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{{dablink|This article is about cells in the nervous system. For other uses of the term neuron, please see [[neuron (disambiguation)]].}}


[[Image:PurkinjeCell.jpg|thumb|250px| Drawing by [[Santiago Ramón y Cajal]] of neurons in the pigeon cerebellum. (A) Denotes [[Purkinje cell]]s, an example of a bipolar neuron. (B) Denotes [[granule cells]] which are multipolar.]]
'''Neurons''' ({{IPAEng|ˈnjuːɹɒn}}, also known as '''neurones''' and '''nerve cells''') are responsive [[cell (biology)|cells]] in the [[nervous system]] that process and transmit information by chemical [[signal]]s within the neuron. They 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 affecting cells of the sensory organs that then send signals to the spinal cord and brain. Motor neurons receive signals from the brain and spinal cord and cause [[muscle contraction]]s and affect [[gland]]s, Inter-neurons connect neurons to other neurons within the brain and spinal cord. Neurons respond to stimulus and communicate the presence of that [[stimuli]] to the central nervous system, which processes that information and sends responses to other parts of the body for action.


==Overview==
== October 2008 ==
[[Image:Information.svg|25px]] Welcome to Wikipedia. Although everyone is welcome to make constructive contributions to Wikipedia, at least one of your recent edits{{#if:Holy Cross School (Papatoetoe)|, such as the one you made to [[:Holy Cross School (Papatoetoe)]],}} did not appear to be constructive and has been removed. Please use [[Wikipedia:Sandbox|the sandbox]] for any test edits you would like to make, and read the [[Wikipedia:Welcoming committee/Welcome to Wikipedia|welcome page]] to learn more about contributing constructively to this encyclopedia. {{#if:|{{{2}}}|Thank you.}}<!-- Template:uw-vandalism1 --> [[User:RoryReloaded|Rory the vandal-fighter]] ([[User talk:RoryReloaded|talk]]) 08:24, 11 October 2008 (UTC)
{{Neuron map|[[Neuron]]}}
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The complexity and diversity in nervous systems is dependent on the interconnections between neurons, which rely on a limited number of different signals transited within the neurons to other neurons or to muscles and glands. The signals are produced and propagated by chemical ions that produce an electrical charge that moves along the neuron.

Neurons exist in a number of different shapes and sizes and can be classified by their morphology and function. The anatomist [[Camillo Golgi]] grouped neurons into two types; type I with long axons used to move signals over long distances and type II without axons. type I cells can be further divided by where the cell body or soma is located. The basic morphology of type I neurons, represented by spinal motor neurons, consists of a cell body called the [[Soma (biology)|soma]], a long thin [[axon]] which is covered by the [[myelin sheath]], one end has a tree branch like [[dendrite|dendritic tree]] that receives signals from other neurons, the other end has branching terminals that release transmitter secretions into an area called the gap junction between the terminals and the dendrites of next neuron.
The anatomy and the properties of the surface membrane determine the behavior of a neuron. The surface membrane is not uniform over the entire length of a neuron, but is modified in specific areas, some regions secrete transmitter substances while other areas respond to transmitter molecules. Other areas of the neuron membrane have passive electrical properties that effect capacitance and resistance. Within the neuron membrane there are gated ion channels that vary in type, the membrane of the axons include fast response sodium channels that are voltage-gated and are used to send rapid signals.
Neurons communicate by [[chemical synapse|chemical]] and [[electrical synapse]]s, in a process known as [[synaptic transmission]]. The fundamental process that triggers synaptic transmission is the [[action potential]], a propagating electrical signal that is generated by exploiting the [[membrane potential|electrically excitable membrane]] of the neuron. This is also known as a wave of depolarization.

