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Neuron
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==History== {{Further|History of neuroscience}} [[File:Golgi Hippocampus.jpg|left|thumb|Drawing by Camillo Golgi of a [[hippocampus]] stained using the [[silver nitrate]] method]] [[File:Purkinje cell by Cajal.png|thumb|Drawing of a Purkinje cell in the [[cerebellar cortex]] done by [[Santiago Ramón y Cajal]], demonstrating the ability of Golgi's staining method to reveal fine detail]] The neuron's place as the primary functional unit of the nervous system was first recognized in the late 19th century through the work of the Spanish anatomist [[Santiago Ramón y Cajal]].<ref name="López-Muñoz">{{cite journal | vauthors = López-Muñoz F, Boya J, Alamo C | 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 | issue = 4–6 | pages = 391–405 | date = October 2006 | pmid = 17027775 | doi = 10.1016/j.brainresbull.2006.07.010 | s2cid = 11273256 }}</ref> To make the structure of individual neurons visible, Ramón y Cajal improved a [[Golgi's method|silver staining process]] that had been developed by [[Camillo Golgi]].<ref name="López-Muñoz" /> The improved process involves a technique called "double impregnation" and is still in use. In 1888 Ramón y Cajal published a paper about the bird cerebellum. In this paper, he stated that he could not find evidence for [[anastomosis]] between axons and dendrites and called each nervous element "an autonomous canton."<ref name="López-Muñoz" /><ref name="finger">{{Cite book|title=Origins of neuroscience: a history of explorations into brain function|last=Finger|first=Stanley|publisher=Oxford University Press|year=1994|url=https://books.google.com/books?id=BdRqAAAAMAAJ&pg=PA47|isbn=9780195146943|oclc=27151391|page=47 |quote=Ramon y Cajal's first paper on the Golgi stain was on the bird cerebellum, and it appeared in the ''Revista''<nowiki> in 1888. He acknowledged that he found the nerve fibers to be very intricate, but stated that he could find no evidence for either axons or dendrites undergoing anastomosis and forming nets. He called each nervous element 'an autonomous canton.'</nowiki>}}</ref> This became known as the [[neuron doctrine]], one of the central tenets of modern [[neuroscience]].<ref name="López-Muñoz" /> In 1891, the German anatomist [[Heinrich Wilhelm Gottfried von Waldeyer-Hartz|Heinrich Wilhelm Waldeyer]] wrote a highly influential review of the neuron doctrine in which he introduced the term ''neuron'' to describe the anatomical and physiological unit of the nervous system.<ref>{{Cite book|title=Origins of neuroscience: a history of explorations into brain function|last=Finger|first=Stanley|publisher=Oxford University Press|year=1994|url=https://books.google.com/books?id=BdRqAAAAMAAJ&pg=PA47|isbn=9780195146943|oclc=27151391|page=47 |quote=... a man who would write a highly influential review of the evidence in favor of the neuron doctrine two years later. In his paper, Waldeyer (1891), ... , wrote that nerve cells terminate freely with end arborizations and that the 'neuron' is the anatomical and physiological unit of the nervous system. The word 'neuron' was born this way.}}</ref><ref>{{cite web|url=http://www.whonamedit.com/doctor.cfm/1846.html|title=Whonamedit - dictionary of medical eponyms|website=www.whonamedit.com|quote=Today, Wilhelm von Waldeyer-Hartz is remembered as the founder of the neurone theory, coining the term "neurone" to describe the cellular function unit of the nervous system and enunciating and clarifying that concept in 1891.}}</ref> The silver impregnation stains are a useful method for [[Neuroanatomy|neuroanatomical]] investigations because, for reasons unknown, it stains only a small percentage of cells in a tissue, exposing the complete micro structure of individual neurons without much overlap from other cells.