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H II region
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NGC 604, a giant H II region in the Triangulum Galaxy
An H II region or HII region is a region of interstellar atomic hydrogen that is ionized.[1] It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.
They may be of any shape, because the distribution of the stars and gas inside them is irregular. The short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds. They often appear clumpy and filamentary, sometimes showing bizarre shapes such as the Horsehead Nebula. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades.
H II regions can be observed a considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distance and chemical composition of galaxies. Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are almost devoid of them. In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically. Some galaxies contain huge H II regions, which may contain tens of thousands of stars. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy.
Contents
[hide]
1 Terminology
2 Observations
3 Origin and lifetime
4 Destruction of stellar nurseries
5 Characteristics
5.1 Physical properties
5.2 Numbers and distribution
5.3 Morphology
6 Notable H II regions
7 Current issues
8 See also
9 References
10 External links
Terminology[edit]
The term H II is pronounced "H two" by astronomers. "H" is the chemical symbol for hydrogen, and "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H+ in other sciences—III for doubly-ionised, e.g. O III is O++, etc.[2] H II, or H+, consists of free protons. An H I region being neutral atomic hydrogen, and a molecular cloud being molecular hydrogen, H2. In spoken discussion with non-astronomers there is sometimes confusion between the identical spoken forms of "H II" and "H2".
Observations[edit]
Dark star-forming regions within the Eagle Nebula commonly referred to as the Pillars of Creation
A few of the brightest H II regions are visible to the naked eye. However, none seem to have been noticed before the advent of the telescope in the early 17th century. Even Galileo did not notice the Orion Nebula when he first observed the star cluster within it (previously cataloged as a single star, θ Orionis, by Johann Bayer). The French observer Nicolas-Claude Fabri de Peiresc is credited with the discovery of the Orion Nebula in 1610.[3] Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies.[4]
William Herschel observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns".[5] In early days astronomers distinguished between "diffuse nebulae" (now known to be H II regions), which retained their fuzzy appearance under magnification through a large telescope, and nebulae that could be resolved into stars, now know to be galaxies external to our own.[6]
Confirmation of Herschel's hypothesis of star formation had to wait another hundred years, when William Huggins together with his wife Mary Huggins turned his spectroscope on various nebulae. Some, such as the Andromeda Nebula, had spectra quite similar to those of stars, but turned out to be galaxies consisting of hundreds of millions of individual stars. Others looked very different. Rather than a strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines.[7] In planetary nebulae, the brightest of these spectral lines was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known chemical element. At first it was hypothesized that the line might be due to an unknown element, which was named nebulium—a similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868.[8] However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, nebulium was not. In the early 20th century, Henry Norris Russell proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions.[9]
Orion Nebula
Interstellar matter, considered dense in an astronomical context, is at high vacuum by laboratory standards. Physicists showed in the 1920s that in gas at extremely low density, electrons can populate excited metastable energy levels in atoms and ions, which at higher densities are rapidly de-excited by collisions.[10] Electron transitions from these levels in doubly ionized oxygen give rise to the 500.7 nm line.[11] These spectral lines, which can only be seen in very low density gases, are called forbidden lines. Spectroscopic observations thus showed that planetary nebulae consisted largely of extremely rarefied ionised oxygen gas (OIII).
In H II regions, however, the dominant spectral line has a wavelength of 656.3 nm. This is the well-known H-alpha line emitted by atomic hydrogen. Specifically, a photon of this wavelength is emitted when the electron of a hydrogen atom changes its excitation state from n=3 to n=2. Such state-changes happen very frequently when an electron is captured by an ionised hydrogen atom (a proton), and the electron cascades down from some higher excitation state to n=1. Thus, it was concluded that H II regions consist of a mix of electrons and ionised hydrogen that are constantly recombining into hydrogen atoms.
During the 20th century, observations showed that H II regions often contained hot, bright stars.[11] These stars are many times more massive than the Sun, and are the shortest-lived stars, with total lifetimes of only a few million years (compared to stars like the Sun, which live for several billion years). Therefore, it was surmised that H II regions must be regions in which new stars were forming.[11] Over a period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse.[12] The Pleiades are an example of a cluster which has 'boiled away' the H II region from which it was formed. Only a trace of reflection nebulosity remains.
