Editing Neuron (section)

== Anatomy and histology ==
{{Multiple image
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| image1            = Complete neuron cell diagram en.svg
| caption1          = Diagram of a typical myelinated vertebrate motor neuron
| image2            = Anatomy of a Neuron with Synapse.png
| caption2          = Schematic of a single [[pyramidal neuron]], with a [[synapse]] from an incoming [[axon]] onto a [[dendritic spine]]
| direction         = vertical
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Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed in different parts of the nervous system, there is a wide variety in their shape, size, and electrochemical properties. For instance, the soma of a neuron can vary from 4 to 100 [[Micrometre|micrometers]] in diameter.<ref>{{cite web |first = Melissa |last = Davies |title = The Neuron: size comparison |url = https://www.ualberta.ca/~neuro/OnlineIntro/NeuronExample.htm |work = Neuroscience: A journey through the brain |date = 2002-04-09 |access-date = 2009-06-20}}</ref>
* The '''[[Soma (biology)|soma]]''' is the body of the neuron. As it contains the [[cell nucleus|nucleus]], most [[protein biosynthesis|protein synthesis]] occurs here. The nucleus can range from 3 to 18 micrometers in diameter.<ref>{{cite web |first = Eric H. |last = Chudler | name-list-style = vanc |title = Brain Facts and Figures |url = http://faculty.washington.edu/chudler/facts.html |work = Neuroscience for Kids |access-date = 2009-06-20 }}</ref>
* The '''[[dendrites]]''' of a neuron are cellular extensions with many branches. This overall shape and structure are referred to metaphorically as a dendritic tree. This is where the majority of input to the neuron occurs via the [[dendritic spine]].
* The '''[[axon]]''' is a finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon primarily carries [[nerve signal]]s away from the soma and carries 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 also 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 electrophysiological terms, it has the most negative [[threshold potential]].
** While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.
* The '''[[axon terminal]]''' is found at the end of the axon farthest from the soma and contains [[synapses]]. Synaptic boutons are specialized structures where [[neurotransmitter]] chemicals are released to communicate with target neurons. In addition to synaptic boutons at the axon terminal, a neuron may have [[en passant boutons]], which are located along the length of the axon.

The accepted view of the neuron attributes dedicated functions to its various anatomical components; however, dendrites and axons often act in ways contrary to their so-called main function.<ref>{{Cite web |date=2021-01-14 |title=16.7: Nervous System |url=https://bio.libretexts.org/Courses/Lumen_Learning/Book%3A_Fundamentals_of_Biology_I_(Lumen)/16%3A_Module_13%3A_Overview_of_Body_Systems/16.7%3A_Nervous_System |access-date=2022-02-28 |website=Biology LibreTexts |language=en}}</ref>

Axons and dendrites in the central nervous system are typically only about one 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 [[motor neuron]] can be over a meter long, reaching from the base of the spine to the toes.

Sensory neurons can have axons that run from the toes to the [[posterior column]] of the spinal cord, over 1.5 meters in adults. [[Giraffe]]s have single axons several meters in length running along the entire length of their necks. Much of what is known about axonal function comes from studying the [[squid giant axon]], an ideal experimental preparation because of its relatively immense size (0.5–1 millimeter thick, several centimeters long).

Fully differentiated neurons are permanently [[G0 phase|postmitotic]]<ref>{{cite journal | vauthors = Herrup K, Yang Y | title = Cell cycle regulation in the postmitotic neuron: oxymoron or new biology? | journal = Nature Reviews. Neuroscience | volume = 8 | issue = 5 | pages = 368–78 | date = May 2007 | pmid = 17453017 | doi = 10.1038/nrn2124 | s2cid = 12908713 }}</ref> however, stem cells present in the adult brain may regenerate functional neurons throughout the life of an organism (see [[neurogenesis]]). [[Astrocyte]]s are star-shaped [[glial cell]]s that have been observed to turn into neurons by virtue of their stem cell-like characteristic of [[pluripotency]].<ref name="Talifu">{{cite journal |vauthors=Talifu Z, Liu JY, Pan YZ, Ke H, Zhang CJ, Xu X, Gao F, Yu Y, Du LJ, Li JJ |title=In vivo astrocyte-to-neuron reprogramming for central nervous system regeneration: a narrative review |journal=Neural Regen Res |volume=18 |issue=4 |pages=750–755 |date=April 2023 |pmid=36204831 |pmc=9700087 |doi=10.4103/1673-5374.353482 |doi-access=free |url=}}</ref>

