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{{Redirect|Myofiber|protein structures inside cells|Myofibril}}
[[File:Synapse diag3.png|thumb|Overall structure of a muscle cell and neuromuscular junction:<br />1. [[Axon]]<br />2. [[Neuromuscular junction]]<br />3. Muscle fiber (Myocyte)<br />4. [[Myofibril]] ]]
A '''myocyte''' (also known as a '''muscle cell''' or '''muscle fiber''')<ref>{{DorlandsDict|five/000069635|myocyte}}</ref> is the type of [[Cell (biology)|cell]] found in [[muscle tissue]]. Myocytes are long, tubular cells that develop from '''myoblasts''' to form [[muscle]]s in a process known as [[myogenesis]].<ref>{{MeshName|Myocytes}}</ref> There are various specialized forms of myocytes: [[cardiac muscle|cardiac]], [[skeletal muscle|skeletal]], and [[smooth muscle]] cells, with various properties. Cardiac myocytes are responsible for generating the electrical impulses that control the [[heart]] rate, and are centronucleated.
Skeletal muscle fibers help support and move the body and tend to have peripheral nuclei. Smooth muscle fibers control involuntary movements such as the peristalsis contractions in the stomach.<ref name=scott>{{cite journal|last1=Scott|first1=W|last2=Stevens|first2=J|last3=Binder-Macleod|first3=SA|title=Human skeletal muscle fiber type classifications.|journal=Physical Therapy|date=2001|volume=81|issue=11|pages=1810–1816|pmid=11694174|url=http://ptjournal.apta.org/content/81/11/1810.full}}</ref><ref>{{cite web|url=http://www.researchgate.net/post/Does_anyone_know_why_skeletal_muscle_fibers_have_peripheral_nuclei_but_the_cardiomyocytes_not_What_are_the_functional_advantages |title=Does anyone know why skeletal muscle fibers have peripheral nuclei, but the cardiomyocytes not? What are the functional advantages? |publisher=ResearchGate}}</ref>
 
==Terminology==
The unusual microstructure of muscle cells has led [[cell biology|cell biologists]] to create specialized terminology. However, each term specific to muscle cells has a counterpart that is used in the terminology applied to other types of cells:
 
{| class="wikitable"
! Muscle cell''' !! Other organismal cells'''
|-
| sarcoplasm || [[cytoplasm]]
|-
| sarcoplasmic reticulum || [[smooth endoplasmic reticulum]] (SER)
|-
| sarcosome || [[mitochondrion]]
|-
| sarcolemma || [[cell membrane]] or [[plasma membrane]]
|}
 
Every muscle is surrounded by [[fascia]]. Fascia is composed of three layers of connective tissue, the [[epimysium]], [[perimysium]], and [[endomysium]]. Each muscle fiber is surrounded by a layer of [[connective tissue]] called the endomysium.<ref name=saladin>{{cite book|last1=Saladin|first1=K|title=Anatomy & Physiology: The Unity of Form and Function|date=2012|publisher=McGraw-Hill|location=New York|isbn=978-0-07-337825-1|pages=403–405|edition=6th}}</ref>
 
The [[sarcoplasm]] is the [[cytoplasm]] of a muscle fiber. Most of the sarcoplasm is filled with myofibrils, which are long protein cords composed of myofilaments. The sarcoplasm is also composed of [[glycogen]], a polysaccharide of glucose monomers, which provides energy to the cell with heightened exercise, and [[myoglobin]], the red pigment that stores oxygen until needed for muscular activity.<ref name=saladin />
 
There are three types of myofilaments:<ref name=saladin />
 
*Thick filaments, composed of protein molecules called [[myosin#myosin II|myosin]]. In striations of muscle bands, these are the dark filaments that make up the A band.
*Thin filaments are composed of protein molecules called [[actin]]. In striations of muscle bands, these are the light filaments that make up the I band.
*Elastic filaments are composed of [[titin]], a large springy protein; these filaments anchor the thick filaments to the Z disc.
 
Together, these myofilaments work to produce a muscle contraction.
 
The [[sarcoplasmic reticulum]], a specialized type of [[endoplasmic reticulum#smooth endoplasmic reticulum|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 transverse tubule, or [[T 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.<ref name=saladin /><ref>{{cite journal|last1=Sugi|first1=Haruo|last2=Abe|first2=T|last3=Kobayashi|first3=T|last4=Chaen|first4=S|last5=Ohnuki|first5=Y|last6=Saeki|first6=Y|last7=Sugiura|first7=S|last8=Guerrero-Hernandez|first8=Agustin|title=Enhancement of force generated by individual myosin heads in skinned rabbit psoas muscle fibers at low ionic strength.|journal=PloS one|date=2013|volume=8|issue=5|pages=e63658|doi=10.1371/journal.pone.0063658|pmid=23691080|accessdate=14 February 2015}}</ref>
 
