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. 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. Download high resolution version (503x800, 66 KB)From Grays Anatomy This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... A diagram of a thoracic vertebra. ... This article or section is in need of attention from an expert on the subject. ... In medical terms, stress is the disruption of homeostasis through physical or psychological stimuli. ... The fight-or-flight response, also called the acute stress response, was first described by Walter Cannon in 1929. ...

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. The Human Nervous System. ... Drawing by Santiago Ramón y Cajal of neurons in the pigeon cerebellum. ... The Peripheral nervous system resides or extends outside the CNS central nervous system (the brain and spinal cord) to serve the limbs and organs. ... A diagram showing the CNS: 1. ... The Spinal cord nested in the vertebral column. ... Autonomic nervous system innervation, showing the sympathetic and parasympathetic (craniosacral) systems, in red and blue, respectively Sympathetic ganglia are the ganglia of the sympathetic nervous system. ... Illustration of the major elements in a prototypical synapse. ...

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. The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... Nicotinic acetylcholine receptors, or nAChRs, are ionotropic receptors that form ion channels in cells plasma membranes. ... Norepinephrine, known as noradrenaline outside the USA, is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ... Norepinephrine (INN)(abbr. ... Epinephrine (INN) or adrenaline (BAN) is a hormone and a neurotransmitter. ... In mammals, the adrenal glands are the triangle-shaped endocrine glands that sit atop the kidneys. ...

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 sweating, increased heart rate, occasional vomiting, and increased blood pressure. Epinephrine Norepinephrine The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that are targets of the catecholamines. ...

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. The human eye The pupil is the central transparent area (showing as black). ... The fight-or-flight response, also called the acute stress response, was first described by Walter Cannon in 1929. ... In the autonomic nervous system, fibers from the CNS to the ganglion are known as preganglionic fibers. ... In mammals, the adrenal glands are the triangle-shaped endocrine glands that sit atop the kidneys. ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... Epinephrine (INN) or adrenaline (BAN) is a hormone and a neurotransmitter. ... Norepinephrine, known as noradrenaline outside the USA, is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ... The circulatory system or cardiovascular system is the organ system which circulates blood around the body of most animals. ... Catecholamines are chemical compounds derived from the amino acid tyrosine that act as hormones or neurotransmitters. ...

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 vertebral column on the left and right. These elongated ganglia are also known as paravertebral ganglia or sympathetic trunks. In these hubs, connections (synapses) are made which then distribute the nerves to major organs, glands, and other parts of the body.^[2] The vertebral column seen from the side Different regions (curvatures) of the vertebral column The vertebral column (backbone or spine) is a column of vertebrae situated in the dorsal aspect of the abdomen. ... The Spinal cord nested in the vertebral column. ... In the thoracic region, the postero-lateral part of the anterior column projects lateralward as a triangular field, which is named the lateral column (lateral cornu, lateral horn). ... In the thoracic region, the postero-lateral part of the anterior column projects lateralward as a triangular field, which is named the lateral column (lateral cornu, lateral horn). ... Diagram of a tsetse fly, showing the head, thorax and abdomen The thorax is a division of an animals body, that lies between the head and the abdomen. ... In anatomy, lumbar is an adjective that means of or pertaining to the abdominal segment of the torso, between the diaphragm and the sacrum (pelvis). ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. ... This is a dorsal root ganglion (DRG) from a chicken embryo (around stage of day 7) after incubation overnight in NGF growth medium stained with anti-neurofilament antibody. ... The sympathetic trunks (sympathetic chain, paravertebral ganglia, gangliated cord) extend from the base of the skull to the coccyx. ... The sympathetic trunks (sympathetic chain, paravertebral ganglia, gangliated cord) extend from the base of the skull to the coccyx. ... Illustration of the major elements in a prototypical synapse. ... This article is about the biological unit. ... A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ...

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. Illustration of the major elements in a prototypical 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. This is a dorsal root ganglion (DRG) from a chicken embryo (around stage of day 7) after incubation overnight in NGF growth medium stained with anti-neurofilament antibody. ... In the autonomic nervous system, fibers from the CNS to the ganglion are known as preganglionic fibers. ... In the autonomic nervous system, fibers from the ganglion to the effector organ are called postganglionic fibers. ...

