Editing Neuron (section)

==Classification==
[[File:GFPneuron.png|thumb|Image of pyramidal neurons in mouse [[cerebral cortex]] expressing [[green fluorescent protein]]. The red staining indicates [[GABA]]ergic interneurons.<ref>{{cite journal | vauthors = Lee WC, Huang H, Feng G, Sanes JR, Brown EN, So PT, Nedivi E | title = Dynamic remodeling of dendritic arbors in GABAergic interneurons of adult visual cortex | journal = PLOS Biology | volume = 4 | issue = 2 | pages = e29 | date = February 2006 | pmid = 16366735 | pmc = 1318477 | doi = 10.1371/journal.pbio.0040029 |doi-access=free }}</ref>]]{{See also|List of distinct cell types in the adult human body#Nervous system}}
Neurons vary in shape and size and can be classified by their [[Morphology (biology)|morphology]] and function.<ref name="Al">{{cite book|last=Al|first=Martini, Frederic Et|title=Anatomy and Physiology' 2007 Ed.2007 Edition|year=2005 |url={{google books |plainurl=y |id=joJb82gVsLoC|page=288}}|publisher=Rex Bookstore, Inc.|isbn=978-971-23-4807-5|pages=288}}</ref> The anatomist [[Camillo Golgi]] grouped neurons into two types; type I with long axons used to move signals over long distances and type II with short axons, which can often be confused with dendrites. Type I cells can be further classified by the location of the soma. The basic morphology of type I neurons, represented by spinal [[motor neurons]], consists of a cell body called the soma and a long thin axon covered by a [[myelin sheath]]. The dendritic tree wraps around the cell body and receives signals from other neurons. The end of the axon has branching [[axon terminal]]s that release neurotransmitters into a gap called the [[synaptic cleft]] between the terminals and the dendrites of the next neuron.{{citation needed|date=July 2022}}

===Structural classification===

====Polarity====
[[File:Neurons uni bi multi pseudouni.svg|thumb|Different kinds of neurons:<br />1 [[Unipolar neuron]]<br />2 [[Bipolar neuron]]<br />3 [[Multipolar neuron]]<br />4 [[Pseudounipolar neuron]] ]]
Most neurons can be anatomically characterized as:<ref name="Betts">{{CC-notice|cc=by4|url=https://openstax.org/books/anatomy-and-physiology/pages/12-2-nervous-tissue}} {{cite book|last1=Betts|first1=J Gordon|last2=Desaix|first2=Peter|last3=Johnson|first3=Eddie|last4=Johnson|first4=Jody E|last5=Korol|first5=Oksana|last6=Kruse|first6=Dean|last7=Poe|first7=Brandon|last8=Wise|first8=James|last9=Womble|first9=Mark D|last10=Young|first10=Kelly A|title=Anatomy & Physiology|location=Houston|publisher=OpenStax CNX|isbn=978-1-947172-04-3|date=June 8, 2023|at=12.2 Nervous tissue}}</ref>
* [[Unipolar neuron|Unipolar]]: single process. Unipolar cells are exclusively sensory neurons. Their dendrites receive sensory information, sometimes directly from the stimulus itself. The cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function, so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system.
* [[Bipolar cell|Bipolar]]: 1 axon and 1 dendrite. They are found mainly in the [[olfactory epithelium]], and as part of the retina.
* [[Multipolar neuron|Multipolar]]: 1 axon and 2 or more dendrites
** [[Golgi I]]: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells
** [[Golgi II]]: neurons whose axonal process projects locally; the best example is the granule cell
* [[Anaxonic neuron|Anaxonic]]: where the axon cannot be distinguished from the dendrite(s)
* [[Pseudounipolar cells|Pseudounipolar]]: 1 process which then serves as both an axon and a dendrite

====Other====
Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:{{citation needed|date=July 2022}}
* [[Basket cell]]s, interneurons that form a dense plexus of terminals around the soma of target cells, found in the cortex and [[cerebellum]]
* [[Betz cell]]s, large motor neurons in [[primary motor cortex]]
* [[Lugaro cell]]s, interneurons of the cerebellum
* [[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
* [[Rosehip neuron|Rosehip cells]], unique human inhibitory neurons that interconnect with Pyramidal cells
* [[Renshaw cell]]s, neurons with both ends linked to [[alpha motor neuron]]s
* [[Unipolar brush cell]]s, interneurons with unique dendrite ending in a brush-like tuft
* [[Granule cell]]s, a type of Golgi II neuron
* [[Anterior horn (spinal cord)|Anterior horn]] cells, [[motoneurons]] located in the spinal cord
* [[Spindle neuron|Spindle cells]], interneurons that connect widely separated areas of the brain

===Functional classification===

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

Afferent and efferent also refer generally to neurons that, respectively, bring information to or send information from the brain.