Origin and lifetime[edit]
See also: Stellar evolution
A small portion of the Tarantula Nebula, a giant H II region in the Large Magellanic Cloud
The precursor to an H II region is a giant molecular cloud (GMC). A GMC is a cold (10–20 K) and dense cloud consisting mostly of molecular hydrogen.[4] GMCs can exist in a stable state for long periods of time, but shock waves due to supernovae, collisions between clouds, and magnetic interactions can trigger its collapse. When this happens, via a process of collapse and fragmentation of the cloud, stars are born (see stellar evolution for a lengthier description).[12]
As stars are born within a GMC, the most massive will reach temperatures hot enough to ionise the surrounding gas.[4] Soon after the formation of an ionising radiation field, energetic photons create an ionisation front, which sweeps through the surrounding gas at supersonic speeds. At greater and greater distances from the ionising star, the ionisation front slows, while the pressure of the newly ionised gas causes the ionised volume to expand. Eventually, the ionisation front slows to subsonic speeds, and is overtaken by the shock front caused by the expansion of the material ejected from the nebula. The H II region has been born.[13]
The lifetime of an H II region is of the order of a few million years.[14] Radiation pressure from the hot young stars will eventually drive most of the gas away. In fact, the whole process tends to be very inefficient, with less than 10 percent of the gas in the H II region forming into stars before the rest is blown off.[12] Contributing to the loss of gas are the supernova explosions of the most massive stars, which will occur after only 1–2 million years.
Destruction of stellar nurseries[edit]
Bok globules in H II region IC 2944
Stars form in clumps of cool molecular gas that hide the nascent stars. It is only when the radiation pressure from a star drives away its 'cocoon' that it becomes visible. The hot, blue stars that are powerful enough to ionize significant amounts of hydrogen and form H II regions will do this quickly, and light up the region in which they just formed. The dense regions which contain younger or less massive still-forming stars and which have not yet blown away the material from which they are forming are often seen in silhouette against the rest of the ionised nebula. These dark patches are known as Bok globules, after astronomer Bart Bok, who proposed in the 1940s that they might be stellar birthplaces,[15] a hypothesis which was shown to be correct in 1990.[16] The hot young stars dissipate these globules, as the radiation from the stars powering the H II region drives the material away. In this sense, the stars which generate H II regions act to destroy stellar nurseries. In doing so, however, one last burst of star formation may be triggered, as radiation pressure and mechanical pressure from supernova may act to squeeze globules, thereby enhancing the density within them.[17]
The young stars in H II regions show evidence for containing planetary systems. The Hubble Space Telescope has revealed hundreds of protoplanetary disks (proplyds) in the Orion Nebula.[18] At least half the young stars in the Orion Nebula appear to be surrounded by disks of gas and dust,[19] thought to contain many times as much matter as would be needed to create a planetary system like the Solar System.
Characteristics[edit]
Physical properties[edit]
Messier 17 is an H II region in the constellation Sagittarius.