<ref name="Grant">{{cite journal | vauthors = Grant G | title = How the 1906 Nobel Prize in Physiology or Medicine was shared between Golgi and Cajal | journal = Brain Research Reviews | volume = 55 | issue = 2 | pages = 490–8 | date = October 2007 | pmid = 17306375 | doi = 10.1016/j.brainresrev.2006.11.004 | s2cid = 24331507 }}</ref> ===Neuron doctrine=== [[File:PurkinjeCell.jpg|thumb|Drawing of neurons in the pigeon [[cerebellum]], by Spanish neuroscientist [[Santiago Ramón y Cajal]] in 1899. (A) denotes [[Purkinje cell]]s and (B) denotes [[granule cells]], both of which are multipolar.]] 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. Later discoveries yielded refinements to the doctrine. For example, [[Neuroglia|glial cells]], which are non-neuronal, play an essential role in information processing.<ref>{{cite journal | vauthors = Witcher MR, Kirov SA, Harris KM | title = Plasticity of perisynaptic astroglia during synaptogenesis in the mature rat hippocampus | journal = Glia | volume = 55 | issue = 1 | pages = 13–23 | date = January 2007 | pmid = 17001633 | doi = 10.1002/glia.20415 | citeseerx = 10.1.1.598.7002 | s2cid = 10664003 }}</ref> Also, electrical synapses are more common than previously thought,<ref>{{cite journal | vauthors = Connors BW, Long MA | title = Electrical synapses in the mammalian brain | journal = Annual Review of Neuroscience | volume = 27 | issue = 1 | pages = 393–418 | year = 2004 | pmid = 15217338 | doi = 10.1146/annurev.neuro.26.041002.131128 | url = https://zenodo.org/record/894386 }}</ref> comprising direct, cytoplasmic connections between neurons; In fact, neurons can form even tighter couplings: the squid giant axon arises from the fusion of multiple axons.<ref>{{cite journal | vauthors = Guillery RW | title = Observations of synaptic structures: origins of the neuron doctrine and its current status | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 360 | issue = 1458 | pages = 1281–307 | date = June 2005 | pmid = 16147523 | pmc = 1569502 | doi = 10.1098/rstb.2003.1459 }}</ref> Ramón y Cajal also postulated the Law of Dynamic Polarization, which states that a neuron receives signals at its dendrites and cell body and transmits them, as action potentials, along the axon in one direction: away from the cell body.<ref name="sabb">{{cite journal | vauthors = Sabbatini RM | date = April–July 2003 | url = http://www.cerebromente.org.br/n17/history/neurons3_i.htm | title = Neurons and Synapses: The History of Its Discovery | journal = Brain & Mind Magazine | pages = 17 }}</ref> The Law of Dynamic Polarization has important exceptions; dendrites can serve as synaptic output sites of neurons<ref>{{cite journal | vauthors = Djurisic M, Antic S, Chen WR, Zecevic D | title = Voltage imaging from dendrites of mitral cells: EPSP attenuation and spike trigger zones | journal = The Journal of Neuroscience | volume = 24 | issue = 30 | pages = 6703–14 | date = July 2004 | pmid = 15282273 | pmc = 6729725 | doi = 10.1523/JNEUROSCI.0307-04.2004 | hdl = 1912/2958 }} </ref> and axons can receive synaptic inputs.<ref>{{cite journal | vauthors = Cochilla AJ, Alford S | title = Glutamate receptor-mediated synaptic excitation in axons of the lamprey | journal = The Journal of Physiology | volume = 499 | issue = Pt 2 | pages = 443–57 | date = March 1997 | pmid = 9080373 | pmc = 1159318 | doi = 10.1113/jphysiol.1997.sp021940 }}</ref> ===Compartmental modelling of neurons=== Although neurons are often described as "fundamental units" of the brain, they perform internal computations. Neurons integrate input within dendrites, and this complexity is lost in models that assume neurons to be a fundamental unit. Dendritic branches can be modeled as spatial compartments, whose activity is related to passive membrane properties, but may also be different depending on input from synapses. [[Compartmental modelling of dendrites]] is especially helpful for understanding the behavior of neurons that are too small to record with electrodes, as is the case for ''Drosophila melanogaster''.<ref>{{cite journal | vauthors = Gouwens NW, Wilson RI | title = Signal propagation in Drosophila central neurons | journal = Journal of Neuroscience | volume = 29 | issue = 19 | pages = 6239–6249 | year = 2009 | pmid = 19439602 | pmc = 2709801 | doi = 10.1523/jneurosci.0764-09.2009 | doi-access = free }}</ref>
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