiple copies of a gene are generated through unequal crossing over, through tandem duplication, or by translocation. Alternative splicing is a mechanism wherein exons are either retained in the mRNA or targeted for removal in different combinations to create a diverse array of mRNAs from a single pre-mRNA.


A vast array of tropomyosin isoforms are generated by using a combination of different genes and alternative splicing. [ 8 ]  In mammals, regardless of the gene, transcription is initiated at the start of either exon 1a or exon 1b. Depending on the  promoter  and initial exon used, tropomyosin isoforms can be categorized as either high- molecular-weight  (HMW, 284 amino acids) or low-molecular-weight (LMW, 248). [ 1 ] [ 9 ]  HMW isoforms express exon 1a and either 2a or 2b, while LMW isoforms express exon 1b. [ 9 ]  To date, all known tropomyosins contain exons 3-9. Alternative splicing can occur at exon 6, with the mutually exclusive choice of exon 6a or 6b. [ 10 ]  At the c-terminus, the transcript is spliced again at exon 9, with the choice of exon 9a, 9b, 9c, or 9d. [ 10 ]


In terms of structure, the genes are very similar, suggesting that they arose through gene duplication of an ancestral gene. The most highly related genes are the α- and γ-genes, utilizing two promoters and differing only with the presence of the unique 2a exon in the α-gene. [ 11 ] [ 12 ]  Although substantial differences between alternative exons from the same gene have been revealed by sequence comparison (1a and 1b, 6a and 6b, and the exon 9s), most exons are, however, highly conserved between the different genes. [ 1 ] [ 8 ] [ 13 ] [ 14 ]  For example, exon 1a and 1b from the α-gene vary considerably in sequence; however, the sequence from exon 1a from the α-, β-, γ-, and δ-genes is highly conserved.


Due to the conservative nature of the genes, it is believed that the genes evolved from a common ancestral gene, giving rise to over 40 functionally distinct isoforms. The expression of these isoforms is highly regulated and variable throughout development. The diversity of tropomyosin expression, both in space and in time, provides the potential not only to regulate actin filament function but to create specialised actin filament populations. [ 3 ]


Numerous reports detail that tropomyosin isoforms are sorted to different intracellular locations, often associating with actin filament populations that are involved in specific processes. [ 15 ] [ 16 ] [ 17 ] [ 18 ]  Direct visualization of spatial segregation of isoforms was initially observed by Burgoyne and Norman and soon after by Lin and co-workers. [ 18 ] [ 19 ] [ 20 ]  They observed that specific isoforms were associated with distinct cellular structures. [ 18 ]  Using specific antibodies, they were able to identify the presence of both HMW and the LMW isoforms of the γ-gene in stress fibers; however, only LMW isoforms were detected in  ruffling membranes . [ 18 ]


These studies have been extended to a number of cell types with similar results. Extensive studies in neuronal cells, [ 21 ]   fibroblasts , [ 16 ] [ 17 ] [ 22 ]  skeletal muscle [ 23 ] [ 24 ]  and  osteoclast  cells has further highlighted the complex association tropomyosin isoforms have with cellular structures. These studies have led to the realization that the regulation of isoform sorting is extremely complex and highly regulated.


Sorting of tropomyosin isoforms in discrete intracellular locations is developmentally regulated. Initial studies reported that the sorting of isoforms changed through development, where Tropomyosin 4 was initially localized to the  growth cone  of growing neurons, but in mature neurons it was relocated to the somatodendritic compartment. [ 25 ]  These observations have been supported by studies on different tropomyosin isoforms, showing how tropomyosin populations were relocated during neuron maturation. This evidence is supportive of the notion that tropomyosin isoforms are subject to temporal regulation.


Additional studies have identified the role the cell cycle plays in isoform sorting. A study that screened a range HMW products from the α- and β-genes and  compared localisation with LMW products from the γ-gene found that the HMW and LMW products are mutually exclusively segregated during the early  G1 phase  of the cell cycle. [ 17 ]


While studies suggest that tropomyosin sorting may be influenced by the sorting of mRNAs, [ 21 ]  there is no absolute correlation between mRNA and protein location. In neurons, Tropomyosin 5NM1 mRNA was found to sort to the pole of the neuron elaborating an axon prior to morphological differentiation. [ 26 ]  The sorting of Tropomyosin 5NM1/2 mRNA to this location correlated with the expression of the Tropomyosin 5NM1/2 protein. In contrast, the mRNA encoding the Tropomyosin Br2 protein was excluded from the pole of the neuron. [ 26 ]


The link between mRNA sorting and protein location has been tested in transgenic mice models. The models were created so that the coding regions of Tropomyosin 5NM1/2 and Tropomyosin 3 were expressed under the control of the β-actin promoter with the a β-actin 3'-untranslated region lacking targeting information. [ 27 ]  The study found that Tropomyosin 3, an isoform that is not normally expressed in neuronal cells, was broadly distributed throughout the neuron, while exogenous expression of the neuronal isoform Tropomyosin 5NM1/2 was found to sort to the growth cone of neurons as does the endogenous Tropomyosin 5NM1/2. As these two transgenes differ only in the tropomyosin coding region yet are localized in two distinct areas, the findings suggest that, in addition to mRNA sorting, the proteins themselves contain sorting information.