Fully differentiated neurons are permanently [[mitosis|amitotic]]<ref>Nature Reviews Neuroscience 8, 368-378 (May 2007) | {{doi|10.1038/nrn2124}}</ref>; however, recent research shows that additional neurons throughout the brain can originate from neural [[stem cells]] found in high concentrations in (but throughout the brain) the [[subventricular zone]] and [[subgranular zone]] through the process of [[neurogenesis]].<ref>[http://www.wsu.edu/DrUniverse/brains.html WSU | Ask Dr. Universe | The BIG Questions<!-- Bot generated title -->]</ref><ref>http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fneuroscience%2Fans_006.html</ref><ref>[http://blog.sciam.com/index.php?title=where_new_neurons_go_to_work_1&more=1&c=1&tb=1&pb=1 Sciam Observations Scientific American Community<!-- Bot generated title -->]</ref><ref>[http://www.brainlightning.com/regen.html Brain Cell Regeneration Studies<!-- Bot generated title -->]</ref><ref>[http://www.princeton.edu/pr/pwb/99/0405/brain.htm Princeton - PWB 040599 - Do brain cells regenerate?<!-- Bot generated title -->]</ref><ref>[http://www.pastpeak.com/archives/2006/06/neurons_regener.htm Past Peak: Neurons Regenerate After All<!-- Bot generated title -->]</ref>

==History==

The neuron's place as the primary functional unit of the nervous system was first recognized in the early 20th century through the work of the Spanish anatomist [[Santiago Ramón y Cajal]].<ref name="López-Muñoz">{{cite journal
| last = López-Muñoz
| first = F.
| coauthors = Boya, J., Alamo, C.
| date = 16 October 2006
| title = Neuron theory, the cornerstone of neuroscience, on the centenary of the Nobel Prize award to Santiago Ramón y Cajal
| journal = Brain Research Bulletin
| volume = 70
| pages = 391–405
| pmid = 17027775
| doi = 10.1016/j.brainresbull.2006.07.010
| url = http://www.sciencedirect.com/science/article/B6SYT-4KMYRRC-1/2/b98a884961498c54452886842389ed72
| accessdate = 2007-04-02
}}</ref> Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells.<ref name="López-Muñoz" /> This became known as the [[neuron doctrine]], one of the central tenets of modern neuroscience.<ref name="López-Muñoz" /> To observe the structure of individual neurons, Cajal used [[Golgi's method|a silver staining method]] developed by his rival, [[Camillo Golgi]].<ref name="López-Muñoz" /> The Golgi stain is an extremely useful method for neuroanatomical investigations because, for reasons unknown, it stains a very small percentage of cells in a tissue, so one is able to see the complete micro structure of individual neurons without much overlap from other cells in the densely packed brain.<ref name="Grant">{{cite journal
| last = Grant
| first = Gunnar
| date = 9 January 2007 (online)
| title = How the 1906 Nobel Prize in Physiology or Medicine was shared between Golgi and Cajal
| journal = Brain Research Reviews
| pmid = 17027775
| doi = 10.1016/j.brainresrev.2006.11.004
| url = http://www.sciencedirect.com/science/article/B6SYS-4MSHXWR-1/2/51f3edfd18b81abbd0a9d56e98de6ceb
| accessdate = 2007-04-02
| volume = 55
| pages = 490
}}</ref>

==Anatomy and histology==
[[Image:Complete neuron cell diagram en.svg|thumb|right|500x350px|Diagram of a typical [[myelin]]ated [[vertebrate]] motoneuron.]]

Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter.<ref>[http://www.ualberta.ca/~neuro/OnlineIntro/NeuronExample.htm The Neuron: Size Comparison]</ref>

*The [[soma (biology)|soma]] is the central part of the neuron. It contains the [[cell nucleus|nucleus]] of the cell, and therefore is where most [[protein biosynthesis|protein synthesis]] occurs. The nucleus ranges from 3 to 18 micrometers in diameter.<ref> [http://faculty.washington.edu/chudler/facts.html Brain Facts and Figures]</ref>

*The [[dendrites]] of a neuron are cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a dendritic tree. This is where the majority of input to the neuron occurs. Information outflow (i.e. from dendrites to other neurons) can also occur, but not across chemical synapses; there, the backflow of a nerve impulse is inhibited by the fact that an axon does not possess chemoreceptors and dendrites cannot secrete neurotransmitter chemicals. This unidirectionality of a chemical synapse explains why nerve impulses are conducted only in one direction.

*The [[axon]] is a finer, cable-like projection which can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve signals away from the soma (and also carry some types of information back to it). Many neurons have only one axon, but this axon may - and usually will - undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the [[axon hillock]]. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in neurological terms it has the most negative [[action potential|action potential threshold]]. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.

