Somatic nervous system 396000 223932930 2008-07-06T16:08:29Z SieBot 4005189 robot Adding: [[nl:Somatisch zenuwstelsel]] :''"Somatic" redirects here; for the musician known as Somatic, see [[Hahn Rowe]].'' The '''somatic nervous system''' is the part of the [[peripheral nervous system]] associated with the [[voluntary]] control of body movements through the action of [[skeletal muscles]], and with [[sensory receptor|reception]] of external [[stimulus|stimuli]], which helps keep the body in touch with its surroundings (e.g., [[tactition|touch]], [[Hearing (sense)|hearing]], and [[sight]]). The system includes all the [[neuron]]s connected with [[muscle]]s, [[skin]] and [[Sensory system|sense organs]]. The somatic nervous system consists of [[efferent nerve]]s responsible for sending brain signals for [[muscle contraction]]. ==Nerve signal transmission== The somatic nervous system processes sensory information and controls all [[voluntary muscle|voluntary muscular]] systems within the body, with the exception of [[reflex arc]]s. The basic route of [[nerve signal]]s within the [[efferent]] somatic nervous system involves a sequence that begins in the upper [[Cell (biology)|cell bodies]] of [[motor neuron]]s ([[upper motor neuron]]s) within the [[Brodmann area 4|precentral gyrus]] (which approximates the [[primary motor cortex]]). Stimuli from the precentral gyrus are transmitted from upper motor neurons and down the [[corticospinal tract]], via [[axon]]s to control skeletal (voluntary) muscles. These stimuli are conveyed from upper motor neurons through the [[ventral horn]] of the [[spinal cord]], and across [[synapse]]s to be received by the [[sensory receptor]]s of [[alpha motor neuron]] (large [[lower motor neuron]]s) of the [[brainstem]] and [[spinal cord]]. Upper motor neurons release a [[neurotransmitter]], [[acetylcholine]], from their axon terminal knobs, which are received by [[nicotinic acetylcholine receptor|nicotinic receptors]] of the alpha motor neurons. In turn, alpha motor neurons relay the [[stimulus (physiology)|stimuli]] received down their axons via the [[ventral root]] of the spinal cord. These signals then proceed to the [[neuromuscular junction]]s of skeletal muscles. From there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic [[receptor (biochemistry)|receptor]]s ([[Nicotinic acetylcholine receptor]]s) of muscles, thereby relaying the stimulus to contract muscle fibers. ==Vertebrate and invertebrate differences== In [[invertebrates]], depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory. For [[vertebrate]]s, however, the response of a muscle fiber to a neurotransmitter (always [[acetylcholine]] (ACh)) can only be excitatory or, in other words, contractile. ==Reflex arcs== A [[reflex arc]] is an automatic reaction that allows an organism to protect itself [[reflex]]ively when an imminent danger is perceived. In response to certain stimuli, such as touching a hot surface, these reflexes are 'hard wired' through the spinal cord. A reflexive impulse travels up afferent nerves, through a spinal [[interneuron]], and back down appropriate efferent nerves. ==See also== *[[Autonomic nervous system]] *[[Enteric nervous system]] {{nervous system}} [[Category:Nervous system]] [[ca:Sistema nerviós somàtic]] [[de:Somatisches Nervensystem]] [[dv:ސޮމެޓިކް ނާރވަސް ސިސްޓަމް]] [[es:Sistema nervioso somático]] [[fr:Système nerveux somatique]] [[is:Viltaugakerfið]] [[nl:Somatisch zenuwstelsel]] [[ja:体性神経系]] [[no:Det somatiske nervesystem]] [[pl:Somatyczny układ nerwowy]] [[pt:Sistema nervoso somático]] [[ru:Соматическая нервная система]] [[fi:Somaattinen hermosto]] [[zh:躯体神经系统]]