Editing Muscular system (section)

== Physiology ==
=== Contraction ===
[[Neuromuscular junctions]] are the focal point where a [[motor neuron]] attaches to a muscle. [[Acetylcholine]], (a [[neurotransmitter]] used in skeletal muscle contraction) is released from the axon terminal of the nerve cell when an action potential reaches the microscopic junction called a [[synapse]]. A group of chemical messengers across the synapse and stimulate the formation of electrical changes, which are produced in the muscle cell when the acetylcholine binds to receptors on its surface. Calcium is released from its storage area in the cell's sarcoplasmic reticulum. An impulse from a nerve cell causes calcium release and brings about a single, short [[muscle contraction]] called a [[muscle twitch]]. If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, such as the muscle contractions that result from [[tetanus]]. Also, a loss of function at the junction can produce [[paralysis]].<ref name=":2" />

Skeletal muscles are organized into hundreds of [[motor unit]]s, each of which involves a motor neuron, attached by a series of thin finger-like structures called [[Chemical synapse#Anatomy and physiology|axon terminals]]. These attach to and control discrete bundles of muscle fibers. A coordinated and fine-tuned response to a specific circumstance will involve controlling the precise number of motor units used. While individual muscle units' contract as a unit, the entire muscle can contract on a predetermined basis due to the structure of the motor unit. Motor unit coordination, balance, and control frequently come under the direction of the [[cerebellum]] of the brain. This allows for complex muscular coordination with little conscious effort, such as when one drives a car without thinking about the process.<ref name=":2" /><ref>{{Cite book|title=Neuroanatomy through clinical cases| vauthors = Blumenfeld H |date=2010 |publisher=Sinauer Associates |isbn=9780878930586|edition=  2nd|location=Sunderland, Mass.|oclc=473478856}}</ref>

=== Tendon ===
{{Main|Tendon}}
A tendon is a piece of connective tissue that connects a muscle to a bone.<ref>{{Cite web|url=https://medlineplus.gov/ency/imagepages/19089.htm|title=Tendon vs. ligament: MedlinePlus Medical Encyclopedia Image|website=medlineplus.gov}}</ref> When a muscle intercepts, it pulls against the skeleton to create movement. A tendon connects this muscle to a bone, making this function possible.

=== Aerobic and anaerobic muscle activity ===
At rest, the body produces the majority of its [[Adenosine triphosphate|ATP]] aerobically in the [[mitochondria]]<ref>{{cite book | vauthors = Abercrombie M, Hickman CJ, Johnson ML |date=1973 |title=A Dictionary of Biology |page=179 |location=Middlesex (England), Baltimore (U.S.A.), Ringwood (Australia) |publisher=Penguin Books |edition=  6th |series= Penguin reference books |oclc=943860 }}</ref> without producing [[lactic acid]] or other fatiguing byproducts. During exercise, the method of ATP production varies depending on the fitness of the individual as well as the duration and intensity of exercise. At lower activity levels, when exercise continues for a long duration (several minutes or longer), energy is produced aerobically by combining oxygen with [[carbohydrate]]s and [[fat]]s stored in the body.<ref name=":3" /><ref>{{cite journal | vauthors = Scott C | title = Misconceptions about Aerobic and Anaerobic Energy Expenditure | journal = Journal of the International Society of Sports Nutrition | volume = 2 | issue = 2 | pages = 32–37 | date = December 2005 | pmid = 18500953 | pmc = 2129144 | doi = 10.1186/1550-2783-2-2-32 | doi-access = free }}</ref>

During activity that is higher in intensity, with possible duration decreasing as intensity increases, ATP production can switch to anaerobic pathways, such as the use of the [[Phosphocreatine|creatine phosphate]] and the phosphagen system or anaerobic [[glycolysis]]. Aerobic ATP production is biochemically much slower and can only be used for long-duration, low-intensity exercise, but produces no fatiguing waste products that cannot be removed immediately from the [[sarcomere]] and the body, and it results in a much greater number of ATP molecules per fat or carbohydrate molecule. Aerobic training allows the oxygen delivery system to be more efficient, allowing aerobic metabolism to begin quicker. Anaerobic ATP production produces ATP much faster and allows near-maximal intensity exercise, but also produces significant amounts of [[lactic acid]] which render high-intensity exercise unsustainable for more than several minutes. The phosphagen system is also anaerobic. It allows for the highest levels of exercise intensity, but intramuscular stores of [[phosphocreatine]] are very limited and can only provide energy for exercises lasting up to ten seconds. Recovery is very quick, with full creatine stores regenerated within five minutes.<ref name=":3" /><ref>{{cite journal | vauthors = Spriet LL | title = Anaerobic metabolism in human skeletal muscle during short-term, intense activity | journal = Canadian Journal of Physiology and Pharmacology | volume = 70 | issue = 1 | pages = 157–165 | date = January 1992 | pmid = 1581850 | doi = 10.1139/y92-023 }}</ref>