*The '''axon terminal''' contains [[synapse]]s, specialized structures where [[neurotransmitter]] chemicals are released in order to communicate with target neurons.

Although the canonical view of the neuron attributes dedicated functions to its various anatomical components, dendrites and axons often act in ways contrary to their so-called main function.

Axons and dendrites in the central nervous system are typically only about one [[micrometre|micrometer]] thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10&ndash;25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human [[motoneuron]] can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the [[dorsal columns]], over 1.5 meters in adults. [[Giraffe]]s have single axons several meters in length running along the entire length of their necks. Much of what is known about axonal function comes from studying the [[squid giant axon]], an ideal experimental preparation because of its relatively immense size (0.5&ndash;1 millimeters thick, several centimeters long).

==Classes==
[[Image:GFPneuron.png|thumb|250px|right|Image of pyramidal neurons in mouse [[cerebral cortex]] expressing [[green fluorescent protein]]. The red staining indicates [[GABA]]ergic interneurons. Source PLoS Biology [http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040029] ]]

[[Image:smi32neuron.jpg|thumb|250px|right|SMI32-stained pyramidal neurons in [[cerebral cortex]].]]
===Structural classification===
====Polarity====
Most neurons can be anatomically characterized as:
*Unipolar or [[Pseudounipolar cells|pseudounipolar]]: dendrite and axon emerging from same process.
*[[Bipolar cell|Bipolar]]: axon and single dendrite on opposite ends of the soma.
*[[Multipolar neuron|Multipolar]]: more than two dendrites:
**[[Golgi I]]: neurons with long-projecting axonal processes; examples are [[pyramidal cell]]s, [[Purkinje cell]]s, and [[Anterior horn (spinal cord)|anterior horn]] cells.
**[[Golgi II]]: neurons whose axonal process projects locally; the best example is the [[granule cell]].

====Other====
Furthermore, some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:
*[[Basket cell]]s, neurons with dilated and knotty dendrites in the [[cerebellum]].
*[[Betz cell]]s, large motor neurons.
*[[Medium spiny neuron]]s, most neurons in the [[corpus striatum]].
*[[Purkinje cell]]s, huge neurons in the [[cerebellum]], a type of Golgi I multipolar neuron.
*[[Pyramidal cell]]s, neurons with triangular soma, a type of Golgi I.
*[[Renshaw cell]]s, neurons with both ends linked to [[alpha motor neuron]]s.
*[[Granule cell]]s, a type of as Golgi II neuron.
*[[Anterior horn (spinal cord)|anterior horn]] cells, [[motoneurons]] located in the spinal cord.

===Functional classification===
====Direction====
*[[Afferent neuron]]s convey information from tissues and organs into the central nervous system and are sometimes also called sensory neurons.
*[[Efferent neuron]]s transmit signals from the central nervous system to the [[effector cell]]s and are sometimes called motor neurons.
*[[Interneuron]]s connect neurons within specific regions of the central nervous system.

''Afferent'' and ''efferent'' can also refer generally to neurons which, respectively, bring information to or send information from the brain region.

====Action on other neurons====
*'''Excitatory neurons''' [[EPSP|excite]] their target neurons. Excitatory neurons in the central nervous system, including the brain, are often [[glutamate]]rgic. Neurons of the peripheral nervous system, such as [[Spinal cord|spinal]] [[motoneuron]]s that synapse onto muscle cells, often use [[acetylcholine]] as their excitatory neurotransmitter. However, this is just a general tendency that may not always be true{{Clarifyme|date=September 2008}}. It is not the neurotransmitter that decides excitatory or inhibitory action, but rather it is the postsynaptic receptor that is responsible for the action of the neurotransmitter{{Fact|date=September 2008}}.
*'''Inhibitory neurons''' [[IPSP|inhibit]] their target neurons. Inhibitory neurons are often interneurons. The output of some brain structures (neostriatum, globus pallidus, cerebellum) are inhibitory. The primary inhibitory neurotransmitters are [[GABA]] and [[glycine]].
*'''Modulatory neurons''' evoke more complex effects termed [[neuromodulation]]. These neurons use such neurotransmitters as [[dopamine]], [[acetylcholine]], [[serotonin]] and others.