Studies suggest that tropomyosin isoform sorting may also be influenced by the actin isoform composition of microfilaments. [ 27 ]  In myoblasts, overexpression of γ-actin resulted in the down-regulation of β–actin and the removal of Tropomyosin 2 but not Tropomyosin 5 from stress fibers. [ 28 ]  It was later found that, when cells were exposed to cytochalasin D, a chemical that results in the disorganization of actin filaments, tropomyosin isoform sorting was disrupted. Upon the washing out of cytochalasin D, tropomyosin isoform sorting was re-established. [ 29 ]  This is suggestive of a strong relationship between the process of tropomyosin isoform sorting and the incorporation of tropomyosin isoforms into organized arrays of actin filaments. There is no evidence for active transport of tropomyosin isoforms to specific locations. Rather, it appears that sorting is the result of local assembly of preferred isoforms at specific intracellular site. The mechanisms that underlie tropomyosin isoform sorting appear to be inherently flexible and dynamic in nature.


Many studies have led to the understanding that tropomyosins perform essential functions and are required in a diverse range of species from yeast, worms, and flies to complex mammals.


The essential role of tropomyosins was discovered in the Bretscher laboratory, where researchers found that, by eliminating the TPM1 gene of budding yeasts, growth rates were reduced, the presence of actin cables disappeared, defects in vesicular transport were observed, and mating of the yeast was poor. [ 30 ]  When a second yeast gene, TPM2, was deleted, no observable changes in the phenotype were recorded; however, when deleted in combination with TPM1, it resulted in lethality. This suggests that TPM1 and -2 genes have overlapping function; however, TPM2 cannot fully compensate of the loss of TPM1, indicating that some functions of TPM1  are unique. Similar results have been observed in flies, worms, amphibians, and mammals, confirming previous results and suggestive of tropomyosin's being involved in a wide range of cellular functions. However, the three co-expressed TMP1, 2, and 4 genes cannot compensate for deletion of the TPM3 gene in embryonic stem cells and preimplantation mouse embryos.


Results from gene knockout experiments can be ambiguous and must be carefully examined. In studies in which the deletion of a gene leads to lethality, it can at first appear that the gene product had a truly unique role. However, lethality can also be the result of the inability of the compromised cell to express other isoforms to rescue the phenotype because the required isoform is not naturally expressed in the cell.


The  actin microfilament  system is the fundamental cytoskeletal system involved in the development and maintenance of cell morphology. The ability of this system to readily respond to cellular cues and undergo structural re-organisation has led to the belief that this system regulates specific structural changes within different cellular regions.


Within humans, there are only six actin isoforms, and these isoforms are responsible for an array of unique and complex cellular structures and key cellular interactions. It is thought that the function and form of the actin cytoskeleton is controlled largely by  actin-binding proteins  (ABP) that are associated with the actin polymer. ABP are a group of proteins that bind to actin. Although tropomyosin is sometimes included as an ABP, it is not a true ABP. The tropomyosin dimer has very low affinity for an actin filament and forms no van der waals contacts with actin. It is only the formation of a tropomyosin polymer's winding around the actin filament that provides stability to the tropomyosin-actin filament interaction.


Many studies suggest that the binding of tropomyosin isoforms to an actin filament may influence the binding of other ABPs, which together alter the structure and convey specific properties and ultimately specific functions to an actin filament. This is demonstrated in neuroepithelial cells, where increased expression of Tropomyosin 5NM1 increases the recruitment of myosin IIB, a myosin motor protein to the growth cone area. [ 31 ]  However, the over-expression of Tropomyosin Br3 had the opposite effect, decreasing myosin activity in the same region.


In a pioneering study by Bernstein and Bamburg, it was observed that the actin-binding protein  actin depolymerisation factor (ADF)/ cofilin , a factor that promotes actin filament depolymerisation, competed with tropomyosin for binding to the actin filament. [ 32 ]  The expression of Tropomyosin 5NM1 in neuronal cells eliminated ADF/cofilin from the growth cone region, leading to more stable actin filaments. [ 31 ]  However, the increased expression of Tropomyosin Br3 was observed to recruit ADF/cofilin to actin filaments bound to the Tropomyosin Br3 isoform within the lamellipodium, which led to the disassembly of actin filaments. [ 31 ]  This phenomenon, whereby a specific tropomyosin isoform directs specific interactions between actin-binding proteins, and the actin filament has been observed in a variety of model systems with a range of different binding proteins (reviewed in Gunning et al., 2008 [ 10 ] ). These interactions, under the influence of tropomyosin isoforms, allow actin filaments to be involved in a diverse range of cellular functions.