===Membrane===
Like all animal cells, the cell body of every neuron is enclosed by a [[plasma membrane]], a bilayer of [[lipid]] molecules with many types of protein structures embedded in it.<ref>{{Cite journal |last=Giménez |first=C. |date=February 1998 |title=[Composition and structure of the neuronal membrane: molecular basis of its physiology and pathology] |url=https://pubmed.ncbi.nlm.nih.gov/9563093/ |journal=Revista de Neurologia |volume=26 |issue=150 |pages=232–239 |issn=0210-0010 |pmid=9563093}}</ref> A lipid bilayer is a powerful electrical [[Insulator (electricity)|insulator]], but in neurons, many of the protein structures embedded in the membrane are electrically active. These include ion channels that permit electrically charged ions to flow across the membrane and ion pumps that chemically transport ions from one side of the membrane to the other. Most ion channels are permeable only to specific types of ions. Some ion channels are [[voltage-gated ion channel|voltage gated]], meaning that they can be switched between open and closed states by altering the voltage difference across the membrane. Others are chemically gated, meaning that they can be switched between open and closed states by interactions with chemicals that diffuse through the extracellular fluid. The [[ion]] materials include [[sodium]], [[potassium]], [[chloride]], and [[calcium]]. The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically a bit less than 1/10 of a volt at baseline. This voltage has two functions: first, it provides a power source for an assortment of voltage-dependent protein machinery that is embedded in the membrane; second, it provides a basis for electrical signal transmission between different parts of the membrane.

===Histology and internal structure===
[[File:Gyrus Dentatus 40x.jpg|thumb|[[Golgi stain|Golgi-stained]] neurons in human [[hippocampal]] tissue]]
[[File:SUM 110913 Cort Neurons 2.5d in vitro 488 Phalloidin no perm 4 cmle-2.png|thumb|Actin filaments in a mouse cortical neuron in culture]]

Numerous microscopic clumps called [[Nissl body|Nissl bodies]] (or Nissl substance) are seen when nerve cell bodies are stained with a basophilic ("base-loving") dye. These structures consist of [[Endoplasmic reticulum#Rough endoplasmic reticulum|rough endoplasmic reticulum]] and associated [[ribosomal RNA]]. Named after German psychiatrist and neuropathologist [[Franz Nissl]] (1860–1919), they are involved in protein synthesis and their prominence can be explained by the fact that nerve cells are very metabolically active. Basophilic dyes such as [[aniline]] or (weakly) [[hematoxylin]]<ref>{{cite book|title=State Hospitals Bulletin|url={{google books |plainurl=y |id=Wp8CAAAAYAAJ|page=378}}|year=1897|publisher=State Commission in Lunacy.|page=378}}</ref> highlight negatively charged components, and so bind to the phosphate backbone of the ribosomal RNA.

The cell body of a neuron is supported by a complex mesh of structural proteins called [[neurofilament]]s, which together with [[neurotubule]]s (neuronal microtubules) are assembled into larger neurofibrils.<ref name="Webster">{{cite web |title=Medical Definition of Neurotubules |url=https://www.merriam-webster.com/medical/neurotubules |website=www.merriam-webster.com}}</ref> Some neurons also contain pigment granules, such as [[neuromelanin]] (a brownish-black pigment that is byproduct of synthesis of [[catecholamine]]s), and [[lipofuscin]] (a yellowish-brown pigment), both of which accumulate with age.<ref>{{cite journal | vauthors = