View source for Biology:Nervous system

{{Short description|Part of an animal that coordinates actions and senses}}
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| Name        = Nervous system
| Latin       = systema nervosum
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| Caption     = The human nervous system
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In [[Biology:Biology|biology]], the '''nervous system''' is the [[Complex system|highly complex]] part of an [[Biology:Animal|animal]] that coordinates its actions and [[Philosophy:Sense|sensory]] information by transmitting [[Biology:Action potential|signals]] to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events.<ref>{{Cite book |title=Principles of Anatomy and Physiology |last1=Tortora |first1=G.J. |last2=Derrickson |first2=B. |publisher=J. Wiley |year=2016 |isbn=978-1-119-34373-8 |edition=15th}}</ref> [[Biology:Nervous tissue|Nervous tissue]] first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates, it consists of two main parts, the [[Biology:Central nervous system|central nervous system]] (CNS) and the [[Biology:Peripheral nervous system|peripheral nervous system]] (PNS). The CNS consists of the [[Biology:Brain|brain]] and [[Biology:Spinal cord|spinal cord]]. The PNS consists mainly of [[Biology:Nerve|nerve]]s, which are enclosed bundles of the long fibers, or [[Biology:Axon|axon]]s, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called [[Biology:Motor nerve|motor nerve]]s or ''[[Medicine:Efferent nerve fiber|efferent]]'' nerves, while those nerves that transmit information from the body to the CNS are called [[Biology:Sensory nerve|sensory nerve]]s or ''[[Biology:Afferent nerve fiber|afferent]]''. Spinal nerves are mixed nerves that serve both functions. The PNS is divided into three separate subsystems, the [[Biology:Somatic nervous system|somatic]], autonomic, and [[Biology:Enteric nervous system|enteric]] nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

At the cellular level, the nervous system is defined by the presence of a special type of cell, called the [[Biology:Neuron|neuron]]. Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical impulses traveling along thin fibers called [[Biology:Axon|axon]]s, which can be directly transmitted to neighboring cells through [[Biology:Electrical synapse|electrical synapse]]s or cause chemicals called [[Biology:Neurotransmitter|neurotransmitter]]s to be released at [[Biology:Chemical synapse|chemical synapse]]s. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise [[Biology:Neuromodulation|modulated]]. The connections between neurons can form [[Biology:Neural pathway|neural pathway]]s, [[Neural circuit|neural circuit]]s, and larger [[Biology:Large-scale brain networks|networks]] that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. Many of the cells and vasculature channels within the nervous system make up the [[Biology:Neurovascular unit|neurovascular unit]], which regulates cerebral blood flow in order to rapidly satisfy the high energy demands of activated neurons.<ref>{{Cite journal |last=Iadecola |first=Costantino |date=2017-09-27 |title=The Neurovascular Unit Coming of Age: A Journey through Neurovascular Coupling in Health and Disease |journal=Neuron |volume=96 |issue=1 |pages=17–42 |doi=10.1016/j.neuron.2017.07.030 |issn=1097-4199 |pmc=5657612 |pmid=28957666}}</ref>

Nervous systems are found in most multicellular animals, but vary greatly in complexity.<ref name=Columbia/> The only multicellular animals that have no nervous system at all are [[Biology:Sponge|sponge]]s, [[Biology:Placozoa|placozoa]]ns, and [[Biology:Mesozoa|mesozoa]]ns, which have very simple body plans. The nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include [[Biology:Sea anemone|anemones]], hydras, corals and [[Biology:Jellyfish|jellyfish]]) consist of a diffuse nerve net. All other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in [[Parallel (geometry)|parallel]]), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants.<ref>{{cite journal |vauthors=Herculano-Houzel S, Avelino-de-Souza K, ''et al''. |title=The elephant brain in numbers |journal=Front Neuroanat |volume=8 |pages=46 |year=2014 |pmid=24971054 |pmc=4053853 |doi=10.3389/fnana.2014.00046 |doi-access=free}}</ref>