H II regions vary greatly in their physical properties. They range in size from so-called ultra-compact (UCHII) regions perhaps only a light-year or less across, to giant H II regions several hundred light-years across.[4] Their size is also known as the Stromgren radius and essentially depends on the intensity of the source of ionising photons and the density of the region. Their densities range from over a million particles per cm³ in the ultra-compact H II regions to only a few particles per cm³ in the largest and most extended regions. This implies total masses between perhaps 100 and 105 solar masses.[20]
There are also "ultra-dense H II" regions (UDHII).[21]
Depending on the size of an H II region there may be several thousand stars within it. This makes H II regions more complicated than planetary nebulae, which have only one central ionising source. Typically H II regions reach temperatures of 10,000 K.[4] They are mostly ionised gases with weak magnetic fields with strengths of several nanoteslas.[22] Nevertheless, H II regions are almost always associated with a cold molecular gas, which originated from the same parent GMC.[4] Magnetic fields are produced by these weak moving electric charges in the ionised gas, suggesting that H II regions might contain electric fields.[23]
Stellar nursery N159 is an HII region over 150 light-years across.[24]
A number of H II regions also show signs of being permeated by a plasma with temperatures exceeding 10,000,000 K, sufficiently hot to emit X-rays. X-ray observatories such as Einstein and Chandra have noted diffuse X-ray emissions in a number of star-forming regions, notably the Orion Nebula, Messier 17, and the Carina Nebula.[25] The hot gas is likely supplied by the strong stellar winds from O-type stars, which may be heated by supersonic shock waves in the winds, through collisions between winds from different stars, or through colliding winds channeled by magnetic fields. This plasma will rapidly expand to fill available cavities in the molecular clouds due to the high speed of sound in the gas at this temperature. It will also leak out through holes in the periphery of the H II region, which appears to be happening in Messier 17.[26]
Chemically, H II regions consist of about 90% hydrogen. The strongest hydrogen emission line at 656.3 nm gives H II regions their characteristic red colour. Most of the rest of an H II region consists of helium, with trace amounts of heavier elements. Across the galaxy, it is found that the amount of heavy elements in H II regions decreases with increasing distance from the galactic centre.[27] This is because over the lifetime of the galaxy, star formation rates have been greater in the denser central regions, resulting in greater enrichment of those regions of the interstellar medium with the products of nucleosynthesis.
Numbers and distribution[edit]
Strings of red H II regions delineate the arms of the Whirlpool Galaxy.
H II regions are found only in spiral galaxies like the Milky Way and irregular galaxies. They are not seen in elliptical galaxies. In irregular galaxies, they may be dispersed throughout the galaxy, but in spirals they are most abundant within the spiral arms. A large spiral galaxy may contain thousands of H II regions.[20]
The reason H II regions rarely appear in elliptical galaxies is that ellipticals are believed to form through galaxy mergers.[28] In galaxy clusters, such mergers are frequent. When galaxies collide, individual stars almost never collide, but the GMCs and H II regions in the colliding galaxies are severely agitated.[28] Under these conditions, enormous bursts of star formation are triggered, so rapid that most of the gas is converted into stars rather than the normal rate of 10% or less.
Galaxies undergoing such rapid star formation are known as starburst galaxies. The post-merger elliptical galaxy has a very low gas content, and so H II regions can no longer form.[28] Twenty-first century observations have shown that a very small number of H II regions exist outside galaxies altogether. These intergalactic H II regions may be the remnants of tidal disruptions of small galaxies, and in some cases may represent a new generation of stars in a galaxy's most recently accreted gas.[29]
Morphology[edit]
See also: Strömgren sphere
H II regions come in an enormous variety of sizes. They are usually clumpy and inhomogeneous on all scales from the smallest to largest.[4] Each star within an H II region ionises a roughly spherical region—known as a Strömgren sphere—of the surrounding gas, but the combination of ionisation spheres of multiple stars within a H II region and the expansion of the heated nebula into surrounding gases creates sharp density gradients that result in complex shapes.[30] Supernova explosions may also sculpt H II regions. In some cases, the formation of a large star cluster within an H II region results in the region being hollowed out from within. This is the case for NGC 604, a giant H II region in the Triangulum Galaxy.[31] For a H II region which cannot be resolved, some information on the spatial structure (the electron density as a function of the distance from the center, and an estimate of the clumpiness) can be inferred by performing an inverse Laplace transform on the frequency spectrum.
Notable H II regions[edit]
An optical image (left) reveals clouds of gas and dust in the Orion Nebula; an infrared image (right) reveals new stars shining within.
