Muscle cell

A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells.[1] A skeletal muscle cell is long and threadlike with many nuclei and is called a muscle fiber.[1] Muscle cells (including myocytes and muscle fibers) develop from embryonic precursor cells called myoblasts.[2]

Muscle cell
Synapse diag3.png
Anatomical terms of microanatomy

Myoblasts fuse to form multinucleated skeletal muscle cells known as syncytia in a process known as myogenesis.[3][4]

Cardiac muscle cells form the chambers of the heart, and have a single central nucleus.[5] Cardiac muscle cells when joined together in a visible unit is described as a cardiac muscle fiber.[6] Cardiac muscle is also striated like skeletal muscle.[7] Smooth muscle cells control involuntary movements such as the peristalsis contractions in the esophagus and stomach.


The unusual microscopic anatomy of a muscle cell gave rise to its own terminology. The cytoplasm in a muscle cell is termed the sarcoplasm; the smooth endoplasmic reticulum of a muscle cell is termed the sarcoplasmic reticulum; and the cell membrane in a muscle cell is termed the sarcolemma.[8] The sarcolemma receives and conducts stimuli.

Skeletal muscle cellsEdit

Diagram of skeletal muscle fiber structure

Skeletal muscle cells are the individual contractile cells within a muscle, and are more usually known as muscle fibers because of their longer threadlike appearance.[9] A single muscle such as the biceps brachii in a young adult human male contains around 253,000 muscle fibers.[10] Skeletal muscle fibers are the only muscle cells that are multinucleated with the nuclei usually referred to as myonuclei. This occurs during myogenesis with the fusion of myoblasts each contributing a nucleus to the newly formed muscle cell or myotube.[11] Fusion depends on muscle-specific proteins known as fusogens called myomaker and myomerger.[12]

A striated muscle fiber contains myofibrils consisting of long protein chains of myofilaments. There are three types of myofilaments: thin, thick, and elastic that work together to produce a muscle contraction.[13] The thin myofilaments are filaments of mostly actin and the thick filaments are of mostly myosin and they slide over each other to shorten the fiber length in a muscle contraction. The third type of myofilament is an elastic filament composed of titin a very large protein.

In striations of muscle bands, myosin forms the dark filaments that make up the A band. Thin filaments of actin are the light filaments that make up the I band. The smallest contractile unit in the fiber is called the sarcomere which is a repeating unit within two Z bands. The sarcoplasm also contains glycogen which provides energy to the cell during heightened exercise, and myoglobin, the red pigment that stores oxygen until needed for muscular activity.[13]

The sarcoplasmic reticulum, a specialized type of smooth endoplasmic reticulum, forms a network around each myofibril of the muscle fiber. This network is composed of groupings of two dilated end-sacs called terminal cisternae, and a single T-tubule (transverse tubule), which bores through the cell and emerge on the other side; together these three components form the triads that exist within the network of the sarcoplasmic reticulum, in which each T-tubule has two terminal cisternae on each side of it. The sarcoplasmic reticulum serves as reservoir for calcium ions, so when an action potential spreads over the T-tubule, it signals the sarcoplasmic reticulum to release calcium ions from the gated membrane channels to stimulate a muscle contraction.[13][14]

In skeletal muscle, at the end of each muscle fiber, the outer layer of the sarcolemma combines with tendon fibers at the myotendinous junction.[15][16] Within the muscle fiber pressed against the sarcolemma are multiple flattened nuclei; embryologically, this multinucleate condition results from multiple myoblasts fusing to produce each muscle fiber, where each myoblast contributes one nucleus.[13]

Cardiac muscle cellsEdit

The cell membrane of a cardiac muscle cell has several specialized regions, which may include the intercalated disc, and transverse tubules. The cell membrane is covered by a lamina coat which is approximately 50 nm wide. The laminar coat is separable into two layers; the lamina densa and lamina lucida. In between these two layers can be several different types of ions, including calcium.[17]

Cardiac muscle like skeletal muscle is also striated and the cells contain myofibrils, myofilaments, and sarcomeres as the skeletal muscle cell. The cell membrane is anchored to the cell's cytoskeleton by anchor fibers that are approximately 10 nm wide. These are generally located at the Z lines so that they form grooves and transverse tubules emanate. In cardiac myocytes this forms a scalloped surface.[17]