The central nervous system functions to send signals from one cell to others, or from one part of the body to others and to receive feedback. Malfunction of the nervous system can occur as a result of genetic defects, physical damage due to trauma or toxicity, infection, or simply [[Biology:Senescence|senescence]]. The medical specialty of [[Biology:Neurology|neurology]] studies disorders of the nervous system and looks for interventions that can prevent or treat them. In the peripheral nervous system, the most common problem is the failure of nerve conduction, which can be due to different causes including [[Medicine:Diabetic neuropathy|diabetic neuropathy]] and demyelinating disorders such as multiple sclerosis and [[Medicine:Amyotrophic lateral sclerosis|amyotrophic lateral sclerosis]]. [[Biology:Neuroscience|Neuroscience]] is the field of science that focuses on the study of the nervous system.

==Structure==
The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the [[Biology:Axon|axon]]s of [[Biology:Neuron|neuron]]s), that emanate from the brain and [[Biology:Spinal cord|spinal cord]], and branch repeatedly to innervate every part of the body.<ref name=KandelCh2/> Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans,<ref name=FingerCh1>{{Cite book |title=Origins of neuroscience: a history of explorations into brain function |author=Finger S |chapter=Ch. 1: The brain in antiquity |publisher=Oxford Univ. Press |year=2001 |isbn=978-0-19-514694-3}}</ref> but their internal structure was not understood until it became possible to examine them using a microscope.<ref>Finger, pp. 43–50</ref> The author Michael Nikoletseas wrote:<ref>Nikoletseas Michael M. (2010) Behavioral and Neural Plasticity. {{ISBN|978-1-4537-8945-2}}</ref>
<blockquote>"It is difficult to believe that until approximately year 1900 it was not known that neurons are the basic units of the brain ([[Biography:Santiago Ramón y Cajal|Santiago Ramón y Cajal]]). Equally surprising is the fact that the concept of chemical transmission in the brain was not known until around 1930 ([[Biography:Henry Hallett Dale|Henry Hallett Dale]] and [[Biography:Otto Loewi|Otto Loewi]]). We began to understand the basic electrical phenomenon that neurons use in order to communicate among themselves, the action potential, in the 1950s ([[Biography:Alan Lloyd Hodgkin|Alan Lloyd Hodgkin]], [[Biography:Andrew Huxley|Andrew Huxley]] and [[Biography:John Eccles (neurophysiologist)|John Eccles]]). It was in the 1960s that we became aware of how basic neuronal networks code stimuli and thus basic concepts are possible (David H. Hubel and [[Biography:Torsten Wiesel|Torsten Wiesel]]). The molecular revolution swept across US universities in the 1980s. It was in the 1990s that molecular mechanisms of behavioral phenomena became widely known (Eric Richard Kandel)."</blockquote> A microscopic examination shows that nerves consist primarily of axons, along with different membranes that wrap around them and segregate them into [[Biology:Nerve fascicle|fascicles]]. The neurons that give rise to nerves do not lie entirely within the nerves themselves—their cell bodies reside within the brain, [[Biology:Spinal cord|spinal cord]], or peripheral [[Biology:Ganglion|ganglia]].<ref name=KandelCh2/>

All animals more advanced than sponges have nervous systems. However, even [[Biology:Sponge|sponge]]s, unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons.<ref name=Sakarya/> In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a nerve net, a diffuse network of isolated cells.<ref name=Ruppert/> In [[Biology:Bilateria|bilateria]]n animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the [[Earth:Ediacaran|Ediacaran]] period, over 550 million years ago.<ref name=Balavoine/><ref>{{Citation |last=Ortega-Hernandez |first=Javier |date=29 February 2016 |title=Our 500 million-year-old nervous system fossil shines a light on animal evolution |publisher=The Conversation US, Inc. |url=https://theconversation.com/our-500-million-year-old-nervous-system-fossil-shines-a-light-on-animal-evolution-55460 |access-date=6 March 2016}}</ref>

===Cells===
The nervous system contains two main categories or types of cells: [[Biology:Neuron|neuron]]s and glial cells.