====Discharge patterns====
Neurons can be classified according to their [[electrophysiology|electrophysiological]] characteristics:
*'''Tonic or regular spiking'''. Some neurons are typically constantly (or tonically) active. Example: interneurons in neurostriatum.
*'''Phasic or bursting'''. Neurons that fire in bursts are called phasic.
*'''Fast spiking'''. Some neurons are notable for their fast firing rates, for example some types of cortical inhibitory interneurons, cells in globus pallidus.
*'''Thin-spike'''. Action potentials of some neurons are more narrow compared to the others. For example, interneurons in prefrontal cortex are thin-spike neurons.

====Neurotransmitter released====
Some examples are

* cholinergic neurons
* GABAergic neurons
* glutamatergic neurons
* dopaminergic neurons
* [[5-hydroxytryptamine]] neurons (5-HT; serotonin)

==Connectivity==
{{main|Synapse}}
Neurons communicate with one another via [[synapse]]s, where the axon terminal or En passant boutons of one cell impinges upon another neuron's dendrite, soma or, less commonly, axon. Neurons such as [[Purkinje cell]]s in the [[cerebellum]] can have over 1000 dendritic branches, making connections with tens of thousands of other cells; other neurons, such as the magnocellular neurons of the [[supraoptic nucleus]], have only one or two dendrites, each of which receives thousands of synapses. Synapses can be [[EPSP|excitatory]] or [[IPSP|inhibitory]] and will either increase or decrease activity in the target neuron. Some neurons also communicate via [[electrical synapse]]s, which are direct, electrically-conductive [[gap junction|junctions]] between cells.

In a chemical synapse, the process of synaptic transmission is as follows: when an action potential reaches the axon terminal, it opens [[Voltage-dependent calcium channel|voltage-gated calcium channels]], allowing [[Calcium in biology|calcium ions]] to enter the terminal. Calcium causes synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane, releasing their contents into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate [[Receptor (biochemistry)|receptors]] on the postsynaptic neuron.

The [[human brain]] has a huge number of synapses. Each of the 10<sup>11</sup> (one hundred billion) neurons has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 10<sup>15</sup> synapses (1 quadrillion). This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 10<sup>14</sup> to 5 x 10<sup>14</sup> synapses (0.1 to 0.5 quadrillion).<ref>{{cite journal | author = Drachman D | title = Do we have brain to spare? | journal = Neurology | volume = 64 | issue = 12 | pages = 2004–5 | year = 2005 | pmid = 15985565 | doi = 10.1212/01.WNL.0000166914.38327.BB <!--Retrieved from CrossRef by DOI bot-->}}http://www.neurology.org/cgi/content/full/64/12/2004?ijkey=76566208aeb98565697fdfc4e202f9d7755a155e&keytype2=tf_ipsecsha</ref>

==Mechanisms for propagating action potentials==
[[Image:Neurons big1.jpg|thumb|right|300px|A signal propagating down an axon to the cell body and dendrites of the next cell.]]
In 1937, [[John Zachary Young]] suggested that the [[squid giant axon]] could be used to study neuronal electrical properties,<ref>[http://faculty.washington.edu/chudler/hist.html Milestones in Neuroscience Research]</ref> being larger but similar in nature to human neurons, they were easier to study. By inserting [[electrophysiology|electrodes]] into the giant squid axons, accurate measurements were made of the [[membrane potential]].

The cell membrane of the axon and soma contain [[voltage-gated ion channel]]s which allow the neuron to generate and propagate an electrical signal (an [[action potential]]). These signals are generated and propagated by charge-carrying ions including [[sodium]] (Na<sup>+</sup>), [[potassium]] (K<sup>+</sup>), [[chloride]] (Cl<sup>-</sup>), and [[calcium]] (Ca<sup>2+</sup>).

There are several stimuli that can activate a neuron leading to electrical activity, including [[Mechanoreceptor|pressure]], stretch, chemical transmitters, and changes of the electric potential across the cell membrane.<ref>[http://physioweb.med.uvm.edu/cardiacep/EP/nervecells.htm Electrical activity of nerves]</ref> Stimuli cause specific ion-channels within the cell membrane to open, leading to a flow of ions through the cell membrane, changing the membrane potential.