The cytoskeleton is what the rest of the cell builds off of and has two primary purposes; the first is to stabilize the topography of the intracellular components and the second is to help control the size and shape of the cell. While the first function is important for biochemical processes, the latter is crucial in defining the surface to volume ratio of the cell. This heavily influences the potential electrical properties of excitable cells. Additionally deviation from the standard shape and size of the cell can have negative prognostic impact.[17]

Smooth muscle cellsEdit

Smooth muscle cells are so-called because they have no striations. They are found in the walls of hollow organs, including the stomach, intestines, urinary bladder and uterus, in the walls of blood vessels, and in the tracts of the respiratory, urinary, and reproductive systems. In the eyes, the ciliary muscles dilate and contract the iris and alter the shape of the lens. In the skin, smooth muscle cells such as those of the arrector pili cause hair to stand erect in response to cold temperature or fear.[18]

Smooth muscle cells are spindle-shaped with wide middles, and tapering ends. They have a single nucleus and range from 30 to 200 micrometres in length. This is thousands of times shorter than skeletal muscle fibers. The diameter of their cells is also much smaller which removes the the need for T-tubules found in striated muscle cells. Smooth muscle cells lack sarcomeres and myofibrils but they do contain large amounts of the contractile proteins actin and myosin. Actin filaments are anchored by dense bodies (similar to the Z discs in sarcomeres) to the sarcolemma.[18]


Portion of a myofibril, showing sarcomere structure:
S = Sarcomere, unit of muscle function
A = A-band, region of myosin
I = I-band, region of just actin
H = H-zone, region of just myosin
Z = Z-line, sarcomere boundary
M = M-line, sarcomere center

Each muscle fiber contains myofibrils, which are very long chains of sarcomeres, the contractile units of the cell. A cell from the biceps muscle may contain 80,000 sarcomeres.[19] The myofibrils of smooth muscle cells are not arranged into sarcomeres. The sarcomeres are composed of thin and thick filaments. Thin filaments are made of actin and attach at Z lines which help them line up correctly with each other.[20] Troponins are found at intervals along the thin filaments. Thick filaments are made of the elongated protein myosin.[21] The sarcomere does not contain organelles or a nucleus. Sarcomeres are marked by Z lines which show the beginning and the end of a sarcomere. Individual muscle cells are surrounded by endomysium.


A myoblast is an embryonic precursor cell that differentiates to give rise to the different muscle cell types.[22] Differentiation is regulated by myogenic regulatory factors, including MyoD, Myf5, myogenin, and MRF4.[23] GATA4 and GATA6 also play a role in myocyte differentiation.[24]

Skeletal muscle fibers are made when myoblasts fuse together; muscle fibers therefore are cells with multiple nuclei, known as myonuclei, with each cell nucleus originating from a single myoblast. The fusion of myoblasts is specific to skeletal muscle (e.g., biceps brachii) and not cardiac muscle or smooth muscle.

Myoblasts in skeletal muscle that do not form muscle fibers dedifferentiate back into myosatellite cells. These satellite cells remain adjacent to a skeletal muscle fiber, situated between the sarcolemma and the basement membrane[25] of the endomysium (the connective tissue investment that divides the muscle fascicles into individual fibers). To re-activate myogenesis, the satellite cells must be stimulated to differentiate into new fibers.

Myoblasts and their derivatives, including satellite cells, can now be generated in vitro through directed differentiation of pluripotent stem cells.[26]

Kindlin-2 plays a role in developmental elongation during myogenesis.[27]


Muscle contractionEdit

When contracting, thin and thick filaments slide with respect to each other by using adenosine triphosphate. This pulls the Z discs closer together in a process called sliding filament mechanism. The contraction of all the sarcomeres results in the contraction of the whole muscle fiber. This contraction of the myocyte is triggered by the action potential over the cell membrane of the myocyte. The action potential uses transverse tubules to get from the surface to the interior of the myocyte, which is continuous within the cell membrane. Sarcoplasmic reticula are membranous bags that transverse tubules touch but remain separate from. These wrap themselves around each sarcomere and are filled with Ca2+.[21]

Excitation of a myocyte causes depolarization at its synapses, the neuromuscular junctions, which triggers action potential. With a singular neuromuscular junction, each muscle fiber receives input from just one somatic efferent neuron. Action potential in a somatic efferent neuron causes the release of the neurotransmitter acetylcholine.[28]