====Neurons====
{{Neuron map}}

The nervous system is defined by the presence of a special type of cell—the [[Biology:Neuron|neuron]] (sometimes called "neurone" or "nerve cell").<ref name=KandelCh2/> Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via [[Biology:Synapse|synapse]]s, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical.<ref name=KandelCh2/> Many types of neuron possess an [[Biology:Axon|axon]], a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts;<ref name=KandelCh4/> axons typically extend throughout the body in bundles called nerves.

Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions.<ref name=KandelCh4/> These include [[Biology:Sensory neuron|sensory neuron]]s that transmute physical stimuli such as light and sound into neural signals, and motor neurons that transmute neural signals into activation of muscles or glands; however in many species the great majority of neurons participate in the formation of centralized structures (the brain and ganglia) and they receive all of their input from other neurons and send their output to other neurons.<ref name=KandelCh2/>

====Glial cells====
Glial cells (named from the Greek for "glue") are non-neuronal cells that provide support and [[Biology:Nutrition|nutrition]], maintain [[Homeostasis|homeostasis]], form [[Biology:Myelin|myelin]], and participate in signal transmission in the nervous system.<ref name=Allen2009/> In the [[Biology:Human brain|human brain]], it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.<ref name=Azevedo/> Among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy [[Biology:Pathogen|pathogen]]s and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets.<ref name=Allen2009/> A very important type of glial cell (oligodendrocytes in the central nervous system, and [[Biology:Schwann cell|Schwann cell]]s in the peripheral nervous system) generates layers of a fatty substance called [[Biology:Myelin|myelin]] that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently. Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within the central nervous system.

===Anatomy in vertebrates===<!-- This section is linked from [[Medicine:Lupus erythematosus|Lupus erythematosus]] -->
[[File:NSdiagram.svg|thumb|right|380px|Diagram showing the major divisions of the vertebrate nervous system.]]
[[File:Visible Human head slice.jpg|thumb|right|Horizontal section of the head of an adult female human, showing skin, skull, and brain with gray matter (brown in this image) and underlying white matter]]
The nervous system of [[Biology:Vertebrate|vertebrate]]s (including humans) is divided into the [[Biology:Central nervous system|central nervous system]] (CNS) and the [[Biology:Peripheral nervous system|peripheral nervous system]] (PNS).<ref name=KandelCh17/>

The (CNS) is the major division, and consists of the [[Biology:Brain|brain]] and the [[Biology:Spinal cord|spinal cord]].<ref name=KandelCh17/> The spinal canal contains the spinal cord, while the cranial cavity contains the brain. The CNS is enclosed and protected by the [[Medicine:Meninges|meninges]], a three-layered system of membranes, including a tough, leathery outer layer called the [[Medicine:Dura mater|dura mater]]. The brain is also protected by the skull, and the spinal cord by the [[Biology:Vertebra|vertebra]]e.

The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS.<ref name=Gray233/> The large majority of the axon bundles called nerves are considered to belong to the PNS, even when the cell bodies of the neurons to which they belong reside within the brain or spinal cord. The PNS is divided into [[Biology:Somatic nervous system|somatic]] and visceral parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal root ganglia of the spinal cord. The visceral part, also known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous system. Some authors also include sensory neurons whose cell bodies lie in the periphery (for senses such as hearing) as part of the PNS; others, however, omit them.<ref name=Hubbardvii>{{Cite book |title=The peripheral nervous system |author=Hubbard JI |publisher=Plenum Press |year=1974 |page=vii |isbn=978-0-306-30764-5 |url=https://archive.org/details/peripheralnervou0000hubb |url-access=registration}}</ref>