Thin neurons and axons require less metabolic expense to produce and carry action potentials, but thicker axons convey impulses more rapidly. To minimize metabolic expense while maintaining rapid conduction, many neurons have insulating sheaths of [[myelin]] around their axons. The sheaths are formed by [[glia]]l cells: [[oligodendrocyte]]s in the central nervous system and [[Schwann cell]]s in the peripheral nervous system. The sheath enables action potentials to travel [[saltatory conduction|faster]] than in unmyelinated axons of the same diameter, whilst using less energy. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1&nbsp;mm long, punctuated by unsheathed [[node of Ranvier|nodes of Ranvier]] which contain a high density of voltage-gated ion channels. [[Multiple sclerosis]] is a neurological disorder that results from demyelination of axons in the [[central nervous system]].

Some neurons do not generate action potentials, but instead generate a graded electrical signal, which in turn causes graded neurotransmitter release. Such nonspiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances.

==All-or-none principle==
The conduction of nerve impulses is an example of an [[all-or-none]] response. In other words, if a neuron responds at all, then it must respond completely. The greater the intensity of stimulation does not produce a stronger signal but can produces ''more'' impulses per second. There are different types of receptor response to stimulus, slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing. These tonic receptors most often respond to increased intensity of stimulus by increasing their firing frequency, usually as a power function of stimulus plotted against impulses per second. This can be likened to an intrinsic property of light where to get higher intensity of a light frequency, for example, color, there has to be more photons, as the photons can't become "stronger" for a specific color.

There are a number of other receptor types that are called quickly adapting or phasic receptors, where firing decreases or stops with steady stimulus, examples include; skin when touched by an object causes the neurons to fire, but if the object maintains even pressure against the skin the neurons stop firing. The neurons of the skin and muscles that are responsive to pressure and vibration have filtering accessory structures that aid their function. The [[pacinian corpuscle]] is one such structure, it has concentric layers like an onion which form around the axon terminal. When pressure is applied and the corpuscle is deformed, mechanical stimulus is transfered to the axon, which fires. If the pressure is steady there is no more stimulus, thus typically these neurons respond with a transient depolarization during the inital deformation and again when the pressure is removed which cause the corpuscle to change shape again. Other types of adaptation are important in extending the funchion of a number of other neurons.<ref>{{cite book | author=Eckert, Roger; Randall, David | authorlink= | coauthors= | title=Animal physiology: mechanisms and adaptations | date=1983 | publisher=W.H. Freeman | location=San Francisco | isbn=0-7167-1423-x | pages=239}}</ref>

== Histology and internal structure ==
[[Image:Gyrus Dentatus 40x.jpg|thumb|250px|Golgi-stained neurons in human [[hippocampus|hippocampal]] tissue.]]

Nerve cell bodies stained with basophilic dyes show numerous microscopic clumps of '''Nissl substance''' (named after German psychiatrist and neuropathologist [[Franz Nissl]], 1860&ndash;1919), which consists of rough [[endoplasmic reticulum]] and associated [[ribosomal RNA]]. The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large amounts of protein synthesis.

The cell body of a neuron is supported by a complex meshwork of structural proteins called '''[[neurofilament]]s''', which are assembled into larger '''neurofibrils'''. Some neurons also contain pigment granules, such as '''neuromelanin''' (a brownish-black pigment, byproduct of synthesis of [[catecholamine]]s) and '''[[lipofuscin]]''' (yellowish-brown pigment that accumulates with age).

There are different internal structural characteristics between axons and dendrites. Axons typically almost never contain [[ribosomes]], except some in the initial segment. Dendrites contain granular [[endoplasmic reticulum]] or [[ribosomes]], with diminishing amounts with distance from the cell body.

