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The '''nervous system''' is a [[network]] of specialized cells that communicate information about an animal's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of [[neuron]]s and other specialized cells called [[glia]], that aid in the function of the neurons. The nervous system is divided broadly into two categories; the [[peripheral nervous system]] and the [[central nervous system]]. Neurons generate and conduct [[impulse]]s between and within the two systems. The peripheral nervous system is composed of sensory neurons and the neurons that connect them to the [[nerve cord]], [[spinal cord]] and [[brain]], which make up the [[central nervous system]]. In response to stimuli, sensory neurons generate and propagate signals to the [[central nervous system]] which then process and conduct back signals to the [[muscle]]s and [[gland]]s. The neurons of the nervous systems of animals are interconnected in complex arrangements and use electrochemical signals and [[neurotransmitter]]s to transmit impulses from one neuron to the next. The interaction of the different neurons form neural circuits that regulate an organisms perception of the world and what is going on with its body, thus regulating its behavior.
Nervous systems are found in many multicellular [[animal|animals]] but differ greatly in complexity between species.<ref>{{cite encyclopedia | title = Nervous System| encyclopedia = Columbia Encyclopedia| volume =| pages =| publisher = Columbia University Press| date = | id = | accessdate = 4/1/08}}</ref>

[[Image:Nervous system diagram.png|thumb|The Human Nervous System. Red is CNS and blue is PNS.]]

== Nervous system in humans ==

[[Image:Structural.gif|thumb|right|Para-sagittal MRI of the brain([[central nervous system]])]]

The human Nervous system can be described both by [[gross anatomy]], (which describes the parts that are large enough to be seen with the [[naked eye]],) and by [[microanatomy]], (which describes the system at a [[cellular]] level.) In gross anatomy, the nervous system can be divided into distinct organs, these being stations through which the [[neural pathways]] cross. These organs can be divided into two systems: the [[central nervous system]] (CNS) and the [[peripheral nervous system]] (PNS).<ref>{{cite book
| last = Maton
| first = Anthea
| authorlink =
| coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright
| title = Human Biology and Health
| publisher = Prentice Hall
| date = 1993
| location = Englewood Cliffs, New Jersey, USA
| pages = 132-144
| url =
| doi =
| id =
| isbn = 0-13-981176-1}}</ref>

=== Central nervous system ===
{{Main|Central nervous system}}

{{See also|List of regions in the human brain}}

The central nervous system (CNS) is the largest part of the nervous system, and includes the [[brain]] and [[spinal cord]]. The [[spinal cavity]] holds and protects the spinal cord, while the [[head]] contains and protects the brain. The CNS is covered by the [[meninges]], a three layered protective coat. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae.
<center>
<table class="prettytable" cellpadding="1" cellspacing="0">
<tr>
<td rowspan="6"> [[Central nervous system|Central <br /> nervous <br /> system]]
<td rowspan="5">[[Brain]]</td>
<td rowspan="2">[[Prosencephalon]]</td>
<td>[[Telencephalon]]</td>
<td colspan="2">
[[Rhinencephalon]],
[[Amygdala]],
[[Hippocampus]],
[[Neocortex]],
[[Lateral ventricles]]
</td>
</tr>
<tr>

<td>[[Diencephalon]]</td>
<td colspan=2>
[[Epithalamus]],
[[Thalamus]],
[[Hypothalamus]],
[[Subthalamus]],
[[Pituitary gland]],
[[Pineal gland]],
[[Third ventricle]]
</td>
</tr>
<tr>
<td rowspan="3">[[Brain stem]]</td>
<td>[[Mesencephalon]]</td>
<td colspan="2">
[[Tectum]],
[[Cerebral peduncle]],
[[Pretectum]],
[[Mesencephalic duct]]
</td>
</tr>
<tr>
<td rowspan=2>[[Rhombencephalon]]</td>
<td> [[Metencephalon]] </td>
<td>
[[Pons]],
[[Cerebellum]],
</td>
</tr>
<tr>
<td>[[Myelencephalon]] </td>
<td>[[Medulla oblongata]]</td>
</tr>
<tr>
<td colspan="5" align="left">[[Spinal cord]] </td>
</tr>
</table>
</center>

=== Peripheral nervous system ===
{{Main|Peripheral nervous system}}

The PNS is a regional term for the collective nervous structures that do not lie in the CNS. The bodies of the nerve cells lie in the CNS, either in the brain or the spinal cord, and the longer of the cellular processes of these cells, known as axons, extend through the limbs and the flesh of the torso. The large majority the axons which are commonly called nerves, are considered to be PNS.