When the acetylcholine is released it diffuses across the synapse and binds to a receptor on the sarcolemma, a term unique to muscle cells that refers to the cell membrane. This initiates an impulse that travels across the sarcolemma.[29]

When the action potential reaches the sarcoplasmic reticulum it triggers the release of Ca2+ from the Ca2+ channels. The Ca2+ flows from the sarcoplasmic reticulum into the sarcomere with both of its filaments. This causes the filaments to start sliding and the sarcomeres to become shorter. This requires a large amount of ATP, as it is used in both the attachment and release of every myosin head. Very quickly Ca2+ is actively transported back into the sarcoplasmic reticulum, which blocks the interaction between the thin and thick filament. This in turn causes the muscle cell to relax.[29]

Kinds of contractionEdit

There are four main different types of muscle contraction: twitch, treppe, tetanus and isometric/isotonic. Twitch contraction is the process in which a single stimulus signals for a single contraction. In twitch contraction the length of the contraction may vary depending on the size of the muscle cell. During treppe (or summation) contraction muscles do not start at maximum efficiency; instead they achieve increased strength of contraction due to repeated stimuli. Tetanus involves a sustained contraction of muscles due to a series of rapid stimuli, which can continue until the muscles fatigue. Isometric contractions are skeletal muscle contractions that do not cause movement of the muscle. However, isotonic contractions are skeletal muscle contractions that do cause movement.[29]

Specialized cardiomyocytes located in the sinoatrial node are responsible for generating the electrical impulses that control the heart rate. These electrical impulses coordinate contraction throughout the remaining heart muscle via the electrical conduction system of the heart. Sinoatrial node activity is modulated, in turn, by nerve fibres of both the sympathetic and parasympathetic nervous systems. These systems act to increase and decrease, respectively, the rate of production of electrical impulses by the sinoatrial node.

Fiber typingEdit

ATPase staining of a muscle cross section. Type II fibers are dark, due to the alkaline pH of the preparation. In this example, the size of the type II fibers is considerably less than the type I fibers due to denervation atrophy.

There are numerous methods employed for fiber-typing, and confusion between the methods is common among non-experts. Two commonly confused methods are histochemical staining for myosin ATPase activity and immunohistochemical staining for myosin heavy chain (MHC) type. Myosin ATPase activity is commonly—and correctly—referred to as simply "fiber type", and results from the direct assaying of ATPase activity under various conditions (e.g. pH).[30] Myosin heavy chain staining is most accurately referred to as "MHC fiber type", e.g. "MHC IIa fibers", and results from determination of different MHC isoforms.[30] These methods are closely related physiologically, as the MHC type is the primary determinant of ATPase activity. However, neither of these typing methods is directly metabolic in nature; they do not directly address oxidative or glycolytic capacity of the fiber.

When "type I" or "type II" fibers are referred to generically, this most accurately refers to the sum of numerical fiber types (I vs. II) as assessed by myosin ATPase activity staining (e.g. "type II" fibers refers to type IIA + type IIAX + type IIXA ... etc.).

Below is a table showing the relationship between these two methods, limited to fiber types found in humans. Subtype capitalization is used in fiber typing vs. MHC typing, and some ATPase types actually contain multiple MHC types. Also, a subtype B or b is not expressed in humans by either method.[31] Early researchers believed humans to express a MHC IIb, which led to the ATPase classification of IIB. However, later research showed that the human MHC IIb was in fact IIx,[31] indicating that the IIB is better named IIX. IIb is expressed in other mammals, so is still accurately seen (along with IIB) in the literature. Non human fiber types include true IIb fibers, IIc, IId, etc.

ATPase Vs. MHC fiber types[30][32][33]
ATPase type MHC heavy chain(s)
Type I MHC Iβ
Type IC MHC Iβ > MHC IIa

Further fiber typing methods are less formally delineated, and exist on more of a spectrum. They tend to be focused more on metabolic and functional capacities (i.e., oxidative vs. glycolytic, fast vs. slow contraction time). As noted above, fiber typing by ATPase or MHC does not directly measure or dictate these parameters. However, many of the various methods are mechanistically linked, while others are correlated in vivo.[34][35] For instance, ATPase fiber type is related to contraction speed, because high ATPase activity allows faster crossbridge cycling.[30] While ATPase activity is only one component of contraction speed, Type I fibers are "slow", in part, because they have low speeds of ATPase activity in comparison to Type II fibers. However, measuring contraction speed is not the same as ATPase fiber typing.

