Difference between revisions of "Neuron" - New World Encyclopedia

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[[Image:PurkinjeCell.jpg|thumb|250px| Drawing by [[Santiago Ramón y Cajal]] of cells in the pigeon cerebellum. (A) Denotes [[Purkinje cell]]s, an example of a bipolar neuron. (B) Denotes [[granule cells]] which are multipolar.]]
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[[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.]]
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'''Neurons''' are a major class of [[cell (biology)|cells]] in the [[nervous system]].  Neurons are sometimes called nerve cells, though this term is technically imprecise since many neurons do not form nerves.  In [[vertebrate]]s, they are found in the [[brain]], the [[spinal cord]] and in the [[nerve]]s and [[ganglion|ganglia]] of the [[peripheral nervous system]], and their primary role is to process and transmit neural information. One important characteristic of neurons is that they have [[membrane potential|excitable membranes]] which allow them to generate and propagate [[electrical]] signals.  
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'''Neurons''' (also known as '''neurones''', '''nerve cells''' and '''nerve fibers''') are electrically excitable [[cell (biology)|cells]] in the [[nervous system]] that function to process and transmit information.  In [[vertebrate]] animals, neurons are the core components of the [[brain]], [[spinal cord]] and peripheral nerves.
  
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The concept of a neuron as the primary computational unit of the nervous system was devised by Spanish anatomist [[Santiago Ramón y Cajal]] in the early 20th century.  Cajal proposed that neurons were discrete cells which communicated with each other via specialized junctions. This became known as the [[Neuron doctrine|Neuron Doctrine]], one of the central tenets of modern neuroscience. However, it is important to note that Cajal would not have been able to observe the structure of individulal neurons and their processes, and in turn devise the Neuron Doctrine, if his rival, [[Camillo Golgi]], (for whom the [[Golgi Apparatus]] is named after) had not developed his highly specific silver staining method.
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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.
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When the Golgi Stain is applied to neurons, it binds the cell's [[microtubules]] and gives stained cells a black outline when light is shone through them.
 
  
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==Anatomy and histology==
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Neurons communicate via [[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.
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[[Image:Neuron.jpg|right|350px]]
 
  
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Many highly-specialized types of neurons exist, and these differ widely in appearance. Neurons have cellular extensions known as ''processes'' which they use to send and receive information.
 
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Neurons are highly asymmetric in shape, and consist of:
 
  
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*The [[soma_(biology)|soma]], or cell-body, is the central part of the cell between the dendrites and the axon. It is where the [[cell nucleus|nucleus]] is located and is where most [[protein synthesis]] occurs.
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==History==
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The neuron's role as the primary functional unit of the nervous system was first recognised in the early 20th century through the work of the Spanish anatomist [[Santiago Ramón y Cajal]]. Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells. This became known as the [[neuron doctrine]], one of the central tenets of modern neuroscience. To observe the structure of individual neurons, Cajal used [[Golgi's method|a silver staining method]] developed by his rival, [[Camillo Golgi]]. 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.
  
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*The [[dendrite]], a branching arbor of cellular extensions. Most neurons have multiple dendrites with profuse dendritic branches. The overall shape and structure of a neuron's dendrites is called its ''dendritic tree''.  The dendritic tree form has traditionally been thought to be the main information receiving network for the neuron.  However, information outflow (i.e. from dendrites to other neurons) can also occur.
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==Anatomy and histology==
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[[Image:Neuron.svg|thumb|350px|Strucure of a typical neuron]]
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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 in size from 4 to 100 micrometers in diameter.  <ref>[http://www.ualberta.ca/~neuro/OnlineIntro/NeuronExample.htm The Neuron: Size Comparison]</ref>
  
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*The [[axon]], a much finer, cable-like projection which may extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. This is the structure that carries nerve signals away from the neuron (and can carry in the other direction also). 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. Thus it has the most [[hyperpolarization | hyperpolarized]] [[action potential|action potential threshold]] of any part of the neuron.  In other words, it is the most easily-excitable part of the neuron, and thus serves as the spike initiation zone for the axon.  While the axon and axon hillock are generally considered places of information outflow, this region can receive input from other neurons as well.
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*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>
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*The '''axon terminal''', a specialized structure at the end of the axon that is used to release [[neurotransmitter]] and communicate with target neurons.
 