The vertebrate nervous system can also be divided into areas called gray matter and [[Biology:White matter|white matter]].<ref name=Purves15/> Gray matter (which is only gray in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons. White matter is composed mainly of [[Biology:Myelin|myelin]]ated axons, and takes its color from the myelin. White matter includes all of the nerves, and much of the interior of the brain and spinal cord. Gray matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. There is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a [[Biology:Nucleus (neuroanatomy)|nucleus]], whereas a cluster of neurons in the periphery is called a [[Biology:Ganglion|ganglion]].<ref name=DorlandsGanglion/> There are, however, a few exceptions to this rule, notably including the part of the forebrain called the [[Biology:Basal ganglia|basal ganglia]].<ref name=Afifi/>

{{Clear}}

===Comparative anatomy and evolution===

====Neural precursors in sponges====
[[Biology:Sponge|Sponge]]s have no cells connected to each other by [[Biology:Synapse|synaptic junctions]], that is, no neurons, and therefore no nervous system. They do, however, have [[Biology:Homology|homologs]] of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a [[Biology:Postsynaptic density|postsynaptic density]] (the signal-receiving part of a synapse).<ref name=Sakarya/> However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction.<ref name=Jacobs/>

====Radiata====
[[Biology:Cnidaria|Jellyfish]], comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete [[Biology:Ganglion|ganglia]].<ref name=Ruppert/>

The development of the nervous system in [[Biology:Radiata|radiata]] is relatively unstructured. Unlike bilaterians, radiata only have two primordial cell layers, [[Biology:Endoderm|endoderm]] and [[Biology:Ectoderm|ectoderm]]. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type.<ref name=Sanes3>{{Cite book |title=Development of the nervous system |url=https://archive.org/details/developmentnervo00sane |url-access=limited |publisher=Academic Press |year=2006 |isbn=978-0-12-618621-5 |pages=[https://archive.org/details/developmentnervo00sane/page/n17 3]–4 |vauthors=Sanes DH, Reh TA, Harris WA}}</ref>

====Bilateria====
[[File:Bilaterian-plan.svg|thumb|right|alt=A rod-shaped body contains a digestive system running from the mouth at one end to the anus at the other. Alongside the digestive system is a nerve cord with a brain at the end, near to the mouth. |Nervous system of a bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front]]

The vast majority of existing animals are [[Biology:Bilateria|bilateria]]ns, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appear as fossils beginning in the Ediacaran period, 550–600 million years ago.<ref name=Balavoine/> The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain".

[[File:Gray797.png|thumb|left|125px|Area of the human body surface innervated by each spinal nerve]]

Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain.<ref name=Ghysen>{{Cite journal |author=Ghysen A |title=The origin and evolution of the nervous system |journal=Int. J. Dev. Biol. |volume=47 |issue=7–8 |pages=555–562 |year=2003 |pmid=14756331 |url=http://www.ijdb.ehu.es/web/paper.php?doi=14756331 |citeseerx=10.1.1.511.5106}}</ref>

Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups (superphyla) called protostomes and [[Biology:Deuterostome|deuterostome]]s.<ref name=Erwin/> Deuterostomes include vertebrates as well as [[Biology:Echinoderm|echinoderm]]s, [[Biology:Hemichordata|hemichordates]] (mainly acorn worms), and [[Biology:Xenoturbellida|Xenoturbellida]]ns.<ref name=Bourlat/> Protostomes, the more diverse group, include [[Biology:Arthropod|arthropod]]s, [[Biology:Mollusc|mollusc]]s, and numerous phyla of "worms". There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve cord is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline.<ref name=Lichtneckert/>

====Worms====
[[File:Earthworm nervous system.png|thumb|250px|right|Earthworm nervous system. ''Top:'' side view of the front of the worm. ''Bottom:'' nervous system in isolation, viewed from above]]

Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, [[Biology:Earthworm|earthworm]]s have dual [[Biology:Ventral nerve cord|nerve cords]] running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by [[Biology:Transverse plane|transverse]] nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two [[Biology:Ganglion|ganglia]] at the head (the "nerve ring") end function similar to a simple [[Biology:Brain|brain]]. Photoreceptors on the animal's eyespots provide sensory information on light and dark.<ref name=Adey>{{Cite journal |author=ADEY WR |title=The nervous system of the earthworm Megascolex |journal=J. Comp. Neurol. |volume=94 |issue=1 |pages=57–103 |date=February 1951 |pmid=14814220 |doi=10.1002/cne.900940104 |s2cid=30827888}}</ref>

The nervous system of one very small roundworm, the [[Biology:Nematode|nematode]] ''[[Biology:Caenorhabditis elegans|Caenorhabditis elegans]]'', has been completely mapped out in a [[Connectome|connectome]] including its synapses. Every neuron and its [[Biology:Fate mapping|cellular lineage]] has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and female hermaphrodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In ''C. elegans'', males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons.<ref name=Wormbook/>

====Arthropods====
[[File:Spider internal anatomy-en.svg|thumb|right|250px|Internal anatomy of a spider, showing the nervous system in blue]]

[[Biology:Arthropod|Arthropod]]s, such as [[Biology:Insect|insect]]s and [[Biology:Crustacean|crustacean]]s, have a nervous system made up of a series of [[Biology:Ganglion|ganglia]], connected by a [[Biology:Ventral nerve cord|ventral nerve cord]] made up of two parallel connectives running along the length of the [[Biology:Abdomen|belly]].<ref name=Chapman>{{Cite book |title=The insects: structure and function |author=Chapman RF |publisher=Cambridge University Press |year=1998 |isbn=978-0-521-57890-5 |chapter=Ch. 20: Nervous system |pages=[https://archive.org/details/insectsstructure0000chap/page/533 533–568] |chapter-url=https://archive.org/details/insectsstructure0000chap/page/533}}</ref> Typically, each body segment has one [[Biology:Ganglion|ganglion]] on each side, though some ganglia are fused to form the brain and other large ganglia. The head segment contains the brain, also known as the [[Biology:Supraesophageal ganglion|supraesophageal ganglion]]. In the [[Biology:Insect#Nervous system|insect nervous system]], the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the [[Biology:Arthropod mouthparts|mouthparts]], the salivary glands and certain [[Biology:Muscle|muscle]]s. Many arthropods have well-developed [[Philosophy:Sense|sensory]] organs, including [[Biology:Compound eye|compound eye]]s for vision and [[Biology:Antenna|antennae]] for [[Biology:Olfaction|olfaction]] and [[Biology:Pheromone|pheromone]] sensation. The sensory information from these organs is processed by the brain.

In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling. A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of the insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called [[Biology:Neuropil|neuropil]], in the interior.<ref>Chapman, p. 546</ref>

==== Molluscs ====
{{excerpt|Mollusca|Nervous system}}

===="Identified" neurons====
A neuron is called ''identified'' if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties.<ref name=Hoyle>{{Cite book |title=Identified neurons and behavior of arthropods |publisher=Plenum Press |year=1977 |isbn=978-0-306-31001-0 |vauthors=Hoyle G, Wiersma CA}}</ref> In vertebrate nervous systems very few neurons are "identified" in this sense—in humans, there are believed to be none—but in simpler nervous systems, some or all neurons may be thus unique. In the roundworm ''[[Biology:Caenorhabditis elegans|C. elegans]]'', whose nervous system is the most thoroughly described of any animal's, every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm. One notable consequence of this fact is that the form of the ''C. elegans'' nervous system is completely specified by the genome, with no experience-dependent plasticity.<ref name=Wormbook/>

The brains of many molluscs and insects also contain substantial numbers of identified neurons.<ref name=Hoyle/> In vertebrates, the best known identified neurons are the gigantic [[Biology:Mauthner cell|Mauthner cell]]s of fish.<ref name=Stein38>{{Cite book |title=Neurons, Networks, and Motor Behavior |author=Stein PSG |publisher=MIT Press |year=1999 |isbn=978-0-262-69227-4 |pages=38–44}}</ref> Every fish has two Mauthner cells, in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a [[Biology:Mauthner cell#The C-start behavior|C-shape]], then straightens, thereby propelling itself rapidly forward. Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response individually, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response.

Mauthner cells have been described as [[Biology:Command neuron|command neuron]]s. A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually.<ref>Stein, p. 112</ref> Such neurons appear most commonly in the fast escape systems of various species—the [[Biology:Squid giant axon|squid giant axon]] and [[Biology:Squid giant synapse|squid giant synapse]], used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances.<ref name=Simmons43>{{Cite book |title=Nerve cells and animal behaviour |url=https://archive.org/details/nervecellsanimal00simm_659 |url-access=limited |publisher=Cambridge University Press |year=1999 |isbn=978-0-521-62726-9 |page=[https://archive.org/details/nervecellsanimal00simm_659/page/n53 43] |vauthors=Simmons PJ, Young D}}</ref>

==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 [[Biology:Hormone|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 [[Biology:Axon|axon]]s, although some types are capable of dendrite-to-dendrite communication. (In fact, the types of neurons called [[Biology:Amacrine cell|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 [[Biology:Action potential|action potential]]s, which produce cell-to-cell signals at points where [[Biology:Axon terminal|axon terminal]]s make [[Biology:Synapse|synaptic]] contact with other cells.<ref name=KandelCh9/>

Synapses may be electrical or chemical. [[Biology:Electrical synapse|Electrical synapse]]s make direct electrical connections between neurons,<ref name=Hormuzdi/> but [[Biology:Chemical synapse|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 [[Biology:Synaptic vesicle|synaptic vesicle]]s, packed with [[Biology:Neurotransmitter|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 [[Biology: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 [[Chemistry:Acetylcholine|acetylcholine]] at a synaptic contact between a motor neuron and a [[Biology:Muscle cell|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 [[Chemistry:Glutamic acid|glutamate]] or [[Chemistry:Gamma-Aminobutyric acid|GABA]], along with one or more [[Chemistry:Peptide|peptide]] neurotransmitters that play slower-acting modulatory roles. Molecular neuroscientists generally divide receptors into two broad groups: [[Biology:Ligand-gated ion channel|chemically gated ion channels]] and [[Biology:Second messenger system|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 [[Biology:Dale's principle|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, [[Chemistry:Glutamic acid|glutamate]] and [[Chemistry: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 traces 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 [[Biology:Long-term potentiation|long-term potentiation]] (abbreviated LTP), which operates at synapses that use the neurotransmitter [[Chemistry:Glutamic acid|glutamate]] acting on a special type of receptor known as the [[Biology:NMDA receptor|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 [[Philosophy:Reward system|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 [[Biology:Dopamine|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. 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 McCulloch and [[Biography:Walter Pitts|Walter Pitts]] showed in 1943 that even [[Artificial neural network|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 [[Biography:René Descartes|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> [[Biography: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 [[Philosophy:Behaviorism|Behaviorism]], the school of thought that dominated [[Social:Psychology|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 [[Biology:Electrophysiology|electrophysiology]], beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for maintaining [[Biology: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 [[Medicine:Pain#Evolutionary and behavioral role|Pain]] -->====
[[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 [[Biology:Reflex arc|reflex arc]], which begins with a sensory input and ends with a motor output, passing through a sequence of neurons connected in [[Engineering: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 receptors 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 [[Biology:Interneuron|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 [[Biology:Reflex|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 brain stem, 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 [[Biology:Feature detection (nervous system)|feature detection]], [[Perception|perceptual]] analysis, memory recall, [[Decision-making|decision-making]], and [[Philosophy:Motor planning|motor planning]].<ref name=KandelCh39/>

[[Biology:Feature detection (nervous system)|Feature detection]] is the ability to extract biologically relevant information from combinations of sensory signals.<ref name=KandelCh21/> In the [[Biology:Visual system|visual system]], for example, sensory receptors in the [[Biology:Retina|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|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|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 [[Biology:Circadian rhythm|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 [[Biology:Suprachiasmatic nucleus|suprachiasmatic nucleus]].

