Editing Muscular system (section)

== Types ==
{{Main|Muscle tissue}}
[[File: Types Of Muscle.jpg|thumb|258x258px|Three distinct types of muscle (L to R): Smooth (non-striated) muscle in internal organs, cardiac or heart muscle, and skeletal muscle.]]
There are three distinct types of muscle: [[skeletal muscle]], [[cardiac muscle|cardiac or heart muscle]], and [[smooth muscle|smooth (non-striated) muscle]]. [[Muscle]]s provide strength, balance, posture, movement, and heat for the body to keep warm.<ref name=":1">{{Cite book|title=Junqueira's basic histology : text and atlas| vauthors = Mescher AL, Junqueira LC |isbn=9780071807203|edition=  Thirteenth |location=New York|oclc=854567882|date = 2013-02-22}}</ref>

There are more than 600 muscles in an adult male human body.<ref>{{cite web|url=https://www.uc.edu/content/dam/uc/ce/images/OLLI/Page%20Content/Muscular%20System%20s.pdf|title=THE MUSCULAR SYSTEM|website=www.uc.edu}}</ref> A kind of elastic tissue makes up each muscle, which consists of thousands, or tens of thousands, of small muscle fibers. Each fiber comprises many tiny strands called fibrils, impulses from nerve cells control the contraction of each muscle fiber.

=== 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>

=== Cardiac ===
{{Main|Cardiac muscle}}
Heart muscle is striated muscle but is distinct from skeletal muscle because the [[muscle fibers]] are laterally connected. Furthermore, just as with smooth muscles, their movement is involuntary. Heart muscle is controlled by the [[sinus node]] influenced by the [[autonomic nervous system]].<ref name=":0" /><ref name=":1" />

=== Smooth ===
{{Main|Smooth muscle}}
Smooth muscle contraction is regulated by the autonomic [[nervous system]], [[hormone]]s, and local chemical signals, allowing for gradual and sustained contractions. This type of [[muscle tissue]] is also capable of adapting to different levels of stretch and tension, which is important for maintaining proper blood flow and the movement of materials through the [[Human digestive system|digestive system]].