Editing Nervous system (section)

==Function==
At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells. One is by releasing chemicals called [[hormone]]s into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells.<ref name=Gray170>{{Cite book |title=Psychology |author=Gray PO |edition=5 |publisher=Macmillan |year=2006 |isbn=978-0-7167-7690-1 |page=[https://archive.org/details/psychology0000gray/page/170 170] |url=https://archive.org/details/psychology0000gray/page/170}}</ref> Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second.

At a more integrative level, the primary function of the nervous system is to control the body.<ref name=KandelCh2/> It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into the central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response. The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain.

===Neurons and synapses===
[[File:Chemical synapse schema cropped.jpg|thumb|350px|Major elements in synaptic transmission. An electrochemical wave called an [[action potential]] travels along the [[axon]] of a [[neuron]]. When the wave reaches a [[synapse]], it provokes release of a small amount of [[neurotransmitter]] molecules, which bind to chemical receptor molecules in the membrane of the target cell.]]

Most neurons send signals via their [[axon]]s, although some types are capable of dendrite-to-dendrite communication. (In fact, the types of neurons called [[amacrine cell]]s have no axons, and communicate only via their dendrites.) Neural signals propagate along an axon in the form of electrochemical waves called [[action potential]]s, which produce cell-to-cell signals at points where [[axon terminal]]s make [[synapse|synaptic]] contact with other cells.<ref name=KandelCh9/>

Synapses may be electrical or chemical. [[Electrical synapse]]s make direct electrical connections between neurons,<ref name=Hormuzdi/> but [[chemical synapse]]s are much more common, and much more diverse in function.<ref name=KandelCh10/> At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny spherical vessels called [[synaptic vesicle]]s, packed with [[neurotransmitter]] chemicals.<ref name=KandelCh9/> When the presynaptic terminal is electrically stimulated, an array of molecules embedded in the membrane are activated, and cause the contents of the vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the [[synaptic cleft]]. The neurotransmitter then binds to [[neurotransmitter receptor|receptors]] embedded in the postsynaptic membrane, causing them to enter an activated state.<ref name=KandelCh10/> Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways. For example, release of the neurotransmitter [[acetylcholine]] at a synaptic contact between a [[motor neuron]] and a [[muscle cell]] induces rapid contraction of the muscle cell.<ref name=KandelCh11/> The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a [[memory trace]]).<ref name=KandelCh4/>

{{Synapse map}}

There are literally hundreds of different types of synapses. In fact, there are over a hundred known neurotransmitters, and many of them have multiple types of receptors.<ref name=KandelCh15/> Many synapses use more than one neurotransmitter—a common arrangement is for a synapse to use one fast-acting small-molecule neurotransmitter such as [[glutamic acid|glutamate]] or [[gamma-Aminobutyric acid|GABA]], along with one or more [[peptide]] neurotransmitters that play slower-acting modulatory roles. Molecular neuroscientists generally divide receptors into two broad groups: [[ligand-gated ion channel|chemically gated ion channels]] and [[second messenger system]]s. When a chemically gated ion channel is activated, it forms a passage that allows specific types of ions to flow across the membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory. When a second messenger system is activated, it starts a cascade of molecular interactions inside the target cell, which may ultimately produce a wide variety of complex effects, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering [[gene transcription]].

According to a rule called [[Dale's principle]], which has only a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses.<ref name=Strata/> This does not mean, though, that a neuron exerts the same effect on all of its targets, because the effect of a synapse depends not on the neurotransmitter, but on the receptors that it activates.<ref name=KandelCh10/> Because different targets can (and frequently do) use different types of receptors, it is possible for a neuron to have excitatory effects on one set of target cells, inhibitory effects on others, and complex modulatory effects on others still. Nevertheless, it happens that the two most widely used neurotransmitters, [[glutamic acid|glutamate]] and [[gamma-Aminobutyric acid|GABA]], each have largely consistent effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory.<ref>There are a number of exceptional situations in which GABA has been found to have excitatory effects, mainly during early development. For a review see {{Cite journal |vauthors=Marty A, Llano I |title=Excitatory effects of GABA in established brain networks |journal=Trends Neurosci. |volume=28 |issue=6 |pages=284–289 |date=June 2005 |pmid=15927683 |doi=10.1016/j.tins.2005.04.003 |s2cid=40022079}}</ref> Because of this consistency, glutamatergic cells are frequently referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". Strictly speaking, this is an abuse of terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is commonly seen even in scholarly publications.