  
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Although the canonical view of the neuron is to assign strictly defined and dedicated functions to its various anatomical components, the fact that dendrites and axons very often act contrary to their so-called main function is but one small glimpse into the complex integrative capacity of every nerve cell. Nervous systems bear little resemblance to simple [[feed-forward]] [[Input/Output]] [[circuits]], and this understanding begins by appreciating the global signaling capacity of individual neurons.
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*The [[dendrites]] of a neuron are cellular extensions with many branches, and are referred to, therefore, as a dendritic tree. The overall shape and structure of a neuron's dendrites is called its ''dendritic tree'', and is where the majority of input to the neuron occurs. However, information outflow (i.e. from dendrites to other neurons) can also occur.
  
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Axons and dendrites in the central nervous system are typically only about a [[micrometre|micrometer]] thick, while some of those in the peripheral nervous system are much thicker. The soma is usually about 10&ndash;25 micrometers in diameter and 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, while [[giraffe]]s have single axons running along the whole length of their necks, several meters in length. Much of what we currently know about axonal function comes from studying the [[squid giant axon]], an ideal experimental preparation for research due to its relatively immense size (0.5&ndash;1 millimeters thick, several centimeters long).
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*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 greatest [[hyperpolarization|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 as well.
  
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==Classes==
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*The '''axon terminal''' a specialized structure at the end of the axon that is used to release [[neurotransmitter]] chemicals and communicate with target neurons.  
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'''Functional classification'''
 
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There are three functional classes of neurons: afferent neurons, efferent neurons, and interneurons.
 
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*[[Afferent neuron]]s convey information from tissues and organs into the [[central nervous system]].
 
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*[[Efferent neuron]]s transmit signals from the [[central nervous system]] to the [[effector cell]]s.
 
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*[[Interneuron]]s connect neurons within specific regions of the [[central nervous system]].
 
  
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''Afferent'' and ''efferent'' can also refer to neurons which convey information from one region of the brain to another.
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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.  
  
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'''Structural classification'''
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Axons and dendrites in the central nervous system are typically only about a [[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)...
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Most neurons can be anatomically characterized into one of three categories:
 
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*Unipolar or [[Pseudounipolar cells|Pseudounipolar]]- dendrite and axon emerging from same process.
 
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*[[Bipolar cell|Bipolar]] - single axon and single dendrite on opposite ends of the soma.
 
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*[[Multipolar neuron|Multipolar]] - more than two dendrites
 
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**[[pyramidal cell|Golgi I]]- Neurons with long-projecting axonal processes.
 
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**[[granule cell|Golgi II]]- Neurons whose axonal process projects locally.
 
  
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==Connectivity==
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==Classes==
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[[Image:SynapseIllustration.png|thumb|350px|Illustration of the major elements in a prototypical '''synapse'''. Synapses allow [[neuron|nerve cells]] to communicate with one another through [[axon]]s and [[dendrite]]s, converting [[action potential|electrical impulses]] into [[neurotransmitter|chemical]] signals.]]
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[[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] ]]
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===Structural classification===
  
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Neurons communicate with one another and to other cells through [[synapse]]s. '''Synapses''' are specialized junctions through which cells of the [[nervous system]] signal to one another and to non-neuronal cells such as [[muscle]]s or [[gland]]s. A synapse between a motor neuron and a muscle cell is called a [[neuromuscular junction]].
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Most neurons can be anatomically characterized as:
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*Unipolar or [[Pseudounipolar cells|Pseudounipolar]]: dendrite and axon emerging from same process.
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*[[Bipolar cell|Bipolar]]: single axon and single dendrite on opposite ends of the soma.
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*[[Multipolar neuron|Multipolar]]: more than two dendrites
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**[[pyramidal cell|Golgi I]]: neurons with long-projecting axonal processes.
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**[[granule cell|Golgi II]]: neurons whose axonal process projects locally.
  