Because of these types of relationships, Type I and Type II fibers have relatively distinct metabolic, contractile, and motor-unit properties. The table below differentiates these types of properties. These types of properties—while they are partly dependent on the properties of individual fibers—tend to be relevant and measured at the level of the motor unit, rather than individual fiber.[30]

Various Properties of Different Fiber Types[30]
Properties Type I fibers Type IIA fibers Type IIX fibers
Motor Unit Type Slow Oxidative (SO) Fast Oxidative/Glycolytic (FOG) Fast Glycolytic (FG)
Twitch speed Slow Fast Fast
Twitch force Small Medium Large
Resistance to fatigue High High Low
Glycogen content Low High High
Capillary supply Rich Rich Poor
Capillary density High Intermediate Low
Myoglobin High High Low
Red color Dark Dark Pale
Mitochondrial density High High Low
Oxidative enzyme capacity High Intermediate-high Low
Z-line width Intermediate Wide Narrow
Alkaline ATPase activity Low High High
Acidic ATPase activity High Medium-high Low

Fiber colorEdit

Traditionally, fibers were categorized depending on their varying color, which is a reflection of myoglobin content. Type I fibers appear red due to the high levels of myoglobin. Red muscle fibers tend to have more mitochondria and greater local capillary density. These fibers are more suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP (adenosine triphosphate). Less oxidative Type II fibers are white due to relatively low myoglobin and a reliance on glycolytic enzymes.

Twitch speedEdit

Fibers can also be classified on their twitch capabilities, into fast and slow twitch. These traits largely, but not completely, overlap the classifications based on color, ATPase, or MHC.

Some authors define a fast twitch fiber as one in which the myosin can split ATP very quickly. These mainly include the ATPase type II and MHC type II fibers. However, fast twitch fibers also demonstrate a higher capability for electrochemical transmission of action potentials and a rapid level of calcium release and uptake by the sarcoplasmic reticulum. The fast twitch fibers rely on a well-developed, anaerobic, short term, glycolytic system for energy transfer and can contract and develop tension at 2–3 times the rate of slow twitch fibers. Fast twitch muscles are much better at generating short bursts of strength or speed than slow muscles, and so fatigue more quickly.[36]

The slow twitch fibers generate energy for ATP re-synthesis by means of a long term system of aerobic energy transfer. These mainly include the ATPase type I and MHC type I fibers. They tend to have a low activity level of ATPase, a slower speed of contraction with a less well developed glycolytic capacity. They contain high mitochondrial volumes, and the high levels of myoglobin that give them a red pigmentation. They have been demonstrated to have high concentrations of mitochondrial enzymes, thus they are fatigue resistant. Slow twitch muscles fire more slowly than fast twitch fibers, but are able to contract for a longer time before fatiguing.[36]

Type distribution

Individual muscles tend to be a mixture of various fiber types, but their proportions vary depending on the actions of that muscle and the species. For instance, in humans, the quadriceps muscles contain ~52% type I fibers, while the soleus is ~80% type I.[37] The orbicularis oculi muscle of the eye is only ~15% type I.[37] Motor units within the muscle, however, have minimal variation between the fibers of that unit. It is this fact that makes the size principal of motor unit recruitment viable.

The total number of skeletal muscle fibers has traditionally been thought not to change. It is believed there are no sex or age differences in fiber distribution; however, proportions of fiber types vary considerably from muscle to muscle and person to person.

Sedentary men and women (as well as young children) have 45% type II and 55% type I fibers.[citation needed] People at the higher end of any sport tend to demonstrate patterns of fiber distribution e.g. endurance athletes show a higher level of type I fibers. Sprint athletes, on the other hand, require large numbers of type IIX fibers. Middle-distance event athletes show approximately equal distribution of the two types. This is also often the case for power athletes such as throwers and jumpers. It has been suggested that various types of exercise can induce changes in the fibers of a skeletal muscle.[38]

It is thought that if you perform endurance type events for a sustained period of time, some of the type IIX fibers transform into type IIA fibers. However, there is no consensus on the subject. It may well be that the type IIX fibers show enhancements of the oxidative capacity after high intensity endurance training which brings them to a level at which they are able to perform oxidative metabolism as effectively as slow twitch fibers of untrained subjects. This would be brought about by an increase in mitochondrial size and number and the associated related changes, not a change in fiber type.