====Action on other neurons====
A neuron affects other neurons by releasing a neurotransmitter that binds to [[receptor (biochemistry)|chemical receptor]]s. The effect on the postsynaptic neuron is determined by the type of receptor that is activated, not by the presynaptic neuron or by the neurotransmitter. Receptors are classified broadly as ''excitatory'' (causing an increase in firing rate), ''inhibitory'' (causing a decrease in firing rate), or ''modulatory'' (causing long-lasting effects not directly related to firing rate).{{citation needed|date=July 2022}}

The two most common (90%+) neurotransmitters in the brain, [[glutamate]] and [[GABA]], have largely consistent actions. Glutamate acts on several types of receptors and has effects that are excitatory at [[ionotropic receptor]]s and a modulatory effect at [[metabotropic receptor]]s. Similarly, GABA acts on several types of receptors, but all of them have inhibitory effects (in adult animals, at least). Because of this consistency, it is common for neuroscientists to refer to cells that release glutamate as "excitatory neurons", and cells that release GABA as "inhibitory neurons". Some other types of neurons have consistent effects, for example, "excitatory" motor neurons in the spinal cord that release [[acetylcholine]], and "inhibitory" [[spinal neuron]]s that release [[glycine]].{{citation needed|date=July 2022}}

The distinction between excitatory and inhibitory neurotransmitters is not absolute. Rather, it depends on the class of chemical receptors present on the postsynaptic neuron. In principle, a single neuron, releasing a single neurotransmitter, can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others still. For example, [[photoreceptor cell]]s in the retina constantly release the neurotransmitter glutamate in the absence of light. So-called OFF [[retinal bipolar cells|bipolar cells]] are, like most neurons, excited by the released glutamate. However, neighboring target neurons called ON bipolar cells are instead inhibited by glutamate, because they lack typical [[ionotropic receptor|ionotropic]] [[glutamate receptors]] and instead express a class of inhibitory [[metabotropic receptor|metabotropic]] glutamate receptors.<ref>{{cite journal | vauthors = Gerber U | title = Metabotropic glutamate receptors in vertebrate retina | journal = Documenta Ophthalmologica. Advances in Ophthalmology | volume = 106 | issue = 1 | pages = 83–7 | date = January 2003 | pmid = 12675489 | doi = 10.1023/A:1022477203420 | s2cid = 22296630 }}</ref> When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes the excitation from the OFF bipolar cells, silencing them.{{citation needed|date=July 2022}}

It is possible to identify the type of inhibitory effect a presynaptic neuron will have on a postsynaptic neuron, based on the proteins the presynaptic neuron expresses. [[Parvalbumin]]-expressing neurons typically dampen the output signal of the postsynaptic neuron in the [[visual cortex]], whereas [[somatostatin]]-expressing neurons typically block dendritic inputs to the postsynaptic neuron.<ref name="pmid22878717">{{cite journal | vauthors = Wilson NR, Runyan CA, Wang FL, Sur M | title = Division and subtraction by distinct cortical inhibitory networks in vivo | journal = Nature | volume = 488 | issue = 7411 | pages = 343–8 | date = August 2012 | pmid = 22878717 | pmc = 3653570 | doi = 10.1038/nature11347 | bibcode = 2012Natur.488..343W | hdl = 1721.1/92709 }}</ref>