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Neurons such as the [[Purkinje cell]]s in the [[cerebellum]], can have over 1000 dendrites each, enabling connections with tens of thousands of other cells. Synapses can either be [[EPSP|excitatory]] or [[IPSP|inhibitory]] and will either respectively increase or decrease activity in the target neuron. Neurons can also communicate via [[electrical synapse]]s, which are direct, electrically-conductive [[gap junction|junctions]] between cells.
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Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are basket, [[Betz cell|Betz]], [[Medium spiny neuron|medium spiny]], [[Purkinje cell|Purkinje]], [[pyramidal cell|pyramidal]] and  [[Renshaw cell|Renshaw]] cells.
  
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Synapses allow the [[neuron]]s of the [[central nervous system]] to form interconnected neural circuits.  They are thus crucial to the biological computations that underlie perception and thought. They also provide the means through which the nervous system connects to and controls the other systems of the body.
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===Functional classification===
  
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The [[human brain]] has a gigantic number of synapses. Each of 100 billion neurons has on average 7,000 synaptic connections to other neurons. Most authorities estimate total number of synapses at 1,000 trillion for a three-year-old child. This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 100 to 500 trillion synapses. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15985565&query_hl=1&itool=pubmed_docsum]
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*[[Afferent neuron]]s convey information from tissues and organs into the central nervous system.
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*[[Efferent neuron]]s transmit signals from the central nervous system to the [[effector cell]]s and are sometimes called motor neurons.
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*[[Interneuron]]s connect neurons within specific regions of the central nervous system.
  
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''Afferent'' and ''efferent'' can also refer to neurons which convey information from one region of the brain to another.
  
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The word "synapse" comes from "synaptein" which Sir [[Charles Scott Sherrington]] and his colleagues coined from the Greek "syn-" meaning "together" and "haptein" meaning "to clasp".
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'''Classification by action on other neurons'''
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*'''Excitatory neurons''' evoke [[EPSP|excitation]] of their target neurons. Excitatory neurons in the brain are often [[glutamate|glutamatergic]]. [[Spinal cord|Spinal]] [[motoneuron]]s use [[acetylcholine]] as their neurotransmitter.
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*'''Inhibitory neurons''' evoke [[IPSP|inhibition]] of 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]].
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*'''Modulatory neurons''' evoke more complex effects termed [[neuromodulation]]. These neurons use such neurotransmitters as [[dopamine]], [[acetylcholine]], [[serotonin]] and others.
  
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===Anatomy===
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'''Classification by discharge patterns'''<br>
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At a prototypical synapse, such as those found at [[dendritic spine]]s, a mushroom-shaped bud projects from each of two cells  and the caps of these buds press flat against one another.  At this interface, the [[biological membrane|membrane]]s of the two cells flank each other across a slender gap, the narrowness of which enables signalling molecules known as [[neurotransmitter]]s to pass rapidly from one cell to the other by [[diffusion]]. This gap, which is about 20 nm wide, is known as the '''synaptic cleft'''.
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Neurons can be classified according to their [[electrophysiology|electrophysiological]] characteristics:
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*'''Tonic or regular spiking'''. Some neurons are typically constantly (or tonically) active. Example: interneurons in neurostriatum.
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*'''Phasic or bursting'''. Neurons that fire in bursts are called phasic.
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*'''Fast spiking'''. Some neurons are notable for their fast firing rates, for example some types of cortical inhibitory interneurons, cells in globus pallidus.
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*'''Thin-spike'''. Action potentials of some neurons are more narrow compared to the others. For example, interneurons in prefrontal cortex are thin-spike neurons.
  
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Such synapses are asymmetric both in structure and in how they operate. Only the so-called '''pre-synaptic''' neuron secretes the neurotransmitter, which binds to [[transmembrane receptor|receptor]]s facing into the synapse from the
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'''Classification by neurotransmitter released'''
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'''post-synaptic''' cell. The pre-synaptic nerve terminal (also called the ''synaptic button'' or ''bouton'') generally buds from the tip of an [[axon]], while the post-synaptic target surface typically appears on a [[dendrite]], a cell body, or another part of a cell. The parts of synapses where neurotransmitter is released are called the '''active zones'''. At active zones the membranes of the two adjacent cells are held in close contact by [[cell adhesion]] proteins. Immediately behind the post-synaptic membrane is an elaborate complex of interlinked proteins called the [[postsynaptic density]].  Proteins in the postsynaptic density serve a myriad of roles, from anchoring and trafficking neurotransmitter receptors into the plasma membrane, to anchoring various proteins which modulate the activity of the receptors. The postsynaptic cell need not be a neuron.  Postsynaptic cells can also be [[gland]] or [[muscle]] cells.
 
