Sympathetic nervous system
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{{Infobox Anatomy |
Name = Sympathetic nervous system |
Image = Gray838.png |
Caption = The sympathetic nervous system extends from the thoracic to lumbar [[vertebra]]e and has connections with the thoracic, abdominal, and pelvic plexuses. |
Mesh = Sympathetic+Nervous+System
}}
The '''Sympathetic Nervous System''' ('''SNS''') is a branch of the [[autonomic nervous system]] along with the [[enteric nervous system]] and [[parasympathetic nervous system]]. It is always active at a basal level (called '''sympathetic tone''') and becomes more active during times of [[stress (medicine)|stress]]. Its actions during the stress response comprise the [[fight-or-flight response]].
==Overview==
Like other parts of the [[nervous system]], the sympathetic nervous system operates through a series of interconnected [[neuron]]s. Sympathetic neurons are frequently considered part of the [[peripheral nervous system]] (PNS), although there are many that lie within the [[central nervous system]] (CNS). Sympathetic neurons of the [[spinal cord]] (which is part of the CNS) communicate with peripheral sympathetic neurons via a series of [[sympathetic ganglia]]. Within the ganglia, spinal cord sympathetic neurons join peripheral sympathetic neurons through [[chemical synapse]]s. Spinal cord sympathetic neurons are therefore called ''presynaptic'' (or ''preganglionic'') neurons, while peripheral sympathetic neurons are called ''postsynaptic'' (or ''postganglionic'') neurons....
At synapses within the sympathetic ganglia, preganglionic sympathetic neurons release [[acetylcholine]], a chemical messenger that binds and activates [[nicotinic acetylcholine receptor]]s on postganglionic neurons. In response to this stimulus, postganglionic neurons principally release [[noradrenaline]] ([[norepinephrine]]). Prolonged activation can elicit the release of [[adrenaline]] from the [[adrenal medulla]].
Once released, noradrenaline and adrenaline bind [[adrenergic receptor]]s on peripheral tissues. Binding to adrenergic receptors causes the effects seen during the fight-or-flight response. These include pupil dilation, increased heart rate, occasional vomiting, and increased blood pressure. Increased sweating is also seen due to binding of [[cholinergic receptor]]s of the sweat glands.
The sympathetic nervous system involves spinal nerves T1 to L2 or L3.
==Function==
The sympathetic nervous system is responsible for up- and down-regulating many homeostatic mechanisms in living organisms. Fibers from the SNS innervate tissues in almost every organ system, providing at least some regulatory function to things as diverse as [[pupil]] diameter, gut motility, and urinary output. It is perhaps best known for mediating the neuronal and hormonal stress response commonly known as the ''fight-or-flight response''. This response is also known as ''sympatho-adrenal response'' of the body, as the [[preganglionic]] sympathetic fibers that end in the adrenal medulla (but also all other sympathetic fibers) secrete acetylcholine, which activates the secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from it. Therefore, this response that acts primarily on the [[cardiovascular system]] is mediated directly via impulses transmitted through the sympathetic nervous system and indirectly via [[catecholamines]] secreted from the adrenal medulla.
Science typically looks at the SNS as an automatic regulation system, that is, one that operates without the intervention of conscious thought. Some evolutionary theorists suggest that the sympathetic nervous system operated in early organisms to maintain survival as the sympathetic nervous system is responsible for priming the body for action.<ref>{{cite book |author=Robert Ornstein |title=The evolution of consciousness: of Darwin, Freud, and cranial fire: the origins of the way we think |publisher=Simon & Schuster |location=New York |year=1992 |pages= |isbn=0-671-79224-5 |oclc= |doi=}}</ref> One example of this priming is in the moments before waking, in which sympathetic outflow spontaneously increases in preparation for action.