==The neuron doctrine==
The '''[[neuron doctrine]]''' is the now fundamental idea that neurons are the basic structural and functional units of the [[nervous system]]. The theory was put forward by [[Santiago Ramón y Cajal]] in the late 19th century. It held that neurons are discrete cells (not connected in a meshwork), acting as metabolically distinct units. Cajal further extended this to the '''Law of Dynamic Polarization''', which states that neural transmission goes only in one direction, from dendrites toward axons.<ref name="sabb">Sabbatini R.M.E. April-July 2003. [http://www.cerebromente.org.br/n17/history/neurons3_i.htm Neurons and Synapses: The History of Its Discovery]. ''Brain & Mind Magazine'', 17. Retrieved on [[March 19]], [[2007]].</ref>
As with all doctrines, there are some exceptions. For example [[glial cell]]s may also play a role in information processing.<ref>{{cite journal | author = Witcher M, Kirov S, Harris K | title = Plasticity of perisynaptic astroglia during synaptogenesis in the mature rat hippocampus | journal = Glia | volume = 55 | issue = 1 | pages = 13–23 | year = 2007 | pmid = 17001633 | doi = 10.1002/glia.20415 <!--Retrieved from CrossRef by DOI bot-->}}</ref> Also, [[electrical synapse]]s are more common than previously thought,<ref> {{cite journal | author = Connors B, Long M | title = Electrical synapses in the mammalian brain | journal = Annu Rev Neurosci | volume = 27 | issue = | pages = 393–418 | year = 2004| pmid = 15217338 | doi = 10.1146/annurev.neuro.26.041002.131128}}</ref> meaning that there are direct-cytoplasmic connections between neurons. In fact, there are examples of neurons forming even tighter coupling; the squid giant axon arises from the fusion of multiple neurons that retain individual cell bodies and the crayfish giant axon consists of a series of neurons with high conductance septate junctions. The Law of Dynamic Polarization also has important exceptions; dendrites can serve as synaptic output sites of neurons<ref>{{cite journal | author = Djurisic M, Antic S, Chen W, Zecevic D | title = Voltage imaging from dendrites of mitral cells: EPSP attenuation and spike trigger zones | journal = J Neurosci | volume = 24 | issue = 30 | pages = 6703–14 | year = 2004 | pmid = 15282273 | doi = 10.1523/JNEUROSCI.0307-04.2004 <!--Retrieved from CrossRef by DOI bot-->}}
</ref> and axons can receive synaptic inputs.

==Neurons in the brain==
The number of neurons in the brain varies dramatically from species to species.<ref name="nervenet">Williams, R and Herrup, K (2001). "The Control of Neuron Number." Originally published in ''The Annual Review of Neuroscience'' '''11''':423–453 (1988). Last revised Sept 28, 2001. Retrieved from http://www.nervenet.org/papers/NUMBER_REV_1988.html on May 12, 2007.</ref> One estimate puts the human brain at about 100 billion (<math>10^{11}</math>) neurons and 100 trillion (<math>10^{14}</math>) synapses.<ref name="nervenet"/> By contrast, the nematode worm ''[[Caenorhabditis elegans]]'' has just 302 neurons making it an ideal experimental subject as scientists have been able to map all of the organism's neurons. By contrast, the fruit fly ''[[Drosophila melanogaster]]'' has around 300,000 neurons and exhibits many complex behaviors. Many properties of neurons, from the type of neurotransmitters used to [[ion channel]] composition, are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems.

==Neurologic diseases==
'''[[Alzheimer's disease]]:''' Alzheimer's disease (AD), also known simply as Alzheimer's, is a neurodegenerative disease characterized by progressive cognitive deterioration together with declining activities of daily living and neuropsychiatric symptoms or behavioral changes. The most striking early symptom is loss of short-term memory (amnesia), which usually manifests as minor forgetfulness that becomes steadily more pronounced with illness progression, with relative preservation of older memories. As the disorder progresses, cognitive (intellectual) impairment extends to the domains of language (aphasia), skilled movements (apraxia), recognition (agnosia), and functions such as decision-making and planning get impaired.

'''[[Parkinson's disease]]:''' Parkinson's disease (also known as Parkinson disease or PD) is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills and speech. Parkinson's disease belongs to a group of conditions called movement disorders. It is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia), and in extreme cases, a loss of physical movement (akinesia). The primary symptoms are the results of decreased stimulation of the motor cortex by the basal ganglia, normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high level cognitive dysfunction and subtle language problems. PD is both chronic and progressive.