The evolutionary origin of muscle cells in animals is highly debated. One view is that muscle cells evolved once and thus all muscles cells have a single common ancestor. Another view is that muscles cells evolved more than once and any morphological or structural similarities are due to convergent evolution and genes that predate the evolution of muscle and even the mesoderm—the germ layer that gives rise to vertebrate muscle cells.

Schmid and Seipel argue that the origin of muscle cells is a monophyletic trait that occurred concurrently with the development of the digestive and nervous systems of all animals and that this origin can be traced to a single metazoan ancestor in which muscle cells are present. They argue that molecular and morphological similarities between the muscles cells in cnidaria and ctenophora are similar enough to those of bilaterians that there would be one ancestor in metazoans from which muscle cells derive. In this case, Schmid and Seipel argue that the last common ancestor of bilateria, ctenophora, and cnidaria was a triploblast or an organism with three germ layers and that diploblasty, meaning an organism with two germ layers, evolved secondarily due to their observation of the lack of mesoderm or muscle found in most cnidarians and ctenophores. By comparing the morphology of cnidarians and ctenophores to bilaterians, Schmid and Seipel were able to conclude that there were myoblast-like structures in the tentacles and gut of some species of cnidarians and in the tentacles of ctenophores. Since this is a structure unique to muscle cells, these scientists determined based on the data collected by their peers that this is a marker for striated muscles similar to that observed in bilaterians. The authors also remark that the muscle cells found in cnidarians and ctenophores are often contests due to the origin of these muscle cells being the ectoderm rather than the mesoderm or mesendoderm. The origin of true muscles cells is argued by others to be the endoderm portion of the mesoderm and the endoderm. However, Schmid and Seipel counter this skepticism about whether or not the muscle cells found in ctenophores and cnidarians are true muscle cells by considering that cnidarians develop through a medusa stage and polyp stage. They observe that in the hydrozoan medusa stage there is a layer of cells that separate from the distal side of the ectoderm to form the striated muscle cells in a way that seems similar to that of the mesoderm and call this third separated layer of cells the ectocodon. They also argue that not all muscle cells are derived from the mesendoderm in bilaterians with key examples being that in both the eye muscles of vertebrates and the muscles of spiralians these cells derive from the ectodermal mesoderm rather than the endodermal mesoderm. Furthermore, Schmid and Seipel argue that since myogenesis does occur in cnidarians with the help of molecular regulatory elements found in the specification of muscles cells in bilaterians that there is evidence for a single origin for striated muscle.[39]

In contrast to this argument for a single origin of muscle cells, Steinmetz et al. argue that molecular markers such as the myosin II protein used to determine this single origin of striated muscle actually predate the formation of muscle cells. This author uses an example of the contractile elements present in the porifera or sponges that do truly lack this striated muscle containing this protein. Furthermore, Steinmetz et al. present evidence for a polyphyletic origin of striated muscle cell development through their analysis of morphological and molecular markers that are present in bilaterians and absent in cnidarians, ctenophores, and bilaterians. Steimetz et al. showed that the traditional morphological and regulatory markers such as actin, the ability to couple myosin side chains phosphorylation to higher concentrations of the positive concentrations of calcium, and other MyHC elements are present in all metazoans not just the organisms that have been shown to have muscle cells. Thus, the usage of any of these structural or regulatory elements in determining whether or not the muscle cells of the cnidarians and ctenophores are similar enough to the muscle cells of the bilaterians to confirm a single lineage is questionable according to Steinmetz et al. Furthermore, Steinmetz et al. explain that the orthologues of the MyHc genes that have been used to hypothesize the origin of striated muscle occurred through a gene duplication event that predates the first true muscle cells (meaning striated muscle), and they show that the MyHc genes are present in the sponges that have contractile elements but no true muscle cells. Furthermore, Steinmetz et all showed that the localization of this duplicated set of genes that serve both the function of facilitating the formation of striated muscle genes and cell regulation and movement genes were already separated into striated myhc and non-muscle myhc. This separation of the duplicated set of genes is shown through the localization of the striated myhc to the contractile vacuole in sponges while the non-muscle myhc was more diffusely expressed during developmental cell shape and change. Steinmetz et al. found a similar pattern of localization in cnidarians with except with the cnidarian N. vectensis having this striated muscle marker present in the smooth muscle of the digestive track. Thus, Steinmetz et al. argue that the pleisiomorphic trait of the separated orthologues of myhc cannot be used to determine the monophylogeny of muscle, and additionally argue that the presence of a striated muscle marker in the smooth muscle of this cnidarian shows a fundamentally different mechanism of muscle cell development and structure in cnidarians.[40]

