Editing Muscle cell (section)

==Structure==
The unusual [[Histology|microscopic anatomy]] of a muscle cell gave rise to its 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]].<ref name="Saladin2011A">{{cite book |last1=Saladin |first1=Kenneth S. |title=Human anatomy |date=2011 |publisher=McGraw-Hill |location=New York |isbn=9780071222075 |pages=244–246 |edition=3rd}}</ref> The sarcolemma receives and conducts stimuli.

===Skeletal muscle cells===
{{Main|Skeletal muscle fibers}}
[[File:Blausen 0801 SkeletalMuscle.png|thumb|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.<ref name="SEER">{{cite web |title=Structure of Skeletal Muscle {{!}} SEER Training |url=https://training.seer.cancer.gov/anatomy/muscular/structure.html |website=training.seer.cancer.gov}}</ref> Broadly there are two types of muscle fiber performing in [[muscle contraction]], either as slow twitch ([[Type I muscle fiber|type I]]) or fast twitch ([[Type II muscle fiber|type II]]).

A single muscle such as the [[biceps brachii]] in a young adult human male contains around 253,000 muscle fibers.<ref name="Klein">{{cite journal |last1=Klein |first1=CS |last2=Marsh |first2=GD |last3=Petrella |first3=RJ |last4=Rice |first4=CL |title=Muscle fiber number in the biceps brachii muscle of young and old men. |journal=Muscle & Nerve |date=July 2003 |volume=28 |issue=1 |pages=62–8 |doi=10.1002/mus.10386 |pmid=12811774|s2cid=20508198 }}</ref> Skeletal muscle fibers are the only muscle cells that are [[multinucleate]]d with the [[Cell nucleus|nuclei]] usually referred to as [[myonuclei]]. This occurs during [[myogenesis]] with the [[cell fusion|fusion]] of [[myoblast]]s each contributing a nucleus to the newly formed muscle cell or [[myotube]].<ref name="Cho">{{cite journal |last1=Cho |first1=CH |last2=Lee |first2=KJ |last3=Lee |first3=EH |title=With the greatest care, stromal interaction molecule (STIM) proteins verify what skeletal muscle is doing. |journal=BMB Reports |date=August 2018 |volume=51 |issue=8 |pages=378–387 |doi=10.5483/bmbrep.2018.51.8.128 |pmid=29898810|pmc=6130827 }}</ref> Fusion depends on muscle-specific proteins known as [[fusogen]]s called ''myomaker'' and ''myomerger''.<ref name="Prasad">{{cite journal |last1=Prasad |first1=V |last2=Millay |first2=DP |title=Skeletal muscle fibers count on nuclear numbers for growth. |journal=Seminars in Cell & Developmental Biology |date=2021-05-08 |volume=119 |pages=3–10 |doi=10.1016/j.semcdb.2021.04.015 |pmid=33972174|pmc=9070318 |s2cid=234362466 }}</ref>

A striated muscle fiber contains [[myofibril]]s consisting of long protein chains of [[myofilament]]s. There are three types of myofilaments: thin, thick, and elastic that work together to produce a [[muscle contraction]].<ref name=saladin /> The thin myofilaments are filaments of mostly [[actin]] and the thick filaments are of mostly [[myosin]] and they [[sliding filament mechanism|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 [[Striated muscle tissue|striations of muscle bands]], myosin forms the dark filaments that make up the [[Sarcomere#Bands|A band]]. Thin filaments of actin are the light filaments that make up the [[Sarcomere#Bands|I band]]. The smallest contractile unit in the fiber is called the sarcomere which is a repeating unit within two [[Sarcomere#Bands|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.<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 [[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 [[T-tubule]] (transverse tubule), which bores through the cell and emerge on the other side; together these three components form the [[Triad (anatomy)|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 a 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 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|pmc=3655179|bibcode=2013PLoSO...863658S|doi-access=free}}</ref>

In skeletal muscle, at the end of each muscle fiber, the outer layer of the sarcolemma combines with tendon fibers at the [[myotendinous junction]].<ref name="Charvet">{{cite journal |last1=Charvet |first1=B |last2=Ruggiero |first2=F |last3=Le Guellec |first3=D |title=The development of the myotendinous junction. A review. |journal=Muscles, Ligaments and Tendons Journal |date=April 2012 |volume=2 |issue=2 |pages=53–63 |pmid=23738275|pmc=3666507 }}</ref><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|pages=a008342|pmc=3281568}}</ref> Within the muscle fiber pressed against the sarcolemma are multiply flattened [[myonuclei|nuclei]]; embryologically, this [[multinucleate]] condition results from multiple myoblasts fusing to produce each muscle fiber, where each myoblast contributes one nucleus.<ref name=saladin />

===Cardiac muscle cells===
{{Main|Cardiac muscle}}
The cell membrane of a [[cardiac muscle cell]] has several specialized regions, which may include the [[intercalated disc]], and [[transverse tubule]]s. The cell 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=oxfordjournals.org/en|publisher=Oxford University Press|access-date=12 February 2015|url-status=dead|archive-url=https://web.archive.org/web/20150219100248/http://eurheartjsupp.oxfordjournals.org/content/ehjsupp/4/suppl_G/G1.full.pdf|archive-date=19 February 2015}}</ref>

Cardiac muscle like the 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&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 cell]]s. Additionally, deviation from the standard shape and size of the cell can have a negative prognostic impact.<ref name=ferrari />

===Smooth muscle cells===
{{Main|Smooth muscle}}{{Further|Basal electrical rhythm}}
[[Smooth muscle cells]] are so-called because they have neither myofibrils nor sarcomeres and therefore no [[striated muscle tissue|striations]]. They are found in the walls of hollow [[organs]], including the [[stomach]], [[intestines]], [[urinary bladder|bladder]] and [[uterus]], in the walls of [[blood vessel]]s, and in the tracts of the [[Respiratory system|respiratory]], [[Urinary system|urinary]], and [[reproductive system]]s. In the [[eye]]s, the [[ciliary muscle]]s dilate and contract the [[iris (anatomy)|iris]] and alter the shape of the [[lens (anatomy)|lens]]. In the [[skin]], smooth muscle cells such as those of the [[arrector pili]] cause [[hair]] to stand erect in response to [[cold|cold temperature]] or [[fear]].<ref name="open">{{cite web |title=Smooth muscle |date=6 March 2013 |url=https://opentextbc.ca/anatomyandphysiologyopenstax/chapter/smooth-muscle/ |access-date=10 June 2021|last1=Betts |first1=J. Gordon |last2=Young |first2=Kelly A. |last3=Wise |first3=James A. |last4=Johnson |first4=Eddie |last5=Poe |first5=Brandon |last6=Kruse |first6=Dean H. |last7=Korol |first7=Oksana |last8=Johnson |first8=Jody E. |last9=Womble |first9=Mark |last10=Desaix |first10=Peter }}</ref>

Smooth muscle cells are spindle-shaped with wide middles, and tapering ends. They have a single nucleus and range from 30 to 200 [[Micrometre|micrometer]]s in length. This is thousands of times shorter than skeletal muscle fibers. The diameter of their cells is also much smaller which removes the need for [[T-tubule]]s found in striated muscle cells. Although smooth muscle cells lack sarcomeres and myofibrils they do contain large amounts of the contractile proteins actin and myosin. Actin filaments are anchored by [[Smooth muscle#Dense bodies|dense bodies]] (similar to the [[Sarcomere#Bands|Z discs]] in sarcomeres) to the sarcolemma.<ref name="open"/>