===Mirror neurons===
{{Main|Biology:Mirror neuron}}
A [[Biology:Mirror neuron|mirror neuron]] is a neuron that [[Biology: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 [[Biology:Primate|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 [[Biology:Premotor cortex|premotor cortex]], the [[Biology:Supplementary motor area|supplementary motor area]], the [[Biology:Primary somatosensory cortex|primary somatosensory cortex]] and the [[Biology: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|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 [[Philosophy:Theory of mind|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 [[Social:Language|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}}</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}}</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>

==Development==
{{Main|Biology:Development of the nervous system|Biology:Biology:Development of the nervous system in humans}}
In vertebrates, landmarks of embryonic neural development include the [[Biology:Neurogenesis|birth]] and [[Biology:Cellular differentiation|differentiation]] of [[Biology:Neuron|neuron]]s from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of [[Biology:Axon|axon]]s from neurons and [[Biology:Axon guidance|guidance]] of the motile [[Biology:Growth cone|growth cone]] through the embryo towards postsynaptic partners, the generation of [[Biology:Synapse|synapse]]s between these axons and their postsynaptic partners, and finally the lifelong [[Biology:Synaptic plasticity|changes]] in synapses which are thought to underlie learning and memory.<ref name=KandelCh52/>

All bilaterian animals at an early stage of development form a gastrula, which is polarized, with one end called the animal pole and the other the vegetal pole. The gastrula has the shape of a disk with three layers of cells, an inner layer called the [[Biology:Endoderm|endoderm]], which gives rise to the lining of most internal organs, a middle layer called the [[Biology:Mesoderm|mesoderm]], which gives rise to the bones and muscles, and an outer layer called the [[Biology:Ectoderm|ectoderm]], which gives rise to the skin and nervous system.<ref name=SanesCh1/>
{| align=center
| [[File:Gray17.png|thumb|center|250px|Human embryo, showing neural groove]]
| [[File:Development of the neural tube.png|thumb|center|425px|Four stages in the development of the neural tube in the human embryo]]
|}

In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate (along the midline) is destined to become the [[Biology:Central nervous system|central nervous system]] (CNS), the outer portion the [[Biology:Peripheral nervous system|peripheral nervous system]] (PNS). As development proceeds, a fold called the neural groove appears along the midline. This fold deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the [[Biology:Neural tube|neural tube]], whereas the future PNS appears as two strips of tissue called the [[Biology:Neural crest|neural crest]], running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and neural crest is known as neurulation.

In the early 20th century, a set of famous experiments by Hans Spemann and Hilde Mangold showed that the formation of nervous tissue is "induced" by signals from a group of mesodermal cells called the ''organizer region''.<ref name=KandelCh52/> For decades, though, the nature of neural induction defeated every attempt to figure it out, until finally it was resolved by genetic approaches in the 1990s. Induction of neural tissue requires inhibition of the gene for a so-called [[Biology:Bone morphogenetic protein|bone morphogenetic protein]], or BMP. Specifically the protein BMP4 appears to be involved. Two proteins called [[Biology:Noggin (protein)|Noggin]] and [[Biology:Chordin|Chordin]], both secreted by the mesoderm, are capable of inhibiting BMP4 and thereby inducing ectoderm to turn into neural tissue. It appears that a similar molecular mechanism is involved for widely disparate types of animals, including arthropods as well as vertebrates. In some animals, however, another type of molecule called Fibroblast Growth Factor or FGF may also play an important role in induction.

Induction of neural tissues causes formation of neural precursor cells, called [[Biology:Neuroblast|neuroblast]]s.<ref name=KandelCh53/> In drosophila, neuroblasts divide asymmetrically, so that one product is a "ganglion mother cell" (GMC), and the other is a neuroblast. A GMC divides once, to give rise to either a pair of neurons or a pair of glial cells. In all, a neuroblast is capable of generating an indefinite number of neurons or glia.