====Discharge patterns====
Neurons have intrinsic electroresponsive properties like intrinsic transmembrane voltage [[Neural oscillation|oscillatory]] patterns.<ref name="llinas2014">{{cite journal | vauthors = Llinás RR | title = Intrinsic electrical properties of mammalian neurons and CNS function: a historical perspective | journal = Frontiers in Cellular Neuroscience | volume = 8 | pages = 320 | date = 2014-01-01 | pmid = 25408634 | pmc = 4219458 | doi = 10.3389/fncel.2014.00320 | doi-access = free }}</ref> So neurons can be classified according to their [[electrophysiology|electrophysiological]] characteristics:
* Tonic or regular spiking. Some neurons are typically constantly (tonically) active, typically firing at a constant frequency. Example: interneurons in [[striatum|neurostriatum]].
* Phasic or [[bursting]]. Neurons that fire in bursts are called phasic.
* Fast-spiking. Some neurons are notable for their high firing rates, for example, some types of cortical inhibitory interneurons, cells in [[globus pallidus]], [[retinal ganglion cells]].<ref>{{cite conference | title = Ion conductances related to shaping the repetitive firing in rat retinal ganglion cells | vauthors = Kolodin YO, Veselovskaia NN, Veselovsky NS, Fedulova SA | conference = Acta Physiologica Congress | url = http://www.blackwellpublishing.com/aphmeeting/abstract.asp?MeetingID=&id=61198 | access-date = 2009-06-20 | archive-url = https://web.archive.org/web/20121007164451/http://www.blackwellpublishing.com/aphmeeting/abstract.asp?MeetingID=&id=61198 | archive-date = 2012-10-07 | url-status = dead }}</ref><ref>{{cite web|url=http://ykolodin.50webs.com/ |title=Ionic conductances underlying excitability in tonically firing retinal ganglion cells of adult rat |publisher=Ykolodin.50webs.com |date=2008-04-27 |access-date=2013-02-16}}</ref>

====Neurotransmitter====
[[File:Neurotransmitters.jpg|thumb|Synaptic vesicles containing neurotransmitters]]
{{Main|Neurotransmitter}}
[[Neurotransmitter]]s are chemical messengers passed from one neuron to another neuron or to a [[muscle cell]] or [[Gland|gland cell]].
* Cholinergic neurons – acetylcholine. [[Acetylcholine]] is released from presynaptic neurons into the synaptic cleft. It acts as a [[ligand]] for both ligand-gated ion channels and [[Metabotropic receptor|metabotropic]] (GPCRs) [[Muscarinic acetylcholine receptor|muscarinic receptors]]. [[Nicotinic receptors]] are pentameric ligand-gated ion channels composed of alpha and beta subunits that bind [[nicotine]]. Ligand binding opens the channel causing the influx of [[Sodium|Na<sup>+</sup>]] depolarization and increases the probability of presynaptic neurotransmitter release. Acetylcholine is synthesized from [[choline]] and [[acetyl coenzyme A]].
* Adrenergic neurons – noradrenaline. [[Noradrenaline]] (norepinephrine) is released from most [[postganglionic]] neurons in the [[sympathetic nervous system]] onto two sets of GPCRs: [[Adrenergic receptor|alpha adrenoceptor]]s and [[beta adrenoceptor]]s. Noradrenaline is one of the three common [[catecholamine]] neurotransmitters, and the most prevalent of them in the [[peripheral nervous system]]; as with other catecholamines, it is synthesized from [[tyrosine]].
* GABAergic neurons – [[gamma aminobutyric acid]]. GABA is one of two neuroinhibitors in the [[central nervous system]] (CNS), along with glycine. GABA has a homologous function to [[Acetylcholine|ACh]], gating anion channels that allow [[Chlorine|Cl<sup>−</sup>]] ions to enter the post synaptic neuron. Cl<sup>−</sup> causes hyperpolarization within the neuron, decreasing the probability of an action potential firing as the voltage becomes more negative (for an action potential to fire, a positive voltage threshold must be reached). GABA is synthesized from glutamate neurotransmitters by the enzyme [[glutamate decarboxylase]].
* Glutamatergic neurons – glutamate. [[Glutamate]] is one of two primary excitatory amino acid neurotransmitters, along with [[Aspartic acid|aspartate]]. Glutamate receptors are one of four categories, three of which are ligand-gated ion channels and one of which is a [[G protein|G-protein]] coupled receptor (often referred to as GPCR).

