Neuron
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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|njˈɒɹɒns}}, also known as '''neurones''' and '''nerve cells''') are electrically excitable [[cell (biology)|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.
==Overview==
{{Neuron map|[[Neuron]]}}Fully developed neurons are permanently [[mitosis|amitotic]]<ref>Nature Reviews Neuroscience 8, 368-378 (May 2007) | {{doi|10.1038/nrn2124}}</ref> (they do not divide); however, additional neurons throughout the brain can originate from neural [[stem cells]] found in 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>
Neurons are typically composed of a [[Soma (biology)|soma]], or cell body, a [[dendrite|dendritic tree]] and an [[axon]]. The majority of vertebrate neurons receive input on the cell body and dendritic tree, and transmit output via the axon. However, there is great heterogeneity throughout the nervous system and the animal kingdom, in the size, shape and function of neurons.
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.
==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 microstructure 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|myelinated]] [[vertebrate]] neuron.]]
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 [[Hyperpolarization (biology)|hyperpolarized]] [[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 10–25 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 (0.5–1 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|GABAergic]] 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|glutamatergic]]. 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. 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.
*'''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 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.]]
The cell membrane in the axon and soma contain [[voltage-gated ion channel]]s which allow the neuron to generate and propagate an electrical impulse (an [[action potential]]).
Substantial early knowledge of neuron electrical activity came from experiments with [[squid giant axon]]s. In 1937, [[John Zachary Young]] suggested that the giant squid axon can be used to study neuronal electrical properties.<ref>[http://faculty.washington.edu/chudler/hist.html Milestones in Neuroscience Research]</ref> As they are much larger than human neurons, but similar in nature, it was easier to study them with the technology of that time. By inserting [[electrophysiology|electrodes]] into the giant squid axons, accurate measurements could be made of the [[membrane potential]].
Electrical activity can be produced in neurons by a number of stimuli. [[Mechanoreceptor|Pressure]], stretch, chemical transmitters, and electrical current passing across the nerve membrane as a result of a difference in voltage can all initiate nerve activity.<ref>[http://physioweb.med.uvm.edu/cardiacep/EP/nervecells.htm Electrical activity of nerves]</ref>
The narrow cross-section of axons lessens the metabolic expense of carrying 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 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. It is important to note that a greater intensity of stimulation produces ''more'' impulses per second, ''not'' a stronger impulse; in this way it is similar to a [[boolean function|boolean]] function in computer programming.{{Fact|date=January 2008}}
== 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]], 1860–1919), which consists of smooth [[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 (which do spike) 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.
==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].
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