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16-September-2008 16:15:11 - Sympathetic nervous system Redirected from Sympathetic system Sympathetic nervous system The sympathetic nervous system extends from the thoracic to lumbar vertebrae and has connections with the thoracic, abdominal, and pelvic plexuses. 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. Its actions during the stress response comprise the fight-or-flight response. Contents 1 Overview 2 Function 3 Organization 4 Information transmission 5 See also 6 References Overview Like other parts of the nervous system, the sympathetic nervous system operates through a series of interconnected neurons. 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 synapses. 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 receptors 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 receptors 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 receptors 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.1 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. Axons 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 ganglia extending alongside the sympathetic nervous system.2 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, middle and inferior, which sends sympathetic nerve fibers to the head and thorax organs, and the celiac and 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 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 v d e Nervous system Central nervous system Brain Spinal cord Autonomic nervous system Sympathetic Parasympathetic Other Peripheral nervous system Somatic nervous system Enteric nervous system Sensory system v d e Nerves - autonomic nervous system sympathetic nervous system/ganglion/trunks and parasympathetic nervous system/ganglion Head/cranial Ciliary ganglion: roots Sensory, Parasympathetic, Sympathetic - Short ciliary Pterygopalatine ganglion: deep petrosal - nerve of pterygoid canal branches of distribution: greater palatine inferior posterior nasal branches - lesser palatine - nasopalatine medial superior posterior nasal branches - pharyngeal Submandibular ganglion Otic ganglion Neck/cervical paravertebral ganglia: Cervical ganglia Superior, Middle, Inferior - Stellate ganglion prevertebral plexus: Cavernous plexus - Internal carotid Chest/thorax paravertebral ganglia: Thoracic ganglia prevertebral plexus: Cardiac plexus - Esophageal plexus - Pulmonary plexus - Thoracic aortic plexus splanchnic nerves: cardiopulmonary - thoracic cardiac nerves: Superior - Middle - Inferior Abdomen/Lumbar paravertebral ganglia: Lumbar ganglia prevertebral ganglia: Celiac ganglia Aorticorenal - Superior mesenteric ganglion - Inferior mesenteric ganglion prevertebral plexus: Celiac plexus - Hepatic, Splenic, Pancreatic - aorticorenal Abdominal aortic plexus, Renal/Suprarenal - Superior mesenteric Gastric - Inferior mesenteric Spermatic, Ovarian - Superior hypogastric hypogastric nerve, Superior rectal - Inferior hypogastric Vesical, Prostatic/Cavernous nerves of penis, Uterovaginal, Middle rectal splanchnic nerves: Lumbar splanchnic nerves enteric nervous system: Meissner's plexus Auerbach's plexus Pelvis/sacral paravertebral ganglia: Sacral ganglia - Ganglion impar splanchnic nerves: Pelvic splanchnic nerves - Sacral splanchnic nerves All Rami communicans White, Gray - Preganglionic fibers - Postganglionic fibers References ^ Robert Ornstein 1992. The evolution of consciousness: of Darwin, Freud, and cranial fire: the origins of the way we think. New York: Simon Schuster. ISBN 0-671-79224-5. ^ /synap/sympathetic.html |title=Sympathetic nervous system, from the University of Chicago Retrieved from http://en..org/wiki/Sympathetic_nervous_system Categories: Sympathetic nervous system Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Dansk Deutsch Þ‹Þ¨ÞˆÞ¬Þ€Þ¨Þ„Þ¦Þ?Þ° Español Français Hrvatski עברית Lietuvių Nederlands Polski Português РуÑ?Ñ?кий SlovenÄ?ina Suomi Svenska ä¸æ–‡ This page was last modified on 27 June 2008, at 15:32
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