One very important subset of synapses are capable of forming [[memory trace]]s by means of long-lasting activity-dependent changes in synaptic strength.<ref name=Paradiso>{{Cite book |vauthors=Paradiso MA, Bear MF, Connors BW |title=Neuroscience: Exploring the Brain |publisher=Lippincott Williams & Wilkins |year=2007 |page=[https://archive.org/details/neuroscienceexpl00mark/page/718 718] |isbn=978-0-7817-6003-4 |url=https://archive.org/details/neuroscienceexpl00mark/page/718}}</ref> The best-known form of neural memory is a process called [[long-term potentiation]] (abbreviated LTP), which operates at synapses that use the neurotransmitter [[glutamic acid|glutamate]] acting on a special type of receptor known as the [[NMDA receptor]].<ref name=Cooke>{{Cite journal |vauthors=Cooke SF, Bliss TV |title=Plasticity in the human central nervous system |journal=Brain |volume=129 |issue=Pt 7 |pages=1659–1673 |year=2006 |pmid=16672292 |doi=10.1093/brain/awl082 |doi-access=free}}</ref> The NMDA receptor has an "associative" property: if the two cells involved in the synapse are both activated at approximately the same time, a channel opens that permits calcium to flow into the target cell.<ref name=Bliss>{{Cite journal |vauthors=Bliss TV, Collingridge GL |title=A synaptic model of memory: long-term potentiation in the hippocampus |journal=Nature |volume=361 |issue=6407 |pages=31–39 |date=January 1993 |pmid=8421494 |doi=10.1038/361031a0 |bibcode=1993Natur.361...31B |s2cid=4326182}}</ref> The calcium entry initiates a second messenger cascade that ultimately leads to an increase in the number of glutamate receptors in the target cell, thereby increasing the effective strength of the synapse. This change in strength can last for weeks or longer. Since the discovery of LTP in 1973, many other types of synaptic memory traces have been found, involving increases or decreases in synaptic strength that are induced by varying conditions, and last for variable periods of time.<ref name=Cooke/> The [[reward system]], that reinforces desired behaviour for example, depends on a variant form of LTP that is conditioned on an extra input coming from a reward-signalling pathway that uses [[dopamine]] as neurotransmitter.<ref name=Kauer>{{Cite journal |vauthors=Kauer JA, Malenka RC |title=Synaptic plasticity and addiction |journal=Nat. Rev. Neurosci. |volume=8 |issue=11 |pages=844–858 |date=November 2007 |pmid=17948030 |doi=10.1038/nrn2234 |s2cid=38811195 |doi-access=free}}</ref> All these forms of synaptic modifiability, taken collectively, give rise to [[neural plasticity]], that is, to a capability for the nervous system to adapt itself to variations in the environment.

===Neural circuits and systems===
The basic neuronal function of sending signals to other cells includes a capability for neurons to exchange signals with each other. [[Neural network (biological)|Networks]] formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation and timing,<ref name=Dayan>{{Cite book |title=Theoretical Neuroscience: Computational and Mathematical Modeling of Neural Systems |publisher=MIT Press |year=2005 |isbn=978-0-262-54185-5 |vauthors=Dayan P, Abbott LF}}</ref> and there are seen to be countless types of information processing possible. [[Warren Sturgis McCulloch|Warren McCulloch]] and [[Walter Pitts]] showed in 1943 that even [[artificial neural network]]s formed from a greatly simplified mathematical abstraction of a neuron are capable of [[universal computation]].<ref name=McCullochPitts/>
[[File:Descartes-reflex.JPG|thumb|right|Illustration of pain pathway, from [[René Descartes]]'s ''Treatise of Man'']]

Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator.<ref name=Sherrington1906/> In this conception, neural processing begins with stimuli that activate sensory neurons, producing signals that propagate through chains of connections in the spinal cord and brain, giving rise eventually to activation of motor neurons and thereby to muscle contraction, i.e., to overt responses. Descartes believed that all of the behaviors of animals, and most of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he also believed that higher cognitive functions such as language were not capable of being explained mechanistically.<ref name=Descartes>{{Cite book |title=Passions of the Soul |author=Descartes R |publisher=Hackett |year=1989 |isbn=978-0-87220-035-7 |others=Voss S}}</ref> [[Charles Scott Sherrington|Charles Sherrington]], in his influential 1906 book ''The Integrative Action of the Nervous System'',<ref name=Sherrington1906>{{Cite book |author=Sherrington CS |title=The Integrative Action of the Nervous System |publisher=Scribner |year=1906 |url=https://books.google.com/books?id=6KwRAAAAYAAJ}}</ref> developed the concept of stimulus-response mechanisms in much more detail, and [[behaviorism]], the school of thought that dominated [[psychology]] through the middle of the 20th century, attempted to explain every aspect of human behavior in stimulus-response terms.<ref name=Baum>{{Cite book |author=Baum WM |year=2005 |title=Understanding behaviorism: Behavior, Culture and Evolution |publisher=Blackwell |isbn=978-1-4051-1262-8}}</ref>

However, experimental studies of [[electrophysiology]], beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for maintaining [[Membrane potential#Cell excitability|cell excitability]] and generating patterns of activity intrinsically, without requiring an external stimulus.<ref name=Piccolino>{{Cite journal |author=Piccolino M |title=Fifty years of the Hodgkin-Huxley era |journal=Trends Neurosci. |volume=25 |issue=11 |pages=552–553 |date=November 2002 |pmid=12392928 |doi=10.1016/S0166-2236(02)02276-2 |s2cid=35465936}}</ref> Neurons were found to be capable of producing regular sequences of action potentials, or sequences of bursts, even in complete isolation.<ref name=Johnston>{{Cite book |title=Foundations of cellular neurophysiology |vauthors=Johnston D, Wu SM |publisher=MIT Press |year=1995 |isbn=978-0-262-10053-3}}</ref> When intrinsically active neurons are connected to each other in complex circuits, the possibilities for generating intricate temporal patterns become far more extensive.<ref name=Dayan/> A modern conception views the function of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact with each other to generate the full repertoire of behavior.<ref name=Simmons>{{Cite book |title=Nerve cells and animal behaviour |url=https://archive.org/details/nervecellsanimal02essimm |url-access=registration |chapter=Ch 1.: Introduction |publisher=Cambridge Univ. Press |year=1999 |isbn=978-0-521-62726-9 |vauthors=Simmons PJ, Young D}}</ref>

====Reflexes and other stimulus-response circuits<!-- This section is linked from [[Pain#Evolutionary and behavioral role]] -->====
[[File:Nervous system organization en.svg|thumb|right|400px|Simplified schema of basic nervous system function: signals are picked up by sensory receptors and sent to the spinal cord and brain, where processing occurs that results in signals sent back to the spinal cord and then out to motor neurons]]

The simplest type of neural circuit is a [[reflex arc]], which begins with a [[sensory system|sensory]] input and ends with a motor output, passing through a sequence of neurons connected in [[Series and parallel circuits|series]].<ref name=KandelCh36/> This can be shown in the "withdrawal reflex" causing a hand to jerk back after a hot stove is touched. The circuit begins with [[sensory receptor]]s in the skin that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes heat to change the electrical field across the membrane. If the change in electrical potential is large enough to pass the given threshold, it evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic contacts with other cells, some of which project (send axonal output) to the same region of the spinal cord, others projecting into the brain. One target is a set of spinal [[interneuron]]s that project to motor neurons controlling the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells. The excitatory signals induce contraction of the muscle cells, which causes the joint angles in the arm to change, pulling the arm away.

In reality, this straightforward schema is subject to numerous complications.<ref name=KandelCh36/> Although for the simplest [[reflex]]es there are short neural paths from sensory neuron to motor neuron, there are also other nearby neurons that participate in the circuit and modulate the response. Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex.

Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain.<ref name=KandelCh38/> For example, when an object in the periphery of the visual field moves, and a person looks toward it many stages of signal processing are initiated. The initial sensory response, in the retina of the eye, and the final motor response, in the [[oculomotor nuclei]] of the [[brainstem]], are not all that different from those in a simple reflex, but the intermediate stages are completely different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the [[thalamus]], [[cerebral cortex]], [[basal ganglia]], [[superior colliculus]], [[cerebellum]], and several brainstem nuclei. These areas perform signal-processing functions that include [[Feature detection (nervous system)|feature detection]], [[perception|perceptual]] analysis, [[memory recall]], [[decision-making]], and [[motor planning]].<ref name=KandelCh39/>

[[feature detection (nervous system)|Feature detection]] is the ability to extract biologically relevant information from combinations of sensory signals.<ref name=KandelCh21/> In the [[visual system]], for example, sensory receptors in the [[retina]] of the eye are only individually capable of detecting "points of light" in the outside world.<ref name=KandelCh25/> Second-level visual neurons receive input from groups of primary receptors, higher-level neurons receive input from groups of second-level neurons, and so on, forming a hierarchy of processing stages. At each stage, important information is extracted from the signal ensemble and unimportant information is discarded. By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties. The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord and in peripheral sensory organs such as the retina.

====Intrinsic pattern generation====
Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity. Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including [[attractor]] dynamics, periodicity, and even [[chaos theory|chaos]]. A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a [[central pattern generator]].

Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. One of the most important types of temporal pattern is [[circadian rhythm]]icity—that is, rhythmicity with a period of approximately 24 hours. All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as the sleep-wake cycle. Experimental studies dating from the 1990s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day. Animals as diverse as insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-day cues are available. The clock genes are expressed in many parts of the nervous system as well as many peripheral organs, but in mammals, all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the [[suprachiasmatic nucleus]].

