Neuron doctrine 3198778 215488329 2008-05-28T12:38:22Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. [[Image:CajalCerebellum.jpg|thumb|right|300px|Ramón y Cajal's drawing of the cells of the chick [[cerebellum]], from "Estructura de los centros nerviosos de las aves", Madrid, 1905.]] The '''neuron doctrine''' is the now fundamental idea that [[neuron]]s are the basic structural and functional units of the [[nervous system]]. The theory was first proposed by [[Santiago Ramón y Cajal]] and completed by the eminent [[Heinrich Wilhelm Gottfried von Waldeyer-Hartz]] in the late 19th century. It holds that neurons are discrete cells (not connected in a meshwork), which are metabolically distinct units with cell bodies (somata), [[axon]]s, and [[dendrite]]s. The '''Law of Dynamic Polarization''' further 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> ==History== Before the neuron doctrine was accepted, it was widely believed that the nervous system was a reticulum, or a connected meshwork, rather than a system made up of discrete [[cell (biology)|cells]].<ref>[[Eric Richard Kandel|Kandel E.R.]], Schwartz, J.H., Jessell, T.M. 2000. ''[[Principles of Neural Science]]'', 4th ed., Page 23. McGraw-Hill, New York.</ref> This theory, the [[reticular theory]], held that neurons' [[soma (biology)|somata]] mainly provided nourishment for the system.<ref>DeFelipe J. 1998. [http://www.psu.edu/nasa/cajal.htm Cajal]. ''MIT Encyclopedia of the Cognitive Sciences'', MIT Press, Cambridge, Mass.</ref> [[Image:Cajal Retina.jpg|thumb|right|200px|Drawing by [[Ramón y Cajal]] from "Structure of the Mammalian [[Retina]]" Madrid, 1900.]] The initial failure to accept the doctrine was due in part to inadequate ability to visualize cells using [[microscopes]], which were not developed enough to provide clear pictures of nerves. With the [[staining (microscopy)|cell staining]] techniques of the day, a slice of neural tissue appeared under a microscope as a complex web and individual cells were difficult to make out. Since neurons have a large number of [[neural process]]es an individual cell can be quite long and complex, and it can be difficult to find an individual cell when it is closely associated with many other cells. Thus, a major breakthrough for the neuron doctrine occurred in the late 1800s when [[Ramón y Cajal]] used a technique developed by [[Camillo Golgi]] to visualize neurons. The staining technique, which uses a silver solution, only stains one in about a hundred cells, effectively isolating the cell visually and showing that cells are separate and do not form a continuous web. Further, the cells that are stained are not stained partially, but rather all their processes are stained as well. [[Ramón y Cajal]] altered the staining technique and used it on samples from younger, less [[myelin]]ated brains, because the technique did not work on myelinated cells.<ref name="sabb"/> He was able to see neurons clearly and produce drawings like the one at right. For their technique and discovery respectively, Golgi and [[Ramón y Cajal]] shared the 1906 [[Nobel Prize in Physiology or Medicine]]. Golgi could not tell for certain that neurons were not connected, and in his acceptance speech he defended the reticular theory. [[Ramón y Cajal]], in ''his'' speech, contradicted that of Golgi and defended the now accepted neuron doctrine. A paper written in 1891 by [[Wilhelm von Waldeyer]], a supporter of [[Ramón y Cajal]], debunked the reticular theory and outlined the Neuron Doctrine. ==Updating the neuron doctrine== While the '''neuron doctrine''' is a central tenet of modern [[neuroscience]], recent studies suggest that there are notable exceptions and important additions to our knowledge about how neurons function. First, electrical synapses are more common in the central nervous system than previously thought. Thus, rather than functioning as individual units, in some parts of the brain large ensembles of neurons may be active simultaneously to process neural information.