The [[sarcolemma]] is the [[plasma membrane]] of a muscle fiber and is designed to receive and conduct stimuli. At the end of each muscle fiber, the outer layer of the sarcolemma combines with tendon fibers.<ref name=Bentzinger2012>{{cite journal|last1=Bentzinger|first1=CF|last2=Wang|first2=YX|last3=Rudnicki|first3=MA|title=Building muscle: molecular regulation of myogenesis.|journal=Cold Spring Harbor perspectives in biology|date=1 February 2012|volume=4|issue=2|doi=10.1101/cshperspect.a008342|pmid=22300977}}</ref> Within the muscle fiber pressed against the sarcolemma are multiple flattened [[cell nucleus|nuclei]]; this multinuclear condition results from multiple myoblasts fusing to produce each muscle fiber, where each myoblast contributes one nucleus.<ref name=saladin />
 
== Structure of myocyte ==
The [[plasma membrane]], plasmalemma, of myocyte has several specialized regions, which may include the [[intercalated disk]] and the [[transverse tubular system]]. The plasma membrane is covered by a lamina coat which is approximately 50&nbsp;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]].<ref name=ferrari>{{cite web|last1=Ferrari|first1=Roberto|title=Healthy versus sick myocytes: metabolism, structure and function|url=http://eurheartjsupp.oxfordjournals.org/content/ehjsupp/4/suppl_G/G1.full.pdf|website=http://oxfordjournals.org/en/|publisher=Oxford University Press|accessdate=12 February 2015}}</ref>
 
The plasma membrane is anchored to the cell's [[cytoskeleton]] by anchor fibers that are approximately 10&nbsp;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.<ref name=ferrari />
 
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.<ref name=ferrari />
 
== Movement of myocyte ==
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 [[plasma 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 plasma 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+.<ref name="courses.washington.edu">{{cite web|title=Structure and Function of Skeletal Muscles|url=http://courses.washington.edu/conj/motor/musclereview.htm|website=http://courses.washington.edu/|accessdate=13 February 2015}}</ref>
 
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]].<ref>{{cite web|title=Muscle Fiber Excitation|url=http://courses.washington.edu/conj/bess/muscle/musclexcitation.html|website=http://courses.washington.edu/|publisher=University of Washington|accessdate=11 February 2015}}</ref>
 
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 [[plasma membrane]]. This initiates an impulse that travels across the sarcolemma.<ref name=ziser>{{cite web|last1=Ziser|first1=Stephen|title=Muscle Cell Anatomy & Function|url=http://www.austincc.edu/sziser/Biol%202404/2404LecNotes/2404LNExII/Muscle%20Physiology.pdf|website=http://www.austincc.edu/|accessdate=12 February 2015}}</ref>
 
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.<ref name=ziser />
 
== Kinds of myocyte contraction ==
 
There are four main different types of muscle contraction: twitch, treppe, tetanus and isometric/isotonic. Twitch contraction is the process previously described, 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 summattion) 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 are skeletal muscle contractions that do not cause movement of the muscle. However isotonic are skeletal muscles contractions that do cause movement.<ref name=ziser />
 
== Myocyte growth ==
Myocyte grow when exercised and shrink when not in use. This is due to the fact that exercise stimulates the increase in [[myofibrils]] which increase the overall size of muscle cells. Well exercised muscles can not only add more size but can also develop more [[mitochondria]], [[myoglobin]], [[glycogen]] and a higher density of [[capillaries]]. However muscle cells cannot not divide to produce new cells, and as a result we have fewer muscle cells as an adult than a newborn.<ref name=ziser />
 
==Myofibrils==
{{main|Myofibril|Sarcomere}}
[[File:Sarcomere.jpg|thumb|Portion of a [[myofibril]], showing [[sarcomere]] structure:<br/>'''S''' = Sarcomere, unit of muscle function<br/>'''A''' = A-band, region of myosin<br/>'''I''' = I-band, region of actin<br/>'''H''' = H-zone, region of just myosin<br/>'''Z''' = Z-line, sarcomere boundary<br/>'''M''' = M-line, sarcomere center]]
 
Each myocyte contains [[myofibril]]s, which are very long chains of [[sarcomere]]s, the contractile units of the cell. A cell from the [[biceps brachii muscle]] may contain 100,000 sarcomeres.<ref>Assuming that the length of biceps is 20 cm and the length of sarcomere is 2 micrometer, there are 100,000 sarcomeres along the length of biceps.</ref>{{Verify source|date=February 2015}} The myofibrils of [[smooth muscle]] cells are not arranged into sarcomeres. The sarcomeres are composed of thin and thick filaments. Thin filaments is made of the protein actin and attach at Z lines which help them line up correctly with each other.<ref>{{cite web|last1=Tamarkin|first1=Dawn|title=Myofibril Composition|url=http://faculty.stcc.edu/AandP/AP/AP1pages/Units5to9/Unit7/myofibri.htm|website=http://www.stcc.edu/faculty/webpages.asp|publisher=STCC Foundation Press|accessdate=12 February 2015}}</ref> Troponins are found at intervals along the thin filaments. Thick filaments are made of the elongated protein myosin.<ref name="courses.washington.edu"/> The sarcomere does not contain organelles or a nucleus. Sacromere are marked by Z lines which show the beginning and the end of a sacromere. Individual myocytes are surrounded by [[endomysium]].
 