==Organization==
Sympathetic nerves originate inside the [[vertebral column]], toward the middle of the spinal cord in the [[intermediolateral cell column]] (or [[lateral horn]]), beginning at the first [[thoracic]] segment of the spinal cord and are thought to extend to the second or third [[lumbar]] segments. Because its cells begin in the thoracic and lumbar regions of the spinal cord, the CNS is said to have a ''thoracolumbar outflow''. [[Axon]]s of these nerves leave the spinal cord in the ventral branches (rami) of the spinal nerves, and then separate out as [[white rami]] (so called from the shiny white sheaths of [[myelin]] around each axon) which connect to two chain [[ganglion|ganglia]] extending alongside the [[sympathetic nervous system]].<ref> /synap/sympathetic.html |title=Sympathetic nervous system, from the University of Chicago}}</ref>
In order to reach the target organs and glands, the axons must travel long distances in the body, and, to accomplish this, many axons link up with the axon of a second cell. The ends of the axons do not make direct contact, but rather link across a space, the [[synapse]].
In the SNS and other components of the peripheral nervous system, these synapses are made at sites called ganglia. The cell that sends its fiber is called a preganglionic cell, while the cell whose fiber leaves the ganglion is called a [[postganglionic]] cell. As mentioned previously, the preganglionic cells of the SNS are located between the first thoracic segment and third lumbar segments of the spinal cord. Postganglionic cells have their cell bodies in the ganglia and send their axons to target organs or glands.
The ganglia include not just the sympathetic trunks but also the cervical ganglia ([[superior cervical ganglion|superior]], [[middle cervical ganglion|middle]] and [[inferior cervical ganglion|inferior]]), which sends sympathetic nerve fibers to the head and thorax organs, and the [[celiac ganglion|celiac]] and [[mesenteric ganglion|mesenteric ganglia]] (which send sympathetic fibers to the gut).
==Information transmission==
Messages travel through the SNS in a bidirectional flow. [[Efferent]] messages can trigger changes in different parts of the body simultaneously. For example, the sympathetic nervous system can accelerate [[heart rate]]; widen [[bronchial]] passages; decrease [[motility]] (movement) of the [[large intestine]]; constrict [[blood vessels]]; increase [[peristalsis]] in the [[esophagus]]; cause pupil [[dilation]], piloerection ([[goose bumps]]) and perspiration ([[sweating]]); and raise [[blood pressure]]. [[Afferent]] messages carry sensations such as heat, cold, or pain.
The first synapse (in the [[sympathetic chain]]) is mediated by [[nicotinic]] receptors physiologically activated by acetylcholine, and the target synapse is mediated by [[adrenergic receptors]] physiologically activated by either noradrenaline (norepinephrine) or adrenaline ([[epinephrine]]). An exception is with sweat glands which receive sympathetic innervation but have [[muscarinic]] acetylcholine receptors which are normally characteristic of [[Peripheral nervous system|PNS]]. Another exception is with certain deep muscle blood vessels, which have acetylcholine receptors and which dilate (rather than constrict) with an increase in sympathetic tone.
==See also==
*[[Autonomic nervous system]]
*[[Parasympathetic nervous system]]
*[[Adrenaline]]
*[[Sympathetic ganglia]]
*[[Noradrenaline]]
{{nervous system}}
{{Autonomic}}
==References==
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[[Category:Sympathetic nervous system|*]]
[[da:Sympatiske nervesystem]]
[[de:Sympathikus]]
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[[es:Sistema nervioso simpático]]
[[fr:Système nerveux orthosympathique]]
[[hr:Simpatikus]]
[[he:המערכת הסימפתטית]]
[[lt:Simpatinė nervų sistema]]
[[nl:Orthosympatisch zenuwstelsel]]
[[ja:交感神経系]]
[[pl:Układ współczulny]]
[[pt:Sistema nervoso simpático]]
[[ru:Симпатическая нервная система]]
[[sk:Sympatický nervový systém]]
[[fi:Sympaattinen hermosto]]
[[sv:Sympatiska nervsystemet]]
[[zh:交感神经]]