Editing Muscular system (section)

=== Skeletal ===
{{Main|Skeletal muscle}}
{{See also|List of muscles of the human body}}

Skeletal muscle, is a type of [[striated muscle]], composed of [[muscle cell]]s, called [[Skeletal muscle#Skeletal muscle cells|muscle fibers]], which are in turn composed of [[myofibrils]]. Myofibrils are composed of [[sarcomere]]s, the basic building blocks of striated muscle tissue. Upon stimulation by an [[action potential]], skeletal muscles perform a coordinated contraction by shortening each sarcomere. The best proposed model for understanding contraction is the [[sliding filament model]] of muscle contraction. Within the sarcomere, [[actin]] and [[myosin]] fibers overlap in a contractile motion towards each other. Myosin filaments have club-shaped [[myosin head]]s that project toward the actin filaments,<ref name=":0" /><ref name=":1" /><ref name=":2">{{Cite book|title=Guyton and Hall textbook of medical physiology| vauthors = Hall JE, Guyton AC |year=2011 |isbn=9781416045748 |edition=  Twelfth |location=Philadelphia, Pa.|oclc=434319356}}</ref> and provide attachment points on binding sites for the actin filaments. The myosin heads move in a coordinated style; they swivel toward the center of the sarcomere, detach, and then reattach to the nearest active site of the actin filament. This is called a ratchet-type drive system.<ref name=":2" />

This process consumes large amounts of [[adenosine triphosphate]] (ATP), the energy source of the cell. ATP binds to the cross-bridges between myosin heads and actin filaments. The release of energy powers the swiveling of the myosin head. When ATP is used, it becomes [[adenosine diphosphate]] (ADP), and since muscles store little ATP, they must continuously replace the discharged   ADP with ATP. Muscle tissue also contains a stored supply of a fast-acting recharge chemical, [[creatine phosphate]], which when necessary can assist with the rapid regeneration of ADP into ATP.<ref name=":3">{{Cite book|title=Marks' basic medical biochemistry : a clinical approach| vauthors = Lieberman M, Peet A |isbn=9781496324818|edition=  Fifth|location=Philadelphia|oclc=981908072|year = 2018}}</ref>

[[Calcium ions]] are required for each cycle of the sarcomere. Calcium is released from the [[sarcoplasmic reticulum]] into the [[sarcomere]] when a muscle is stimulated to contract. This calcium uncovers the actin-binding sites. When the muscle no longer needs to contract, the calcium ions are pumped from the sarcomere and back into storage in the [[sarcoplasmic reticulum]].<ref name=":2"/>

There are approximately 639 skeletal muscles in the human body.{{cn|date=February 2025}}

<gallery>
Image: Muscles anterior labeled.png|Skeletal muscles, viewed from the front
Image: Muscle posterior labeled.png|Skeletal muscles, viewed from the back
</gallery>