Notable Galactic H II regions include the Orion Nebula, the Eta Carinae Nebula, and the Berkeley 59 / Cepheus OB4 Complex.[32] The Orion Nebula, about 500 pc (1,500 light-years) from Earth, is part of OMC-1, a GMC that, if visible, would be seen to fill most of the constellation of Orion.[11] The Horsehead Nebula and Barnard's Loop are two other illuminated parts of this cloud of gas.[33] The Orion Nebula is actually a thin layer of ionised gas on the outer border of the OMC-1 cloud. The stars in the Trapezium cluster, and especially θ1 Orionis, are responsible for this ionisation.[11]
The Large Magellanic Cloud, a satellite galaxy of the Milky Way at about 50 kpc (160 thousand light years), contains a giant H II region called the Tarantula Nebula. Measuring at about 200 pc (650 light years) across, this nebula is the most massive and the second-largest H II region in the Local Group.[34] It is much bigger than the Orion Nebula, and is forming thousands of stars, some with masses of over 100 times that of the sun—OB and Wolf-Rayet stars. If the Tarantula Nebula were as close to Earth as the Orion Nebula, it would shine about as brightly as the full moon in the night sky. The supernova SN 1987A occurred in the outskirts of the Tarantula Nebula.[30]
Another giant H II region—NGC 604 is located in M33 spiral galaxy, which is at 817 kpc (2.66 million light years). Measuring at approximately 240 × 250 pc (800 × 830 light years) across, NGC 604 is the second-most-massive H II region in the Local Group after the Tarantula Nebula, although it is slightly larger in size than the latter. It contains around 200 hot OB and Wolf-Rayet stars, which heat the gas inside it to millions of degrees, producing bright X-ray emissions. The total mass of the hot gas in NGC 604 is about 6,000 Solar masses.[31]
Current issues[edit]
Trifid Nebula seen at different wavelengths
As with planetary nebulae, estimates of the abundance of elements in H II regions are subject to some uncertainty.[35] There are two different ways of determining the abundance of metals (metals in this case are elements other than hydrogen and helium) in nebulae, which rely on different types of spectral lines, and large discrepancies are sometimes seen between the results derived from the two methods.[34] Some astronomers put this down to the presence of small temperature fluctuations within H II regions; others claim that the discrepancies are too large to be explained by temperature effects, and hypothesise the existence of cold knots containing very little hydrogen to explain the observations.[35]
The full details of massive star formation within H II regions are not yet well known. Two major problems hamper research in this area. First, the distance from Earth to large H II regions is considerable, with the nearest H II (California Nebula) region at 300 pc (1,000 light-years);[36] other H II regions are several times that distance from Earth. Secondly, the formation of these stars is deeply obscured by dust, and visible light observations are impossible. Radio and infrared light can penetrate the dust, but the youngest stars may not emit much light at these wavelengths.[33]
See also[edit]
Emission nebula
Reflection nebula
Astronomical object
H I region
Planetary nebula
Protoplanetary nebula
Astronomical spectroscopy
Interstellar medium
Astronomy portal
References[edit]
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External links[edit]
Wikimedia Commons has media related to H II regions.
Hubble images of nebulae including several H II regions
Information from SEDS
Harvard astronomy course notes on H II regions
H II region at Encyclopædia Britannica
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text:
Sympathetic nervous system
From Wikipedia, the free encyclopedia
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Sympathetic nervous system
Schematic illustration showing the sympathetic nervous system with sympathetic cord and target organs.
Details
Identifiers
Latin
pars sympathica divisionis autonomici systematis nervosi
Acronym(s)
SNS
MeSH
D013564
TA
A14.3.01.001
FMA
9906
Anatomical terminology
[edit on Wikidata]
The sympathetic nervous system (SNS) is one of the two main divisions of the autonomic nervous system, the other being the parasympathetic nervous system. (The enteric nervous system (ENS) is now usually referred to as separate from the autonomic nervous system since it has its own independent reflex activity.)[1][2]
The autonomic nervous system functions to regulate the body's unconscious actions. The sympathetic nervous system's primary process is to stimulate the body's fight-or-flight response. It is, however, constantly active at a basic level to maintain homeostasis homeodynamics.[3] The sympathetic nervous system is described as being complementary to the parasympathetic nervous system which stimulates the body to "feed and breed" and to (then) "rest-and-digest".