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). The superior cervical ganglion, the largest of the cervical ganglia, is placed opposite the second and third cervical vertebræ. It is of a reddish-gray color, and usually fusiform in shape; sometimes broad and flattened, and occasionally constricted at intervals; it is believed to be formed by the coalescence of... The middle cervical ganglion is the smallest of the three cervical ganglia, and is occasionally absent. ... It has been suggested that this article or section be merged into cervical ganglia. ... The Celiac Ganglia (semilunar ganglia) are two large irregularly shaped masses having the appearance of lymph glands and placed one on either side of the middle line in front of the crura of the diaphragm close to the suprarenal glands, that on the right side being placed behind the inferior... Mesenteric ganglion can refer to: inferior mesenteric ganglion superior mesenteric ganglion Category: ...

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. Efferent nerve fibers carry information away from the brain. ... Heart rate is a term used to describe the frequency of the cardiac cycle. ... The bronchioles are the first airway branches that no longer contain cartilage. ... Motility is a biological term which refers to the ability to move spontaneously and independently. ... The large intestine, an organ which is now more commonly referred to by its Greek name, the colon, is the last part of the digestive system: the final stage of the alimentary canal in vertebrate animals. ... The blood vessels are part of the circulatory system and function to transport blood throughout the body. ... In much of the digestive tract, muscles contract in sequence to produce a peristaltic wave which forces food (called bolus while in the esophagus and chyme below the esophagus) along the alimentary canal. ... The esophagus (also spelled oesophagus/Å“sophagus, Greek ), or gullet is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. ... The human eye The pupil is the central transparent area (showing as black). ... Dilation in physiological context may mean: pupil dilation (mydriasis) dilation of blood vessels (vasodilation) cervical dilation (or dilation of the cervix) in childbirth Dilation and curettage (surgical dilation) In mathematics: Dilation This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same... Goose bumps on a human Goose bumps, also called goose pimples, goose flesh, chill bumps, chicken skin, or the medical term cutis anserina, are the bumps on a persons skin at the base of body hairs which involuntarily develop when a person is cold or experiences strong emotions such... Perspiration (also called sweating or sometimes transpiration) is the production and evaporation of a fluid, consisting primarily of water as well as a smaller amount of sodium chloride (the main constituent of table salt), that is excreted by the sweat glands in the skin of mammals. ... A sphygmomanometer, a device used for measuring arterial pressure. ... In nervous systems, afferent signals or nerve fibers carry information toward the brain. ...

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 or adrenaline. 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. The Sympathetic Chain is a series of interconnected sympathetic ganglia, belonging to the autonomic nervous system. ... Nicotinic Receptors form ion channels present in the plasma membrane of cells. ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... The adrenergic receptors (or adrenoceptors) are a class of G_protein coupled receptors that is the target of catecholamines. ... Muscarinic receptors are those membrane bound acetylcholine receptors that are more sensitive to muscarine than to nicotine. ... The Peripheral nervous system resides or extends outside the CNS central nervous system (the brain and spinal cord) to serve the limbs and organs. ...

See also

• Autonomic nervous system
• Parasympathetic nervous system
• Adrenaline
• Sympathetic ganglia
• Noradrenaline

The enteric nervous system (ENS) is an interdependent part of the autonomic nervous system. ... The nerves of the small intestines are derived from the plexuses of sympathetic nerves around the superior mesenteric artery. ... Part of the enteric nervous system, Auerbachs plexus exists between the longitudinal and circular layers of muscle in the gastrointestinal tract and provides motor innervation to both layers and secretomotor innervation to the mucosa. ... The pelvis (pl. ... The sympathetic trunks (sympathetic chain, paravertebral ganglia, gangliated cord) extend from the base of the skull to the coccyx. ... As the sympathetic trunk heads inferiorly down the sacram, it turns medially. ... The pelvic portion of each sympathetic trunk is situated in front of the sacrum, medial to the anterior sacral foramina. ... The splanchnic nerves are part of the autonomic nervous system. ... Pelvic splanchnic nerves are splanchnic nerves that arise from sacral spinal nerves S2, S3, S4 to provide parasympathetic innervation to the hindgut. ... Sacral splanchnic nerves are nerves that connect the inferior hypogastric plexus to the sympathetic trunk in the pelvis. ... Rami communicans (plural rami communicantes) is the term used for a nerve which connects two other nervers. ... The thoracic, and the first and second lumbar nerves each contribute a branch, white ramus communicans to the adjoining sympathetic ganglion. ... Each spinal nerve receives a branch, gray ramus communicans, from the adjacent ganglion of the sympathetic trunk. ... In the autonomic nervous system, fibers from the CNS to the ganglion are known as preganglionic fibers. ... Acetylcholine Norepinephrine In the autonomic nervous system, fibers from the ganglion to the effector organ are called postganglionic fibers. ...

References

1. ^ 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. 
2. ^ Sympathetic nervous system, from the University of Chicago.