== Microanatomy ==

The nervous system is, on a small scale, primarily made up of [[neurons]]. However, [[glial cell]]s also play a major role.

=== Neurons ===
{{Main|Neuron}}

Neurons are electrically excitable cells in the nervous system that process and transmit information. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves. A number of different types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli effecting sensory organs and send signals to the spinal cord and brain, motor neurons receive signals from the brain and spinal cord and cause muscle contractions and affect glands, Interneurons connect neurons to other neurons within the brain and spinal cord.

=== Glial cells ===
{{Main|Glial cell}}

Glial cells are non-[[neuron]]al [[cell (biology)|cells]] that provide support and nutrition, maintain [[homeostasis]], form [[myelin]], and participate in signal transmission in the [[nervous system]]. In the [[human brain]], glia are estimated to outnumber neurons by about 10 to 1.<ref name="sfn">[http://www.sfn.org/index.cfm?pagename=brainBriefings_astrocytes sfn.org Society for Neuroscience, 2000]</ref>

Glial cells provide support and protection for [[neurons]]. They are thus known as the "glue" of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply [[nutrients]] and [[oxygen]] to neurons, to insulate one neuron from another, and to destroy [[pathogens]] and remove dead neurons.

=== Physiological division ===

[[Image:NSdiagram.png|thumb|right|400px|Diagram showing the major divisions of the nervous system.]]

A less anatomical but much more functional way of dividing of the human nervous system is classification according to the role that the different neural pathways play, regardless of whether or not they cross through the CNS/PNS:

The [[somatic nervous system]] is responsible for coordinating voluntary body movements (i.e. activities that are under conscious control).

The [[autonomic nervous system]] is responsible for coordinating involuntary functions, such as breathing and digestion.

In turn, these divisions of the nervous system can be further divided according to the direction in which they conduct nerve impulses:

* Afferent system by [[sensory neuron]]s, which carries impulses from a somatic receptor to the CNS
* Efferent system by [[motor neuron]]s, which carries impulses from the CNS to an effector
* Relay system by [[interneurons]] (also called "relay neurons"), which transmit impulses between the sensory and motor neurons (both in the CNS and PNS).

The junction between two neurons is called a [[synapse]]. There is a very narrow gap (about 20nm in width) between the neurons called the [[synaptic cleft]]. This is where an action potential (the "message" being carried by the neurons, also known as the nerve impulse) is transmitted from one neuron to the next. This is achieved by relaying the message across the synaptic cleft using [[neurotransmitters]], which [[Diffusion|diffuse]] across the gap. The neurotransmitters then bind to receptor sites on the neighboring ([[postsynaptic neuron|postsynaptic]]) neuron, which in turn produces its own electrical/nerve impulse. This impulse is sent to the next synapse, and the cycle repeats itself.

Nerve impulses are a change in ion balance between the inside and outside of a neuron. Because the nervous system uses a combination of electrical and chemical signals, it is incredibly fast. Although the chemical aspect of signaling is much slower than the electrical aspect, a nerve impulse is still fast enough that it is perceived as being instantaneous.<!-- Ed.: Original research, perhaps, but the reason a nerve impulse is perceived as instantaneous would appear to be due to the speed of the nervous system itself. That is, our nervous system cannot perceive the speed of its own transmissions. At least, most of the time. --> Speed is a necessary characteristic in order for an organism to quickly identify the presence of danger, and thus avoid injury/death. For example, a hand touching a hot stove. If the nervous system was only comprised of chemical signals, the nervous system would not be able to signal the arm to move fast enough to escape dangerous burns. Thus, the speed of the nervous system is evolutionarily valuable, and is in fact a necessity for life.