Contents
[hide]
1 Structure
1.1 Organization
1.2 Information transmission
2 Function
2.1 Sensation
2.2 Relationship with the parasympathetic nervous system
3 Disorders
4 History and etymology
5 See also
6 References
Structure[edit]
There are two kinds of neurons involved in the transmission of any signal through the sympathetic system: pre-ganglionic and post-ganglionic. The shorter preganglionic neurons originate from the thoracolumbar region of the spinal cord specifically at T1 to L2~L3, and travel to a ganglion, often one of the paravertebral ganglia, where they synapse with a postganglionic neuron. From there, the long postganglionic neurons extend across most of the body.[4]
At the synapses within the ganglia, preganglionic neurons release acetylcholine, a neurotransmitter that activates nicotinic acetylcholine receptors on postganglionic neurons. In response to this stimulus, the postganglionic neurons release norepinephrine, which activates adrenergic receptors that are present on the peripheral target tissues. The activation of target tissue receptors causes the effects associated with the sympathetic system. However, there are three important exceptions:[5]
Postganglionic neurons of sweat glands and chromaffin cells of the adrenal medulla. Postganglionic neurons of sweat glands release acetylcholine for the activation of muscarinic receptors, except for areas of thick skin, the palms and the plantar surfaces of the feet, where norepinephrine is released and acts on adrenergic receptors.
Chromaffin cells of the adrenal medulla are analogous to post-ganglionic neurons; the adrenal medulla develops in tandem with the sympathetic nervous system and acts as a modified sympathetic ganglion. Within this endocrine gland, pre-ganglionic neurons synapse with chromaffin cells, triggering the release of two transmitters: a small proportion of norepinephrine, and more substantially, epinephrine. The synthesis and release of epinephrine as opposed to norepinephrine is another distinguishing feature of chromaffin cells compared to postganglionic sympathetic neurons.[6]
Postganglionic sympathetic nerves terminating in the kidney release dopamine, which acts on dopamine D1 receptors of blood vessels to control how much blood the kidney filters. Dopamine is the immediate metabolic precursor to norepinephrine, but is nonetheless a distinct signaling molecule.[7]
Organization[edit]
The sympathetic nervous system extends from the thoracic to lumbar vertebrae and has connections with the thoracic, abdominal, and pelvic plexuses.
Sympathetic nerves arise from near the middle of the spinal cord in the intermediolateral nucleus of the lateral grey column, beginning at the first thoracic vertebra of the vertebral column and are thought to extend to the second or third lumbar vertebra. Because its cells begin in the thoracic and lumbar regions of the spinal cord, the sympathetic nervous system is said to have a thoracolumbar outflow. Axons of these nerves leave the spinal cord through the anterior root. They pass near the spinal (sensory) ganglion, where they enter the anterior rami of the spinal nerves. However, unlike somatic innervation, they quickly separate out through white rami connectors (so called from the shiny white sheaths of myelin around each axon) that connect to either the paravertebral (which lie near the vertebral column) or prevertebral (which lie near the aortic bifurcation) ganglia extending alongside the spinal column.
To reach target organs and glands, the axons must travel long distances in the body, and, to accomplish this, many axons relay their message to a second cell through synaptic transmission. The ends of the axons link across a space, the synapse, to the dendrites of the second cell. The first cell (the presynaptic cell) sends a neurotransmitter across the synaptic cleft where it activates the second cell (the postsynaptic cell). The message is then carried to the final destination.
Presynaptic nerves' axons terminate in either the paravertebral ganglia or prevertebral ganglia. There are four different paths an axon can take before reaching its terminal. In all cases, the axon enters the paravertebral ganglion at the level of its originating spinal nerve. After this, it can then either synapse in this ganglion, ascend to a more superior or descend to a more inferior paravertebral ganglion and synapse there, or it can descend to a prevertebral ganglion and synapse there with the postsynaptic cell.
The postsynaptic cell then goes on to innervate the targeted end effector (i.e. gland, smooth muscle, etc.). Because paravertebral and prevertebral ganglia are relatively close to the spinal cord, presynaptic neurons are generally much shorter than their postsynaptic counterparts, which must extend throughout the body to reach their destinations.