Steinmetz et al. continue to argue for multiple origins of striated muscle in the metazoans by explaining that a key set of genes used to form the troponin complex for muscle regulation and formation in bilaterians is missing from the cnidarians and ctenophores, and of 47 structural and regulatory proteins observed, Steinmetz et al. were not able to find even on unique striated muscle cell protein that was expressed in both cnidarians and bilaterians. Furthermore, the Z-disc seemed to have evolved differently even within bilaterians and there is a great deal diversity of proteins developed even between this clade, showing a large degree of radiation for muscle cells. Through this divergence of the Z-disc, Steimetz et al. argue that there are only four common protein components that were present in all bilaterians muscle ancestors and that of these for necessary Z-disc components only an actin protein that they have already argued is an uninformative marker through its pleisiomorphic state is present in cnidarians. Through further molecular marker testing, Steinmetz et al. observe that non-bilaterians lack many regulatory and structural components necessary for bilaterians muscle formation and do not find any unique set of proteins to both bilaterians and cnidarians and ctenophores that are not present in earlier, more primitive animals such as the sponges and amoebozoans. Through this analysis the authors conclude that due to the lack of elements that bilaterians muscles are dependent on for structure and usage, nonbilaterian muscles must be of a different origin with a different set regulatory and structural proteins.[40]

In another take on the argument, Andrikou and Arnone use the newly available data on gene regulatory networks to look at how the hierarchy of genes and morphogens and other mechanism of tissue specification diverge and are similar among early deuterostomes and protostomes. By understanding not only what genes are present in all bilaterians but also the time and place of deployment of these genes, Andrikou and Arnone discuss a deeper understanding of the evolution of myogenesis.[41]

In their paper Andrikou and Arnone argue that to truly understand the evolution of muscle cells the function of transcriptional regulators must be understood in the context of other external and internal interactions. Through their analysis, Andrikou and Arnone found that there were conserved orthologues of the gene regulatory network in both invertebrate bilaterians and in cnidarians. They argue that having this common, general regulatory circuit allowed for a high degree of divergence from a single well functioning network. Andrikou and Arnone found that the orthologues of genes found in vertebrates had been changed through different types of structural mutations in the invertebrate deuterostomes and protostomes, and they argue that these structural changes in the genes allowed for a large divergence of muscle function and muscle formation in these species. Andrikou and Arnone were able to recognize not only any difference due to mutation in the genes found in vertebrates and invertebrates but also the integration of species specific genes that could also cause divergence from the original gene regulatory network function. Thus, although a common muscle patterning system has been determined, they argue that this could be due to a more ancestral gene regulatory network being coopted several times across lineages with additional genes and mutations causing very divergent development of muscles. Thus it seems that myogenic patterning framework may be an ancestral trait. However, Andrikou and Arnone explain that the basic muscle patterning structure must also be considered in combination with the cis regulatory elements present at different times during development. In contrast with the high level of gene family apparatuses structure, Andrikou and Arnone found that the cis regulatory elements were not well conserved both in time and place in the network which could show a large degree of divergence in the formation of muscle cells. Through this analysis, it seems that the myogenic GRN is an ancestral GRN with actual changes in myogenic function and structure possibly being linked to later coopts of genes at different times and places.[41]

Evolutionarily, specialized forms of skeletal and cardiac muscles predated the divergence of the vertebrate/arthropod evolutionary line.[42] This indicates that these types of muscle developed in a common ancestor sometime before 700 million years ago (mya). Vertebrate smooth muscle was found to have evolved independently from the skeletal and cardiac muscle types.

See alsoEdit


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