As shown in a 2008 study, one factor common to all bilateral organisms (including humans) is a family of secreted signaling molecules called [[Biology:Neurotrophin|neurotrophin]]s which regulate the growth and survival of [[Biology:Neuron|neuron]]s.<ref name=Zhu/> Zhu et al. identified DNT1, the first neurotrophin found in [[Biology:Fly|flies]]. DNT1 shares structural similarity with all known neurotrophins and is a key factor in the fate of neurons in [[Biology:Drosophila|Drosophila]]. Because neurotrophins have now been identified in both vertebrate and invertebrates, this evidence suggests that neurotrophins were present in an ancestor common to bilateral organisms and may represent a common mechanism for nervous system formation.

==Pathology==
{{Main|Biology:Neurology}}

[[File:CNS protection.jpg|thumb|267x267px|Layers protecting the brain and spinal cord.]]
The central nervous system is protected by major physical and chemical barriers. Physically, the brain and spinal cord are surrounded by tough [[Medicine:Meninges|meningeal]] membranes, and enclosed in the bones of the [[Biology:Skull|skull]] and [[Biology:Vertebral column|vertebral column]], which combine to form a strong physical shield. Chemically, the brain and spinal cord are isolated by the [[Biology:Blood–brain barrier|blood–brain barrier]], which prevents most types of chemicals from moving from the bloodstream into the interior of the CNS. These protections make the CNS less susceptible in many ways than the PNS; the flip side, however, is that damage to the CNS tends to have more serious consequences.

Although nerves tend to lie deep under the skin except in a few places such as the ulnar nerve near the elbow joint, they are still relatively exposed to physical damage, which can cause pain, loss of sensation, or loss of muscle control. Damage to nerves can also be caused by swelling or bruises at places where a nerve passes through a tight bony channel, as happens in [[Medicine:Carpal tunnel syndrome|carpal tunnel syndrome]]. If a nerve is completely transected, it will often regenerate, but for long nerves this process may take months to complete. In addition to physical damage, [[Medicine:Peripheral neuropathy|peripheral neuropathy]] may be caused by many other medical problems, including genetic conditions, metabolic conditions such as [[Medicine:Diabetes|diabetes]], inflammatory conditions such as [[Medicine:Guillain–Barré syndrome|Guillain–Barré syndrome]], vitamin deficiency, infectious diseases such as [[Medicine:Leprosy|leprosy]] or [[Medicine:Shingles|shingles]], or poisoning by toxins such as heavy metals. Many cases have no cause that can be identified, and are referred to as idiopathic. It is also possible for nerves to lose function temporarily, resulting in numbness as stiffness—common causes include mechanical pressure, a drop in temperature, or chemical interactions with [[Medicine:Local anesthetic|local anesthetic]] drugs such as [[Chemistry:Lidocaine|lidocaine]].

Physical damage to the spinal cord may result in [[Engineering:Paralysis|loss of sensation or movement]]. If an injury to the spine produces nothing worse than swelling, the symptoms may be transient, but if nerve fibers in the spine are actually destroyed, the loss of function is usually permanent. Experimental studies have shown that spinal nerve fibers attempt to regrow in the same way as nerve fibers, but in the spinal cord, tissue destruction usually produces scar tissue that cannot be penetrated by the regrowing nerves.

==See also==
* [[Biology:Circulatory system|Circulatory system]]
* Digestive system
* [[Biology:Muscular system|Muscular system]]
* [[Philosophy:Sentience|Sentience]]

==References==
{{Reflist|30em|refs=

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}}

==Further reading==
*[http://www.scholarpedia.org/article/Nervous_system Nervous system] William E. Skaggs, [[Scholarpedia]]

==External links==
{{Library resources box
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* (human)
* (non-human)
*[https://web.archive.org/web/20170708165531/http://thehumanbrainproject.org/ The Human Brain Project Homepage]

{{nervous system}}
{{Nervous tissue}}
{{Human systems and organs}}


{{DEFAULTSORT:Nervous System}}
[[Category:Nervous system| ]]

{{Sourceattribution|Nervous system}}