Skeletal muscle  is composed of large, multi-nucleated cells ( muscle fibers ). Each muscle fiber is packed with longitudinal arrays of  myofibrils . Myofibrils are composed of repeating protein structures or  sarcomeres , the basic functional unit of skeletal muscle. The sarcomere is a highly structured protein array, consisting of interdigitating thick and thin filaments, where the thin filaments are tethered to a protein structure, the  Z-line . The dynamic interaction between the thick and thin filaments results in muscle contraction.


Myosin belongs to a family of motor proteins, and the muscle isoforms of this family comprise the thick filament. The thin filament is made of the skeletal muscle isoforms of actin. Each myosin protein 'paddles' along the thin actin filament, repeatedly binding to myosin-binding sites along the actin filament, ratcheting and letting go. In effect, the thick filament moves or slides along the thin filament, resulting in  muscle contraction . This process is known as the  sliding filament model .


The binding of the myosin heads to the muscle actin is a highly regulated process. The thin filament is made of actin, tropomyosin, and troponin. The contraction of skeletal muscle is triggered by nerve impulses that in turn stimulate the release of Ca 2+ . The release of Ca 2+  from the  sarcoplasmic reticulum  causes an increase in the concentration of Ca 2+  in the cytosol. Calcium ions then bind to troponin, which is associated with tropomyosin. Binding causes changes in the shape of troponin and subsequently causes the tropomyosin isoform to shift its position on the actin filament. This shifting in position exposes the myosin-binding sites on the actin filament, allowing the myosin heads of the thick filament to bind to the thin filament.


Structural and biochemical studies suggest that the position of tropomyosin and troponin on the thin filament regulates the interactions between the myosin heads of the thick filament and the binding sites on the actin of the thin filament.  X-ray diffraction  and  cryoelectron microscopy  suggest that tropomyosin  sterically  blocks the access of myosin to the actin filament.


Although this model is well-established, it is unclear as to whether the movement of tropomyosin directly causes the myosin head to engage the actin filament. As such, an alternative model has emerged, whereby the movement of the tropomyosin in the filament functions as an  allosteric  switch that is modulated by activating myosin binding but does not function solely by regulating myosin binding.


Smooth muscle  is a type of non-striated muscle, and, unlike striated muscle, contraction of smooth muscle is not under conscious control. Smooth muscle may contract spontaneously or rhythmically and be induced by a number of physiochemical agents (hormones, drugs, neurotransmitters). Smooth muscle is found within the walls of various organs and tubes in the body such as the esophagus, stomach, intestines, bronchi, urethra, bladder, and blood vessels.


Although smooth muscles do not form regular arrays of thick and thin filaments like the sarcomeres of striated muscles, contraction is still due to the same sliding filament mechanism controlled by myosin crossbridges interacting with actin filaments. The thin filament of smooth muscle is made of actin, tropomyosin,  caldesmon , and  calmodulin . Within this type of muscle, caldesmon and calmodulin control the tropomyosin-mediated transition between on and off activity states. Caldesmon binds to actin, tropomyosin, calmodulin, and myosin, of which its interactions with actin are most important. The binding of caldesmon is strongly influenced by tropomyosin. Caldesmon is an inhibitor of actinomyosin ATPase and motility, and both actin binding and caldesmon inhibition are greatly enhanced in the presence of tropomyosin.


Smooth muscle contraction is initiated by the release of Ca 2+ . Ca 2+  binds to and activates calmodulin, which then binds to caldesmon. This binding causes the caldesmon protein to disengage from the actin filament, exposing the myosin-binding sites on the actin filament. Myosin motor heads are phosphorylated by  myosin light-chain kinase , allowing the myosin head to interact with the actin filament and cause contraction.


The cytoskeleton is an elaborate network of filaments required for the proper functioning of a range of cellular processes including cell motility, cell division, intracellular trafficking, and the maintenance of cell shape.  The cytoskeleton is composed of three distinct filament systems: microtubules, intermediate filaments, and microfilaments (also known as the actin cytoskeleton). It is the dynamic interactions between these filaments that provide cells with unique structures and functions.


A number of regulatory mechanisms, employing many actin-binding proteins, have evolved to control the dynamics of the actin filament system. It is believed that tropomyosins play a pivotal role in this regulatory system, influencing the associations the actin filament has with other ABPs. Together, these associations confer specific properties on the filament, allowing these structures to be involved in a wide range of cellular processes, but also to rapidly respond to cellular stimuli.


Many studies have shown that there are specific changes to the repertoire of tropomyosins expressed in cells that are undergoing cellular transformation. These highly reproducible results suggest that, during the process of cellular transformation, a process whereby a normal cell becomes malignant, there is a decreased synthesis of HMW tropomyosin isoforms. In the initial studies, transformation of rat embryo fibroblast cell line REF-52 and of normal rat kidney cells led to decreased synthesis of HMW tropomyosins. [ 33 ] [ 34 ] [ 35 ]  In both of these systems, the down regulation was contributed to a decrease in the mRNA levels. These early results suggested that tropomyosins played a critical role in facilitating certain processes that occurred during cell transformation, such as actin filament re-organisation and changes in cell shape. These studies have been reproduced in other laboratories and in other cell lines, with similar results (reviewed in Gunning et al., 2008 [ 10 ] ).