A notable exception to the routes mentioned above is the sympathetic innervation of the suprarenal (adrenal) medulla. In this case, presynaptic neurons pass through paravertebral ganglia, on through prevertebral ganglia and then synapse directly with suprarenal tissue. This tissue consists of cells that have pseudo-neuron like qualities in that when activated by the presynaptic neuron, they will release their neurotransmitter (epinephrine) directly into the bloodstream.
In the sympathetic nervous system 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 sympathetic nervous system 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).

Autonomic nervous supply to organs in the human body edit
Organ
Nerves[8]
Spinal column origin[8]
stomach
PS: anterior and posterior vagal trunks
S: greater splanchnic nerves
T5,T6, T7, T8, T9, sometimes T10
duodenum
PS: vagus nerves
S: greater splanchnic nerves
T5, T6, T7, T8, T9, sometimes T10
jejunum and ileum
PS: posterior vagal trunks
S: greater splanchnic nerves
T5, T6, T7, T8, T9
spleen
S: greater splanchnic nerves
T6, T7, T8
gallbladder and liver
PS: vagus nerve
S: celiac plexus
right phrenic nerve
T6, T7, T8, T9
colon
PS: vagus nerves and pelvic splanchnic nerves
S: lesser and least splanchnic nerves
T10, T11, T12 (proximal colon)
L1, L2, L3, (distal colon)
pancreatic head
PS: vagus nerves
S: thoracic splanchnic nerves
T8, T9
vermiform appendix
nerves to superior mesenteric plexus
T10
kidneys and ureters
PS: vagus nerve
S: thoracic and lumbar splanchnic nerves
T11, T12
Information transmission[edit]
Sympathetic Nervous System - Information transmits through it affecting various organs.
Messages travel through the sympathetic nervous system in a bi-directional 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 oesophagus; cause pupillary dilation, piloerection (goose bumps) and perspiration (sweating); and raise blood pressure. One exception is with certain blood vessels such as those in the cerebral and coronary arteries, which dilate (rather than constrict) with an increase in sympathetic tone. This is because of a proportional increase in the presence of β2 adrenergic receptors rather than α1 receptors. β2 receptors promote vessel dilation instead of constriction like α1 receptors. An alternative explanation is that the primary (and direct) effect of sympathetic stimulation on coronary arteries is vasoconstriction followed by a secondary vasodilation caused by the release of vasodilatory metabolites due to the sympathetically increased cardiac inotropy and heart rate. This secondary vasodilation caused by the primary vasoconstriction is termed functional sympatholysis, the overall effect of which on coronary arteries is dilation.[9]
The target synapse of the postganglionic neuron is mediated by adrenergic receptors and is activated by either norepinephrine (noradrenaline) or epinephrine (adrenaline).
Function[edit]
Examples of sympathetic system action on various organs[6] except where otherwise indicated.
Organ
Effect
Eye Dilates
Heart Increases rate and force of contraction
Lungs Dilates bronchioles via circulating adrenaline[10]
Blood vessels Dilate in skeletal muscle (in animals).[11]
Constricts in gastrointestinal organs
Sweat glands Activates sweat secretion
Digestive tract Inhibits peristalsis
Kidney Increases renin secretion
Penis Inhibits Tumescence
Ductus deferens Promotes emission prior to ejaculation
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 regulation of functions as diverse as pupil diameter, gut motility, and urinary system output and function.[12] 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 great 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 sympathetic nervous system is responsible for priming the body for action, particularly in situations threatening survival.[13] One example of this priming is in the moments before waking, in which sympathetic outflow spontaneously increases in preparation for action.
Sympathetic nervous system stimulation causes vasoconstriction of most blood vessels, including many of those in the skin, the digestive tract, and the kidneys. This occurs as a result of activation of alpha-1 adrenergic receptors by norepinephrine released by post-ganglionic sympathetic neurons. These receptors exist throughout the vasculature of the body but are inhibited and counterbalanced by beta-2 adrenergic receptors (stimulated by epinephrine release from the adrenal glands) in the skeletal muscles, the heart, the lungs, and the brain during a sympathoadrenal response. The net effect of this is a shunting of blood away from the organs not necessary to the immediate survival of the organism and an increase in blood flow to those organs involved in intense physical activity.