Myocytes are bound together by [[perimysium]] into bundles called [[Muscle fascicle|fascicles]]; the bundles are then grouped together to form [[muscle tissue]], which is enclosed in a sheath of [[epimysium]]. The perimysium contains blood vessels and nerves which provide for the muscle fibers. [[Muscle spindle]]s are distributed throughout the muscles and provide sensory feedback information to the [[central nervous system]]. Myosin is a shaped like long shaft with a rounded end pointed out towards the surface. This structure forms the cross bridge that connects with the thin filaments.<ref name="courses.washington.edu"/>
 
==Functional control==
[[Kindlin-2]] plays a role in elongation.<ref name="pmid18611274">{{cite journal |author=Dowling JJ, Vreede AP, Kim S, Golden J, Feldman EL |title=Kindlin-2 is required for myocyte elongation and is essential for myogenesis |journal=BMC Cell Biol. |volume=9|pages=36 |year=2008 |pmid=18611274 |pmc=2478659 |doi=10.1186/1471-2121-9-36 |url=http://www.biomedcentral.com/1471-2121/9/36}}</ref>
 
[[GATA4]] and [[GATA6]] play a role in differentiation.<ref name="pmid18400219">{{cite journal |author=Zhao R, Watt AJ, Battle MA, Li J, Bondow BJ, Duncan SA |title=Loss of both GATA4 and GATA6 blocks cardiac myocyte differentiation and results in acardia in mice |journal=Dev. Biol. |volume=317 |issue=2 |pages=614–9 |date=May 2008 |pmid=18400219 |pmc=2423416 |doi=10.1016/j.ydbio.2008.03.013 |url=http://linkinghub.elsevier.com/retrieve/pii/S0012-1606(08)00219-4}}</ref>
 
==Myoblasts==
A '''myoblast''' is a type of embryonic [[progenitor cell]] that [[Cellular differentiation|differentiates]] to give rise to muscle cells.<ref>page 395, Biology, Fifth Edition, Campbell, 1999</ref> Differentiation is regulated by [[myogenic regulatory factors]], including [[MyoD]], [[Myf5]], [[myogenin]], and [[MRF4]].<ref>{{cite journal |author=Perry R, Rudnick M |title=Molecular mechanisms regulating myogenic determination and differentiation |journal=Front Biosci |volume=5 |issue= |pages=D750–67 |year= 2000|pmid=10966875 |doi=10.2741/Perry}}</ref>
 
[[Skeletal striated muscle|Skeletal muscle fibers]] are made when myoblasts fuse together; muscle fibers therefore have multiple nuclei (each 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 that do not form muscle fibers [[Cellular differentiation|dedifferentiate]] back into [[satellite cell]]s. These satellite cells remain adjacent to a muscle fiber, situated between the sarcolemma and the [[endomysium]] (the connective tissue investment that divides the muscle fascicles into individual fibers). To re-activate myogenesis, the satellite cells must be stimulated so they differentiate into new fibers.
 
Myoblasts and their derivatives, including satellite cells, can now be generated in vitro through [[directed differentiation]] of [[Induced pluripotent stem cells|pluripotent stem cells]]<ref name="pmid26237517">{{cite journal |vauthors = Chal J, Oginuma M, Al Tanoury Z, Gobert B, Sumara O, Hick A, Bousson F, Zidouni Y, Mursch C, Moncuquet P, Tassy O, Vincent S, Miyanari A, Bera A, Garnier JM, Guevara G, Hestin M, Kennedy L, Hayashi S, Drayton B, Cherrier T, Gayraud-Morel B, Gussoni E, Relaix F, Tajbakhsh S, Pourquié O|title = Differentiation of pluripotent stem cells to muscle fiber to model Duchenne muscular dystrophy |journal = [[Nat. Biotechnol.|Nature Biotechnology]]|date = August 2015|pmid = 26237517|doi = 10.1038/nbt.3297}} {{Closed access}}</ref>
 
==See also==
* [[Sinoatrial node]]
 
==References==
{{Reflist|30em}}
 
==External links==
* [http://www.ivy-rose.co.uk/HumanBody/Muscles/Muscle_Cell.php Structure of a Muscle Cell]
 
{{Muscle tissue}}
{{Human cell types}}
 
[[Category:Contractile cells]]
[[Category:Animal cells]]
[[Category:Non-terminally differentiated (blast) cells]]
[[Category:Myoblasts|*]]
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