  
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There also exists a less elaborate form of junction called an [[electrical synapse]], in which neurons are electrically coupled to each other via protein complexes called [[gap junction]]s.
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Some examples are cholinergic, GABA-ergic, glutamatergic and dopaminergic neurons.
  
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=== Signaling across chemical synapses ===
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==Connectivity==
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{{main|Synapse}}
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[[Image:Characteristics of AD.jpg|thumb|200px|Loss of connections between neurons (top middle) in [[Alzheimer's disease]].]]
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Neurons communicate with one another via [[synapse]]s, where the axon terminal of one cell impinges upon a dendrite or soma of another (or less commonly to an 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.
  
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The release of neurotransmitter is triggered by the arrival of a nerve impulse (or [[action potential]]) and occurs through an unusually rapid process of [[cellular secretion]]: Within the pre-synaptic nerve terminal, [[vesicle (biology)|vesicle]]s containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of [[second messenger|calcium ions]] through voltage-dependent, calcium-selective [[ion channel]]s. Calcium ions then trigger a biochemical cascade which results in vesicles fusing with the presynaptic-membrane and release their contents to the synaptic cleft. Receptors on the opposite side of the synaptic gap bind neurotransmitter molecules and respond by opening nearby ion channels in the post-synaptic cell membrane, causing ions to rush in or out and changing the local [[transmembrane potential]] of the cell. The resulting change in voltage is called a [[postsynaptic potential]]. The result is ''excitatory'', in the case of [[Depolarization|depolarizing]] currents, or ''inhibitory'' in the case of [[Hyperpolarization|hyperpolarizing]] currents.  Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel conduct the post-synaptic current, which in turn is a function of the type of receptors and neurotransmitter employed at the synapse.
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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.
  
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The [[human brain]] has a huge number of synapses. Each of the 10<sup>12</sup> neurons (1,000 billion, i.e. 1 trillion) has on average 7,000 synaptic connections to other neurons. Most authors estimate that the brain of a three-year-old child has about 10<sup>16</sup> synapses (10,000 trillion). This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 10<sup>15</sup> to 5 x 10<sup>15</sup> synapses (1,000 to 5,000 trillion). <ref>{{cite journal | author = Drachman D | title = Do we have brain to spare? | journal = Neurology | volume = 64 | issue = 12 | pages = 2004-5 | year = 2005 | id = PMID 15985565}} </ref>
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=== Integration of synaptic inputs ===
 
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Generally, if an excitatory synapse is strong, an action potential in the pre-synaptic neuron will trigger another in the post-synaptic cell; whereas at a weak synapse the [[excitatory postsynaptic potential|excitatory post-synaptic potential ("EPSP")]] will not reach the [[action potential|threshold]] for action potential initiation. In the brain, however, each neuron typically connects or synapses to many others, and likewise each receives synaptic inputs from many others. When action potentials fire simultaneously in several neurons that weakly synapse on a single cell, they may initiate an impulse in that cell even though the synapses are weak. On the other hand, a pre-synaptic neuron releasing an inhibitory neurotransmitter such as [[GABA]] can cause [[inhibitory postsynaptic potential]] in the post-synaptic neuron, decreasing its excitability and therefore decreasing the neuron's likelihood to fire an action potential. In this way the output of a neuron may depend on the input of many others, each of which may have a different degree of influence, depending on the strength of its synapse with that neuron. [[John Carew Eccles]] performed some of the important early experiments on synaptic integration, for which he received the [[Nobel Prize for Physiology or Medicine]] in 1963. Complex input/output relationships form the basis of [[transistor]]-based computations in [[computer]]s, and are thought to figure similarly in neural circuits.
 