Sensation[edit]
The afferent fibers of the autonomic nervous system, which transmit sensory information from the internal organs of the body back to the central nervous system (or CNS), are not divided into parasympathetic and sympathetic fibers as the efferent fibers are.[14] Instead, autonomic sensory information is conducted by general visceral afferent fibers.
General visceral afferent sensations are mostly unconscious visceral motor reflex sensations from hollow organs and glands that are transmitted to the CNS. While the unconscious reflex arcs normally are undetectable, in certain instances they may send pain sensations to the CNS masked as referred pain. If the peritoneal cavity becomes inflamed or if the bowel is suddenly distended, the body will interpret the afferent pain stimulus as somatic in origin. This pain is usually non-localized. The pain is also usually referred to dermatomes that are at the same spinal nerve level as the visceral afferent synapse.[citation needed]
Relationship with the parasympathetic nervous system[edit]
Together with the other component of the autonomic nervous system, the parasympathetic nervous system, the sympathetic nervous system aids in the control of most of the body's internal organs. Reaction to stress—as in the flight-or-fight response—is thought to counteract the parasympathetic system, which generally works to promote maintenance of the body at rest. The comprehensive functions of both the parasympathetic and sympathetic nervous systems are not so straightforward, but this is a useful rule of thumb.[3][15]
Disorders[edit]
In heart failure, the sympathetic nervous system increases its activity, leading to increased force of muscular contractions that in turn increases the stroke volume, as well as peripheral vasoconstriction to maintain blood pressure. However, these effects accelerate disease progression, eventually increasing mortality in heart failure.[16]
Sympathicotonia is a stimulated[17] condition of the sympathetic nervous system, marked by vascular spasm,[18] elevated blood pressure,[18] and goose bumps.[18] A recent study has shown the expansion of Foxp3+ natural Treg in the bone marrow of mice after brain ischemia and this myeloid Treg expansion is related to sympathetic stress signaling after brain ischemia.[19]
History and etymology[edit]
The name of this system can be traced to the concept of sympathy, in the sense of "connection between parts", first used medically by Galen.[20] In the 18th century, Jacob B. Winslow applied the term specifically to nerves.[21]
See also[edit]
Epinephrine
History of catecholamine research
Limbic system
Norepinephrine
Sympathetic ganglia
Sympathetic trunk
References[edit]
Jump up ^ Dorland's (2012). Dorland's Illustrated Medical Dictionary (32nd ed.). Elsevier Saunders. p. 1862. ISBN 978-1-4160-6257-8.
Jump up ^ Pocock G, Richards C (2006). Human Physiology The Basis of Medicine (Third ed.). Oxford University Press. p. 63. ISBN 978-0-19-856878-0.
^ Jump up to: a b Brodal, Per (2004). The Central Nervous System: Structure and Function (3 ed.). Oxford University Press US. pp. 369–396. ISBN 0-19-516560-8.
Jump up ^ Drake, Richard L.; Vogl, Wayne; Mitchell, Adam W.M., eds. (2005). Gray's Anatomy for Students (1 ed.). Elsevier. pp. 76–84. ISBN 0-443-06612-4.
Jump up ^ Rang, H.P.; Dale, M.M.; Ritter, J.M.; Flower, R.J. (2007). Rang and Dale's Pharmacology (6 ed.). Elsevier. p. 135. ISBN 0-443-06911-5.
^ Jump up to: a b Silverthorn, Dee Unglaub (2009). Human Physiology: An Integrated Approach (4 ed.). Pearson/Benjamin Cummings. pp. 379–386. ISBN 0-321-54130-8.
Jump up ^ Santiago Cuevas; Van Anthony Villar; Pedro A. Jose; Ines Armando (2013). "Renal Dopamine Receptors, Oxidative Stress, and Hypertension". International Journal of Molecular Sciences. 14 (9): 17553–17572. doi:10.3390/ijms140917553. PMC 3794741  . PMID 23985827.
^ Jump up to: a b Unless specified otherwise in the boxes, the source is: Moore, Keith L.; Agur, A. M. R. (2002). Essential Clinical Anatomy (2nd ed.). Lippincott Williams & Wilkins. p. 199. ISBN 978-0-7817-5940-3.
