Editing Nervous system (section)

===Comparative anatomy and evolution===
{{Main|Evolution of nervous systems}}

====Neural precursors in sponges====
[[Sponge]]s have no cells connected to each other by [[synapse|synaptic junctions]], that is, no neurons, and therefore no nervous system. They do, however, have [[homology (biology)|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 [[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====
[[Cnidaria|Jellyfish]], [[ctenophore|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 [[ganglion|ganglia]].<ref name=Ruppert/>

The development of the nervous system in [[radiata]] is relatively unstructured. Unlike [[bilaterians]], radiata only have two primordial cell layers, [[endoderm]] and [[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 [[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 ([[superphylum|superphyla]]) called [[protostomia|protostomes]] and [[deuterostome]]s.<ref name=Erwin/> Deuterostomes include vertebrates as well as [[echinoderm]]s, [[hemichordata|hemichordates]] (mainly acorn worms), and [[Xenoturbellida]]ns.<ref name=Bourlat/> Protostomes, the more diverse group, include [[arthropod]]s, [[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 [[Étienne Geoffroy Saint-Hilaire|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]]

[[Worm]]s are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, [[earthworm]]s have dual [[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 [[transverse plane|transverse]] nerves like the rungs of a ladder. These transverse nerves help [[coordinate]] the two sides of the animal. Two [[ganglion|ganglia]] at the head (the "[[nerve ring]]") end function similar to a simple [[brain]]. [[Simple eyes in arthropods|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 [[nematode]] ''[[Caenorhabditis elegans]]'', has been completely mapped out in a [[connectome]] including its synapses. Every neuron and its [[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]]

[[Arthropod]]s, such as [[insect]]s and [[crustacean]]s, have a nervous system made up of a series of [[ganglion|ganglia]], connected by a [[ventral nerve cord]] made up of two parallel connectives running along the length of the [[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 [[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 [[supraesophageal ganglion]]. In the [[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 [[Arthropod mouthparts|mouthparts]], the [[salivary glands]] and certain [[muscle]]s. Many arthropods have well-developed [[sense|sensory]] organs, including [[compound eye]]s for vision and [[antenna (biology)|antennae]] for [[olfaction]] and [[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 [[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 ''[[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 [[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 [[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 [[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 [[squid giant axon]] and [[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>