<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 = | pmid = 15217338 | doi = 10.1146/annurev.neuro.26.041002.131128}} </ref> Electrical synapses are formed by gap junctions that allow molecules to directly pass between neurons, creating a cytoplasm-to-cytoplasm connection. Second, dendrites, like axons, also have voltage-gated ion channels and can generate electrical potentials that carry information to and from the soma. This challenges the view that dendrites are simply passive recipients of information and axons the sole transmitters. It also suggests that the neuron is not simply active as a single element, but that complex computations can occur within a single neuron.<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}} </ref> Third, the role of [[Glial cell|glia]] in processing neural information has begun to be appreciated. Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons: glia outnumber neurons by as many as 10:1. Recent experimental results have suggested that glia play a vital 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}}</ref> Finally, recent research has challenged the historical view that [[neurogenesis]], or the generation of new neurons, does not occur in adult mammalian brains. It is now known that the adult brain continuously creates new neurons in the [[hippocampus]] and in an area contributing to the [[olfactory bulb]]. This research has shown that neurogenesis is environment-dependent (eg. exercise, diet, interactive surroundings), age-related, upregulated by a number of growth factors, and halted by survival-type stress factors.<ref>[http://www.seedmagazine.com/news/2006/02/the_reinvention_of_the_self.php?page=all&p=y The reinvention of the self]</ref><ref>[http://www.princeton.edu/pr/news/99/q4/1014-brain.htm Scientists Discover Addition of New Brain Cells in Highest Brain Area]</ref> Of particularly compelling interest, Charles Gross and Elizabeth Gould provided evidence suggesting that neurogenesis occurred in neocortex after birth, in areas of the brain known to be important for cognitive function.<ref>{{cite journal | author = Gould E, Reeves A, Graziano M, Gross C | title = Neurogenesis in the neocortex of adult primates. | journal = Science | volume = 286 | issue = 5439 | pages = 548–52 | year = 1999 | pmid = 10521353 | doi = 10.1126/science.286.5439.548}}</ref> Strong challenges to this work have come from more well-controlled studies by [[Pasko Rakic]] and others which support Rakic's original hypothesis that neurogenesis after birth is restricted to the olfactory bulb and hippocampus.<ref>{{cite journal | author = Bhardwaj R, Curtis M, Spalding K, Buchholz B, Fink D, Björk-Eriksson T, Nordborg C, Gage F, Druid H, Eriksson P, Frisén J | title = Neocortical neurogenesis in humans is restricted to development. | journal = Proc Natl Acad Sci U S A | volume = 103 | issue = 33 | pages = 12564–8 | year = 2006 | pmid = 16901981 | doi = 10.1073/pnas.0605177103}}</ref><ref>{{cite journal | author = Rakic P | title = Neurons in rhesus monkey visual cortex: systematic relation between time of origin and eventual disposition. | journal = Science | volume = 183 | issue = 123 | pages = 425–7 | year = 1974 | pmid = 4203022 | doi = 10.1126/science.183.4123.425}}</ref><ref>{{cite journal | author = Kornack D, Rakic P | title = Cell proliferation without neurogenesis in adult primate neocortex. | journal = Science | volume = 294 | issue = 5549 | pages = 2127–30 | year = 2001 | pmid = 11739948 | doi = 10.1126/science.1065467}}</ref> Rakic argues that the Princeton group's work has not been substantiated by multiple other groups.<ref>{{cite journal | author = Rakic P | title = Neuroscience. No more cortical neurons for you. | journal = Science | volume = 313 | issue = 5789 | pages = 928–9 | year = 2006 | pmid = 16917050 | doi = 10.1126/science.1131713}}</ref> ==References== <references/> * Bullock, T.H., Bennett, M.V.L., Johnston, D., Josephson, R., Marder, E., Fields R.D. 2005. "The Neuron Doctrine, Redux", ''Science'', Volume 310, Issue 5749, Pages 791-793. PMID 16272104. Retrieved on [[March 19]], [[2007]]. ==External links== * [http://zlab.rutgers.edu/classes/behaviorCogNeuro/HonorsLecture%201.htm The Neuron Doctrine] * [http://neurophilosophy.wordpress.com/2006/08/29/the-discovery-of-the-neuron/ The discovery of the neuron] [[Category:Neurons]] [[Category:History of neuroscience]] [[es:Doctrina de la neurona]] [[fr:Théorie du neurone]] [[sr:неуронска доктрина]]