'''[[Myasthenia Gravis]]:''' Myasthenia gravis is a neuromuscular disease leading to fluctuating muscle weakness and fatigability. Weakness is typically caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy.

==Demyelination==

Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves. When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. This leads to certain neurodegenerative disorders like multiple sclerosis, chronic inflammatory demyelinating polyneuropathy.

==Axonal degeneration==

Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute axonal degeneration (AAD), which is rapid separation of the proximal and distal ends within 30 minutes of injury. Degeneration follows with swelling of the axolemma, and eventually leads to bead like formation. Granular disintegration of the axonal cytoskeleton and inner organelles occurs after axolemma degradation. Early changes include accumulation of mitochondria in the paranodal regions at the site of injury. Endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate. The disintegration is dependent on Ubiquitin and Calpain proteases (caused by influx of calcium ion), suggesting that axonal degeneration is an active process. Thus the axon undergoes complete fragmentation. The process takes about roughly 24 hrs in the PNS, and longer in the CNS. The signaling pathways leading to axolemma degeneration are currently unknown.

==Nerve regeneration==
Although neurons do not divide or replicate in most parts of the adult vertebrate brain, it is often possible for axons to regrow if they are severed. This can take a long time: #after a nerve injury to the human arm, for example, it may take months for feeling to return to the hands and fingers.{{Fact|date=October 2008}}

==References==
<div class="references-small">
<references />
</div>

==Sources==
* Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. ''Principles of Neural Science'', 4th ed., McGraw-Hill, New York.
* Bullock, T.H., Bennett, M.V.L., Johnston, D., Josephson, R., Marder, E., Fields R.D. 2005. ''The Neuron Doctrine, Redux'', Science, V.310, p. 791-793.
* Ramón y Cajal, S. 1933 ''Histology'', 10th ed., Wood, Baltimore.
* Roberts A., Bush B.M.H. 1981. ''Neurones Without Impulses''. Cambridge University Press, Cambridge.
* Peters, A., Palay, S.L., Webster, H, D., 1991 ''The Fine Structure of the Nervous System'', 3rd ed., Oxford, New York.

==External links ==
* [http://NeuronBank.org NeuronBank]an online neuromics tool for cataloging neuronal types and synaptic connectivity.
* [http://brainmaps.org High Resolution Neuroanatomical Images of Primate and Non-Primate Brains].

{{Commons}}
{{Nervous tissue}}

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[[Category:Nervous system]]
[[Category:Medical terms]]

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[[fr:Neurone]]
[[ga:Cillín néarach]]
[[ko:신경 세포]]
[[hr:Neuron]]
[[io:Neurono]]
[[id:Sel saraf]]
[[is:Taugafruma]]
[[it:Neurone]]
[[he:תא עצב]]
[[ka:ნეირონი]]
[[la:Neuron]]
[[lv:Neirons]]
[[lt:Neuronas]]
[[hu:Idegsejt]]
[[mk:Неврон]]
[[mr:चेतापेशी]]
[[nl:Zenuwcel]]
[[ja:神経細胞]]
[[no:Nevron]]
[[oc:Neuròna]]
[[pl:Neuron]]
[[pt:Neurónio]]
[[ro:Neuron]]
[[ru:Нейроны]]
[[simple:Neuron]]
[[sk:Neurón]]
[[sl:Nevron]]
[[sr:Неурон]]
[[fi:Neuroni]]
[[sv:Nervcell]]
[[th:เซลล์ประสาท]]
[[tr:Sinir hücresi]]
[[uk:Нейрон]]
[[ur:عصبون]]
[[yi:ניוראן]]
[[zh:神經元]]

Revision as of 08:24, 11 October 2008


October 2008

Welcome to Wikipedia. Although everyone is welcome to make constructive contributions to Wikipedia, at least one of your recent edits, such as the one you made to Holy Cross School (Papatoetoe), did not appear to be constructive and has been removed. Please use the sandbox for any test edits you would like to make, and read the welcome page to learn more about contributing constructively to this encyclopedia. Thank you. Rory the vandal-fighter (talk) 08:24, 11 October 2008 (UTC)

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