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=== Detailed properties and regulation ===
 
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Following fusion of the synaptic vesicles and release of transmitter molecules into the synaptic cleft, the neurotransmitter is rapidly cleared from the space for recycling by specialized membrane proteins in the pre-synaptic or post-synaptic membrane.  This "[[reuptake|re-uptake]]" prevents "[[desensitization]]" of the post-synaptic receptors and ensures that succeeding action potentials will elicit the same size EPSP. The necessity of re-uptake and the phenomenon of desensitization in receptors and ion channels means that the strength of a synapse may in effect diminish as a train of action potentials arrive in rapid succession—a phenomenon that gives rise to the so-called '''frequency dependence''' of synapses. The nervous system exploits this property for computational purposes, and can tune its synapses through such means as [[phosphorylation]] of the proteins involved. The size, number and replenishment rate of vesicles also are subject to regulation, as are many other elements of synaptic transmission. For example, a class of drugs known as selective serotonin re-uptake inhibitors or [[Selective serotonin reuptake inhibitor|SSRI]]s affect certain synapses by inhibiting the re-uptake of the neurotransmitter [[serotonin]].  In contrast, one important excitatory neurotransmitter, [[acetylcholine]], does not undergo re-uptake, but instead is removed from the synapse by the action of the enzyme [[acetylcholinesterase]].
 
  
 
==Adaptations to carrying action potentials==
 
==Adaptations to carrying action potentials==
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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 known as an [[action potential]].  
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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'').
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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]].  
  
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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 might be used to better understand nerve cells [http://faculty.washington.edu/chudler/hist.html]. Since they are much larger than human neurons, but similar in nature, it was easier to study them with less advanced technology at that time. By inserting [[electrophysiology|electrodes]] into the giant squid axons, accurate measurements could be made of the [[membrane potential]].
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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>
  
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Electrical activity can be produced in neurons by a number of stimuli. [[Mechanoreceptor|Pressure]], stretch, [[neurotransmitter|chemical transmitters]], and electrical current passing across the nerve membrane as a result of a potential difference in voltage all can initiate nerve activity [http://physioweb.med.uvm.edu/cardiacep/EP/nervecells.htm].
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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&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 abnormal demyelination of peripheral nerves. Neurons with demyelinated axons do not conduct electrical signals properly.
  
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The narrow cross-section of axons lessens the metabolic expense of carrying [[action potential]]s, however thicker axons convey the impulses more rapidly. In order to minimize metabolic expense yet maintain a rapid conduction velocity, 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 the action potentials to travel [[saltatory conduction|faster]] than in unmyelinated axons of the same diameter whilst simultaneously spending less energy to "recharge" the action potential after. 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 which results from abnormal demyelination of peripheral nerves.  Neurons with demyelinated axons do not conduct electrical signals properly.
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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.
  
 
== Histology and internal structure ==
 
== Histology and internal structure ==
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[[Image:NisslHippo2.jpg|thumb|250px|Image of a Nissl-stained histological section through the [[rodent]] [[hippocampus]] showing various classes of cells.]]
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[[Image:NisslHippo2.jpg|thumb|250px|Image of a Nissl-stained histological section through the rodent [[hippocampus]] showing various classes of cells.]]
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[[Image:NeuronGolgi.png|thumb|250px|Golgi-stained neurons in the somatosensory cortex of the macaque monkey.]]
  
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Nerve cell bodies stained with basophilic dyes will 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 [[ribosomes]]. 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]].
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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 [[ribosomes]]. 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).
 
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).
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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.
  
 
==Challenges to the neuron doctrine==
 
==Challenges to the neuron doctrine==
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While the '''neuron doctrine''' has remained a central tenet of modern neuroscience, recent studies challenging this view have suggested that the narrow confines of this doctrine need to be expanded.
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The '''[[neuron doctrine]]''' is a central tenet of modern neuroscience, but recent studies suggest that this doctrine needs to be revised.  
  
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Among the most serious challenges to the neuron doctrine is the fact that [[electrical synapse]]s are more common in the central nervous system than previously thought. This means that rather than functioning as individual units, in some parts of the brain large ensembles of neurons may be active together in order to process neural information.  
+
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 = | id = PMID 15217338}} </ref>
  
−
A second challenge comes from the fact that [[dendrites]], like [[axons]], also have [[voltage-gated ion channel]]s and can generate [[membrane potential|electrical potentials]] which convey 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.  
+
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 | id = PMID 15282273}}
 +
</ref>.
  