===Mirror neurons===
{{Main|Mirror neuron}}
A [[mirror neuron]] is a neuron that [[action potential|fires]] both when an animal acts and when the animal observes the same action performed by another.<ref name="RizzolattiCraighero2004">{{Cite journal |last1=Rizzolatti |first1=Giacomo |last2=Craighero |first2=Laila |year=2004 |title=The mirror-neuron system |journal=Annual Review of Neuroscience |volume=27 |pages=169–192 |url=http://www.kuleuven.be/mirrorneuronsystem/readinglist/Rizzolatti%20&%20Craighero%202004%20-%20The%20MNS%20-%20ARN.pdf |doi=10.1146/annurev.neuro.27.070203.144230 |pmid=15217330|s2cid=1729870 }}</ref><ref name="Keysers 2009">{{Cite journal |doi=10.1016/j.cub.2009.08.026 |last=Keysers |first=Christian |year=2010 |title=Mirror Neurons |journal=Current Biology |volume=19 |issue=21 |pages=R971–973 |url=http://www.bcn-nic.nl/txt/people/publications/2009_Keysers_CurrentBiology.pdf |pmid=19922849 |s2cid=12668046 |url-status=dead |archive-url=https://web.archive.org/web/20130119224448/http://www.bcn-nic.nl/txt/people/publications/2009_Keysers_CurrentBiology.pdf |archive-date=19 January 2013}}</ref><ref name="EmpathicBrain">{{cite book |last=Keysers |first=Christian |title=The Empathic Brain |url=https://www.facebook.com/theempathicbrain |date=2011-06-23 |publisher=Kindle}}</ref> Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Such neurons have been directly observed in [[primate]] species.<ref name="architalbiol.org">{{Cite journal |last1=Rizzolatti |first1=Giacomo |last2=Fadiga |first2=Luciano |year=1999 |title=Resonance Behaviors and Mirror Neurons |journal=Italiennes de Biologie |volume=137 |issue=2–3 |pages=85–100 |pmid=10349488 |url=http://www.architalbiol.org/aib/article/view/575/532}}</ref> Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system.<ref name="architalbiol.org"/><ref>{{cite journal |last1=Akins |first1=Chana |last2=Klein |first2=Edward |year=2002 |title=Imitative Learning in Japanese Quail using Bidirectional Control Procedure |journal=Animal Learning & Behavior |volume=30 |issue=3 |pages=275–281 |doi=10.3758/bf03192836 |pmid=12391793 |doi-access=free}}</ref> In humans, brain activity consistent with that of mirror neurons has been found in the [[premotor cortex]], the [[supplementary motor area]], the [[primary somatosensory cortex]] and the [[parietal lobe|inferior parietal cortex]].<ref name="Molenberghs P, Cunnington R, Mattingley J 975–980">{{cite journal |vauthors=Molenberghs P, Cunnington R, Mattingley J |title=Is the mirror neuron system involved in imitation? A short review and meta-analysis. |journal=Neuroscience & Biobehavioral Reviews |volume=33 |issue=1 |pages=975–980 |date=July 2009 |doi=10.1016/j.neubiorev.2009.03.010 |pmid=19580913 |s2cid=25620637|url=https://research.monash.edu/en/publications/d8f4e6d1-41d6-4646-9012-2b7b39c8f89d }}</ref> The function of the mirror system is a subject of much speculation. Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception/action coupling (see the [[common coding theory]]).<ref name="EmpathicBrain"/> They argue that mirror neurons may be important for understanding the actions of other people, and for learning new skills by imitation. Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to [[theory of mind]] skills,<ref>{{cite web |first1=Christian |last1=Keysers |first2=Valeria |last2=Gazzola |title=Progress in Brain Research |year=2006 |url=http://www.bcn-nic.nl/txt/people/publications/keysersgazzolapbr.pdf |publisher=Bcn-nic.nl |url-status=dead |archive-url=https://web.archive.org/web/20070630021020/http://www.bcn-nic.nl/txt/people/publications/keysersgazzolapbr.pdf |archive-date=30 June 2007}}</ref><ref>Michael Arbib, ''[http://www.interdisciplines.org/coevolution/papers/11 The Mirror System Hypothesis. Linking Language to Theory of Mind] {{webarchive |url=https://web.archive.org/web/20090329093003/http://www.interdisciplines.org/coevolution/papers/11 |date=29 March 2009 }}'', 2005, retrieved 2006-02-17</ref> while others relate mirror neurons to [[language]] abilities.<ref>{{cite journal |last1=Théoret |first1=Hugo |last2=Pascual-Leone |first2=Alvaro |title=Language Acquisition: Do as You Hear |journal=Current Biology |volume=12 |issue=21 |pages=R736–R737 |year=2002 |pmid=12419204 |doi=10.1016/S0960-9822(02)01251-4 |s2cid=12867585 |doi-access=free|bibcode=2002CBio...12.R736T }}</ref> However, to date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions such as imitation.<ref name=Dinstein>{{cite journal |journal=Curr Biol |year=2008 |volume=18 |issue=1 |pages=R13–R18 |title=A mirror up to nature |vauthors=Dinstein I, Thomas C, Behrmann M, Heeger DJ |pmid=18177704 |doi=10.1016/j.cub.2007.11.004 |pmc=2517574|bibcode=2008CBio...18..R13D }}</ref> There are neuroscientists who caution that the claims being made for the role of mirror neurons are not supported by adequate research.<ref name="ncbi.nlm.nih.gov">{{cite journal |last=Hickok |first=G. |title=Eight Problems for the Mirror Neuron Theory of Action Understanding in Monkeys and Humans |journal=Journal of Cognitive Neuroscience |date=21 July 2009 |volume=21 |issue=7 |pages=1229–1243 |pmc=2773693 |pmid=19199415 |doi=10.1162/jocn.2009.21189}}</ref><ref name="else.econ.ucl.ac.uk">{{cite web |last=Heyes |first=Cecilia |title=Where do mirror neurons come from? |work=Neuroscience and Biobehavioral Reviews |year=2009 |url=http://else.econ.ucl.ac.uk/papers/uploaded/362.pdf |access-date=14 January 2015 |archive-url=https://web.archive.org/web/20120426062918/http://else.econ.ucl.ac.uk/papers/uploaded/362.pdf |archive-date=26 April 2012 |url-status=dead}}</ref>