Editing Muscle cell (section)

===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 />