−
Third, the role of [[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: It has been estimated that glial cells outnumber neurons by as many as 10:1. Recent experimental results have suggested that glial cells play a vital role in information processing among neurons, indicating that neurons may not be the sole information processing cells in the nervous system.
+
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 | id = PMID 17001633}}</ref>
  
−
Recent research has challenged the view that [[neurogenesis]], or the generation of new neurons, does not occur in adult primate brains. This research has shown that neurogenesis can be environment-dependent in addition to being age-related and is halted by survival-type stress factors. [http://www.seedmagazine.com/news/2006/02/the_reinvention_of_the_self.php?page=all&p=y] [http://www.princeton.edu/pr/news/99/q4/1014-brain.htm]
+
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 suggestive 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 | id = PMID 10521353}}</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 | id = PMID 16901981}}</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 | id = PMID 4203022}}</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 | id = PMID 11739948}}</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 | id = PMID 16917050}}</ref>.
  
 
==Neurons in the brain==
 
==Neurons in the brain==
−
The number of neurons contained within the brain varies dramatically across [[species]]. For example the human brain has about 100 billion (<math>10^{11}</math>) neurons and 100 trillion (<math>10^{14}</math>) connections ([[synapse|synapses]]) between them. In contrast, the nematode worm (''[[Caenorhabditis elegans]]'') has 302 neurons. Scientists have mapped all of the nematode's neurons. As a result, such worms are ideal candidates for neurobiological experiments and tests.  Many properties of neurons, ranging from the type of [[neurotransmitter]] used to [[ion channel]] composition are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems.
+
The number of neurons in the brain varies dramatically from species to species. The human brain has about 100 billion (<math>10^{11}</math>) neurons and 100 trillion (<math>10^{14}</math>) synapses. 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. 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.
  
  
 +
==See also==
 +
* [[Artificial neuron]]
 +
* [[Interneuron|Interneurons]]
 +
* [[Neural oscillations]]
 +
* [[Mirror neuron]]
 +
* [[Neuroscience]]
 +
* [[Neural network]]
 +
* [[Spindle neuron]]
  
 
==References==
 
==References==
 +
<div class="references-small">
 +
<references />
 +
</div>
 +
 
* Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. ''Principles of Neural Science'', 4th ed., McGraw-Hill, New York.
 
* 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.
 
* 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.
 
* Ramón y Cajal, S. 1933 ''Histology'', 10th ed., Wood, Baltimore.
−
* M.F. Bear, B.W. Connors, and M.A. Paradiso. 2001. ''Neuroscience: Exploring the Brain''. Baltimore: Lippincott. ISBN 0781739446
+
* Roberts A., Bush B.M.H. 1981. ''Neurones Without Impulses''. Cambridge University Press, Cambridge.
−
* [[Eric R. Kandel|Kandel ER]], Schwartz JH, Jessell TM. ''[[Principles of Neural Science]]'', 4th ed. McGraw-Hill, New York (2000). ISBN 0838577016
+
* Peters, A., Palay, S.L., Webster, H, D., 1991 ''The Fine Structure of the Nervous System'', 3rd ed., Oxford, New York.
−
* J.G. Nicholls, A.R. Martin, B.G. Wallace and P.A. Fuchs. "From Neuron to Brain". 4th ed. Sinauer Associates, Sunderland, MA. ISBN 0878924391
 
−
 
 
  
 +
==External links ==
  
−
==External links ==
 
 
* [http://ccdb.ucsd.edu/CCDB/index.shtml Cell Centered Database] UC San Diego images of neurons.
 
* [http://ccdb.ucsd.edu/CCDB/index.shtml Cell Centered Database] UC San Diego images of neurons.
 
* [http://brainmaps.org High Resolution Neuroanatomical Images of Primate and Non-Primate Brains].
 
* [http://brainmaps.org High Resolution Neuroanatomical Images of Primate and Non-Primate Brains].
 +
{{Nervous tissue}}
  
  
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{{credit2|Neuron|52680407|Synapse}}
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{{credit|101396606}}
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]

Revision as of 21:39, 17 January 2007

Drawing by Santiago Ramón y Cajal of neurons in the pigeon cerebellum. (A) Denotes Purkinje cells, an example of a bipolar neuron. (B) Denotes granule cells which are multipolar.

Neurons (also known as neurones, nerve cells and nerve fibers) are electrically excitable cells in the nervous system that function to process and transmit information. In vertebrate animals, neurons are the core components of the brain, spinal cord and peripheral nerves.

