Editing Muscular system (section)

=== 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>