Jump up ^ Klabunde, Richard E. (2012). Cardiovascular Physiology Concepts (2 ed.). Lippincott Williams & Wilkins. p. 160.
Jump up ^ Berger, Michael P. Hlastala; Albert J. (2001). Physiology of respiration (2. ed.). Oxford [u.a.]: Oxford Univ. Press. p. 177. ISBN 0195138465.
Jump up ^ Jänig, Wilfrid (2006). The integrative action of the autonomic nervous system : neurobiology of homeostasis. Cambridge: UK. pp. 132–135. ISBN 9780521845182.
Jump up ^ Moro, C; Tajouri, L; Chess-Williams, R (January 2013). "Adrenoceptor function and expression in bladder urothelium and lamina propria". Urology. 81 (1): 211.e1–7. doi:10.1016/j.urology.2012.09.011. PMID 23200975.
Jump up ^ 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.
Jump up ^ Moore, K.L., & Agur, A.M. (2007). Essential Clinical Anatomy: Third Edition. Baltimore: Lippincott Williams & Wilkins. 34-35. ISBN 978-0-7817-6274-8
Jump up ^ Sherwood, Lauralee (2008). Human Physiology: From Cells to Systems (7 ed.). Cengage Learning. p. 240. ISBN 0-495-39184-0.
Jump up ^ Triposkiadis F, Karayannis G, Giamouzis G, Skoularigis J, Louridas G, Butler J (2009). "The sympathetic nervous system in heart failure physiology, pathophysiology, and clinical implications". J. Am. Coll. Cardiol. 54 (19): 1747–62. doi:10.1016/j.jacc.2009.05.015. PMID 19874988.
Jump up ^ thefreedictionary.com Citing: Dorland's Medical Dictionary for Health Consumers. © 2007
^ Jump up to: a b c thefreedictionary.com Citing: The American Heritage Medical Dictionary Copyright © 2007
Jump up ^ Wang J, Yu L, Jiang C, Fu X, Liu X, Wang M, Ou C, Cui X, Zhou C, Wang J (August 2015). "Cerebral ischemia increases bone marrow CD4+CD25+FoxP3+ regulatory T cells in mice via signals from sympathetic nervous system". Brain Behav Immun. 43: 172–83. doi:10.1016/j.bbi.2014.07.022. PMC 4258426  . PMID 25110149.
Jump up ^ Sympathy, Sympathetic. Evolution of a Concept and Relevance to Current Understanding of Autonomic Disorders (2013)
Jump up ^ Olry, R. (1996). "Winslow's contribution to our understanding of the cervical portion of the sympathetic nervous system". J Hist Neurosci. 5 (2): 190–6. doi:10.1080/09647049609525666. PMID 11619046.
[show]
v
t
e
Nervous system
Central nervous system
Meninges
Spinal cord
Brain
Rhombencephalon
Medulla
Pons
Cerebellum
Midbrain
Forebrain
Diencephalon
Retina
Optic nerve
Cerebrum
Limbic system
Peripheral nervous system
Somatic
Sensory nerve
Motor nerve
Cranial nerve
Spinal nerve
Autonomic
Sympathetic
Parasympathetic
Enteric
[show]
v
t
e
Anatomy of the autonomic nervous system
Head
Sympathetic
Cervical ganglia: roots
long ciliary
Deep petrosal
Parasympathetic
Ciliary ganglion: roots
Sensory
Parasympathetic
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
Sympathetic
paravertebral ganglia: Cervical ganglia
Superior
Middle
Inferior
Stellate ganglion
prevertebral plexus: Cavernous plexus
Internal carotid
Chest
Sympathetic
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
Sympathetic
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
Submucous plexus
Myenteric plexus
Pelvis
Sympathetic
paravertebral ganglia: Sacral ganglia
Ganglion impar
splanchnic nerves: Sacral splanchnic nerves
Parasympathetic
splanchnic nerves: Pelvic splanchnic nerves
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Sympathetic nervous system
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This page was last edited on 17 May 2018, at 09:58.
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