Neurons are typically composed of a soma, or cell body, a 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 via chemical and electrical synapses, 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 electrically excitable membrane of the neuron.


History

The neuron's role as the primary functional unit of the nervous system was first recognised in the early 20th century through the work of the Spanish anatomist Santiago Ramón y Cajal. Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells. This became known as the neuron doctrine, one of the central tenets of modern neuroscience. To observe the structure of individual neurons, Cajal used a silver staining method developed by his rival, Camillo Golgi. 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.

Anatomy and histology

Strucure of a typical 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 in size from 4 to 100 micrometers in diameter. [1]

  • The soma, is the central part of the neuron. It contains the nucleus of the cell, and therefore is where most protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter. [2]
  • The dendrites of a neuron are cellular extensions with many branches, and are referred to, therefore, as a dendritic tree. The overall shape and structure of a neuron's dendrites is called its dendritic tree, and is where the majority of input to the neuron occurs. However, information outflow (i.e. from dendrites to other neurons) can also occur.
  • 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 greatest hyperpolarized action potential threshold. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons as well.
  • The axon terminal a specialized structure at the end of the axon that is used to release neurotransmitter chemicals and 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 a 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. Giraffes 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 of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons. Source PLoS Biology [1]

Structural classification

Most neurons can be anatomically characterized as:

  • Unipolar or Pseudounipolar: dendrite and axon emerging from same process.
  • Bipolar: single axon and single dendrite on opposite ends of the soma.
  • Multipolar: more than two dendrites
    • Golgi I: neurons with long-projecting axonal processes.
    • Golgi II: neurons whose axonal process projects locally.

Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are basket, Betz, medium spiny, Purkinje, pyramidal and Renshaw cells.

Functional classification

  • Afferent neurons convey information from tissues and organs into the central nervous system.
  • Efferent neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor neurons.
  • Interneurons connect neurons within specific regions of the central nervous system.

Afferent and efferent can also refer to neurons which convey information from one region of the brain to another.

Classification by action on other neurons

  • Excitatory neurons evoke excitation of their target neurons. Excitatory neurons in the brain are often glutamatergic. Spinal motoneurons use acetylcholine as their neurotransmitter.
  • Inhibitory neurons evoke inhibition of 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.

Classification by discharge patterns
Neurons can be classified according to their 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.

Classification by neurotransmitter released

Some examples are cholinergic, GABA-ergic, glutamatergic and dopaminergic neurons.

Connectivity

File:Characteristics of AD.jpg
Loss of connections between neurons (top middle) in Alzheimer's disease.

Neurons communicate with one another via synapses, where the axon terminal of one cell impinges upon a dendrite or soma of another (or less commonly to an axon). Neurons such as Purkinje cells 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 excitatory or inhibitory and will either increase or decrease activity in the target neuron. Some neurons also communicate via electrical synapses, which are direct, electrically-conductive 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-gated calcium channels, allowing 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 receptors on the postsynaptic neuron.

The human brain has a huge number of synapses. Each of the 1012 neurons (1,000 billion, i.e. 1 trillion) has on average 7,000 synaptic connections to other neurons. Most authors estimate that the brain of a three-year-old child has about 1016 synapses (10,000 trillion). This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 1015 to 5 x 1015 synapses (1,000 to 5,000 trillion). [3]

Adaptations to carrying action potentials

The cell membrane in the axon and soma contain voltage-gated ion channels 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 axons. In 1937, John Zachary Young suggested that the giant squid axon can be used to study neuronal electrical properties. [4] 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 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. 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. [5]

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 glial cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The sheath enables action potentials to travel 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 nodes of Ranvier which contain a high density of voltage-gated ion channels. Multiple sclerosis is a neurological disorder that results from abnormal demyelination of peripheral nerves. Neurons with demyelinated axons do not conduct electrical signals properly.

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.

Histology and internal structure

File:NisslHippo2.jpg
Image of a Nissl-stained histological section through the rodent hippocampus showing various classes of cells.
File:NeuronGolgi.png
Golgi-stained neurons in the somatosensory cortex of the macaque monkey.

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 rough endoplasmic reticulum and associated ribosomes. 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 neurofilaments, which are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of catecholamines) 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.