:#[[AMPA]] and [[Kainic acid|Kainate]] receptors function as [[Ion|cation]] channels permeable to Na<sup>+</sup> cation channels mediating fast excitatory synaptic transmission.
:#[[N-Methyl-D-aspartic acid|NMDA]] receptors are another cation channel that is more permeable to [[Calcium in biology|Ca<sup>2+</sup>]]. The function of NMDA receptors depends on glycine receptor binding as a co-[[agonist]] within the channel pore. NMDA receptors do not function without both ligands present.
:#Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability.
:: Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in [[brain damage]]. When blood flow is suppressed, glutamate is released from presynaptic neurons, causing greater NMDA and AMPA receptor activation than normal outside of stress conditions, leading to elevated Ca<sup>2+</sup> and Na<sup>+</sup> entering the post synaptic neuron and cell damage. Glutamate is synthesized from the amino acid glutamine by the enzyme [[Glutamine oxoglutarate aminotransferase|glutamate synthase]].
* Dopaminergic neurons—[[dopamine]]. [[Dopamine]] is a neurotransmitter that acts on D1 type (D1 and D5) Gs-coupled receptors, which increase cAMP and PKA, and D2 type (D2, D3, and D4) receptors, which activate Gi-coupled receptors that decrease cAMP and PKA. Dopamine is connected to mood and behavior and modulates both pre- and post-synaptic neurotransmission. Loss of dopamine neurons in the [[substantia nigra]] has been linked to [[Parkinson's disease]]. Dopamine is synthesized from the amino acid [[tyrosine]]. Tyrosine is catalyzed into levodopa (or [[L-DOPA]]) by [[tyrosine hydroxylase]], and levodopa is then converted into dopamine by the aromatic amino acid [[Carboxy-lyases|decarboxylase]].
* Serotonergic neurons—[[serotonin]]. [[Serotonin]] (5-Hydroxytryptamine, 5-HT) can act as excitatory or inhibitory. Of its four 5-HT receptor classes, 3 are GPCR and 1 is a ligand-gated cation channel. Serotonin is synthesized from [[tryptophan]] by [[tryptophan hydroxylase]], and then further by decarboxylase. A lack of 5-HT at postsynaptic neurons has been linked to depression. Drugs that block the presynaptic [[serotonin transporter]] are used for treatment, such as [[Prozac]] and [[Zoloft]].
* Purinergic neurons—ATP. [[Adenosine triphosphate|ATP]] is a neurotransmitter acting at both ligand-gated ion channels ([[P2X]] receptors) and GPCRs ([[P2Y receptor|P2Y]]) receptors. ATP is, however, best known as a [[cotransmitter]]. Such [[purinergic signaling]] can also be mediated by other [[purine]]s like [[adenosine]], which particularly acts at P2Y receptors.
* Histaminergic neurons—[[histamine]]. [[Histamine]] is a [[monoamine neurotransmitter]] and [[neuromodulator]]. Histamine-producing neurons are found in the [[tuberomammillary nucleus]] of the [[hypothalamus]].<ref>{{cite journal | vauthors = Scammell TE, Jackson AC, Franks NP, Wisden W, Dauvilliers Y | title = Histamine: neural circuits and new medications | journal = Sleep | volume = 42 | issue = 1 | date = January 2019 | pmid = 30239935 | pmc = 6335869 | doi = 10.1093/sleep/zsy183 }}</ref> Histamine is involved in [[arousal]] and regulating sleep/wake behaviors.

====Multimodel classification====
Since 2012 there has been a push from the cellular and [[computational neuroscience]] community to come up with a universal classification of neurons that will apply to all neurons in the brain as well as across species. This is done by considering the three essential qualities of all neurons: electrophysiology, morphology, and the individual transcriptome of the cells. Besides being universal this classification has the advantage of being able to classify astrocytes as well. A method called [[patch-sequencing]] in which all three qualities can be measured at once is used extensively by the [[Allen Institute for Brain Science]].<ref>{{cite web |url=https://www.news-medical.net/news/20201203/Patch-seq-technique-helps-depict-the-variation-of-neural-cells-in-the-brain.aspx |title=Patch-seq technique helps depict the variation of neural cells in the brain |work=News-medical.net |date=3 December 2020 |access-date=26 August 2021 }}</ref> In 2023, a comprehensive cell atlas of the adult, and developing human brain at the transcriptional, epigenetic, and functional levels was created through an international collaboration of researchers using the most cutting-edge molecular biology approaches.<ref>{{Cite web
| last = Science AAAS
| title = BRAIN CELL CENSUS 
| url = https://www.science.org/collections/brain-cell-census 
| access-date = 2023-10-17
}}
</ref>