=== Development ===
{{Main|Neural development in humans}}

Some landmarks of [[embryo]]nic neural development include the birth and [[cellular differentiation|differentiation]] of [[neuron]]s from [[stem cells|stem cell]] precursors, the [[cellular migration|migration]] of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of [[axon]]s from neurons and [[axon guidance|guidance]] of the motile [[growth cone]] through the embryo towards postsynaptic partners, the generation of [[synapse]]s between these axons and their postsynaptic partners, and finally the lifelong [[synaptic plasticity|changes]] in synapses which are thought to underlie learning and memory.

=== Importance ===

The evolution of a complex nervous system makes it possible for various animal species to have advanced perception abilities like sight, complex social interactions, rapid coordination of other organ systems, and integrated processing of many concurrent signals. In humans, the advanced development of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other outcomes of human society that would not be possible without our brains.

Many people have lost basic motor skills and other skills because of spinal cord injuries. If this portion is damaged, the biggest nerve and the most important one get damaged. This leads to paralysis or other permanent damage. Physical lesions or genetic abnormalities of the brain can also lead to major harm.

=== Abilities ===

The nervous system is able to make basic motor skills and other skills possible. The basic [[5 senses]] of texture, taste, sight, smell, and hearing are powered by the nervous system. If disabled, basic motor skills may be lost.

== Non-humans ==

===Vertebrates===<!-- This section is linked from [[Lupus erythematosus]] -->
The nervous system of all [[vertebrate|vertebrate animals]], is often divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord.

=== Worms ===

[[Planaria]], a type of [[flatworm]], have dual [[Ventral nerve cord|nerve cord]]s running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by [[transverse]] nerves like the rungs of a ladder. These transverse nerves help [[coordinate]] the two sides of the animal. Two large [[ganglia]] at the head end function similar to a simple brain. [[Photoreceptor]]s on the animal's eyespots provide sensory information on light and dark.

The nervous system of the [[roundworm]] ''[[Caenorhabditis elegans]]'' has been mapped out to the cellular level. 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 [[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 [http://www.wormbook.org/chapters/www_specnervsys/specnervsys.html]

=== Arthropoda ===

[[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 [[belly]] [http://www.entomology.umn.edu/cues/4015/morpology/]. Typically, each body segment has one [[ganglion]] on each side, though some ganglia are fused to form the brain and other large ganglia [http://www.cals.ncsu.edu/course/ent425/tutorial/nerves.html].

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 [[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.

=== Development ===
{{Main|Neural development}}

Neural development in most species have many similarities [[neural development in humans]].

== References ==

<references/>

== External links ==

* [http://faculty.washington.edu/chudler/introb.html Neuroscience for Kids]
* [http://www.thehumanbrainproject.org The Human Brain Project Homepage]
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CNS.html Kimball's Biology Pages, CNS]
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html Kimball's Biology Pages, PNS]

{{Systems}}
{{organ systems}}
{{nervous system}}
{{Somatosensory system}}
{{Nervous tissue}}
{{Nervous system physiology}}
{{Diseases of the nervous system}}
{{Development of nervous system}}
{{Neurosurgical procedures}}

[[Category:Nervous system]]

[[ar:جهاز عصبي]]
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[[ca:Sistema nerviós]]
[[cs:Nervová soustava]]
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[[dv:ނާރުތަކުގެ ނިޒާމް]]
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[[el:Νευρικό σύστημα]]
[[es:Sistema nervioso]]
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[[it:Sistema nervoso]]
[[he:מערכת העצבים]]
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[[mk:Нервен систем]]
[[nl:Zenuwstelsel]]
[[ja:神経系]]
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[[pl:Układ nerwowy człowieka]]
[[pt:Sistema nervoso]]
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[[qu:Ankucha llika]]
[[ru:Нервная система]]
[[sq:Sistemi nervor]]
[[simple:Nervous system]]
[[sk:Nervové tkanivo]]
[[sl:Živčni sistem]]
[[sr:Нервни систем]]
[[fi:Hermosto]]
[[sv:Nervsystemet]]
[[tl:Sistemang nerbiyos]]
[[th:ระบบประสาท]]
[[tr:Sinir sistemi]]
[[uk:Нервова система]]
[[ur:عصبی نظام]]
[[yi:נערווען סיסטעם]]
[[zh:神经系统]]

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