Challenges to the neuron doctrine

The neuron doctrine is a central tenet of modern neuroscience, but recent studies suggest that this doctrine needs to be revised.

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. [6]

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. [7].

Third, the role of 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. [8]

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. [9] [10]. Of particularly compelling interest, Charles Gross and Elizabeth Gould provided evidence suggestive that neurogenesis occurred in neocortex after birth, in areas of the brain known to be important for cognitive function.[11] 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[12][13][14]. Rakic argues that the Princeton group's work has not been substantiated by multiple other groups[15].

Neurons in the brain

The number of neurons in the brain varies dramatically from species to species. The human brain has about 100 billion () neurons and 100 trillion () synapses. 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. 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.


See also

  • Artificial neuron
  • Interneurons
  • Neural oscillations
  • Mirror neuron
  • Neuroscience
  • Neural network
  • Spindle neuron

References
ISBN links support NWE through referral fees

  1. ↑ The Neuron: Size Comparison
  2. ↑ Brain Facts and Figures
  3. ↑ Drachman D (2005). Do we have brain to spare?. Neurology 64 (12): 2004-5. PMID 15985565.
  4. ↑ Milestones in Neuroscience Research
  5. ↑ Electrical activity of nerves
  6. ↑ Connors B, Long M. Electrical synapses in the mammalian brain.. Annu Rev Neurosci 27: 393-418. PMID 15217338.
  7. ↑ Djurisic M, Antic S, Chen W, Zecevic D (2004). Voltage imaging from dendrites of mitral cells: EPSP attenuation and spike trigger zones.. J Neurosci 24 (30): 6703-14. PMID 15282273.
  8. ↑ Witcher M, Kirov S, Harris K (2007). Plasticity of perisynaptic astroglia during synaptogenesis in the mature rat hippocampus.. Glia 55 (1): 13-23. PMID 17001633.
  9. ↑ The reinvention of the self
  10. ↑ Scientists Discover Addition of New Brain Cells in Highest Brain Area
  11. ↑ Gould E, Reeves A, Graziano M, Gross C (1999). Neurogenesis in the neocortex of adult primates.. Science 286 (5439): 548-52. PMID 10521353.
  12. ↑ Bhardwaj R, Curtis M, Spalding K, Buchholz B, Fink D, Björk-Eriksson T, Nordborg C, Gage F, Druid H, Eriksson P, Frisén J (2006). Neocortical neurogenesis in humans is restricted to development.. Proc Natl Acad Sci U S A 103 (33): 12564-8. PMID 16901981.
  13. ↑ Rakic P (1974). Neurons in rhesus monkey visual cortex: systematic relation between time of origin and eventual disposition.. Science 183 (123): 425-7. PMID 4203022.
  14. ↑ Kornack D, Rakic P (2001). Cell proliferation without neurogenesis in adult primate neocortex.. Science 294 (5549): 2127-30. PMID 11739948.
  15. ↑ Rakic P (2006). Neuroscience. No more cortical neurons for you.. Science 313 (5789): 928-9. PMID 16917050.
  • 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

Histology: nervous tissue

Neurons (gray matter): soma, axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament), dendrite (Nissl body, dendritic spine)
types (bipolar, pseudounipolar, multipolar, pyramidal, Purkinje, granule)

Afferent nerve/Sensory nerve/Sensory neuron (GSA, GVA, Type Ia sensory fiber), Efferent nerve/Motor nerve/Motor neuron (GSE, GVE, Alpha motor neuron, Gamma motoneurons, Upper motor neuron, Lower motor neuron), Interneuron (Renshaw)

Synapses: neuropil, boutons, synaptic vesicle, neuromuscular junction, electrical synapse

Sensory receptors: Free nerve ending, Meissner's corpuscle, Merkel nerve ending, Muscle spindle, Pacinian corpuscle, Ruffini ending, Olfactory receptor neuron, Photoreceptor, Hair cell, Taste bud

Glial cells: astrocyte, ependymal cells, microglia, radial glia

Myelination (white matter): Schwann cell, oligodendrocyte, nodes of Ranvier, internode, Schmidt-Lanterman incisures, neurolemma

closely related Connective tissue: epineurium, perineurium, endoneurium, nerve fascicle, meninges


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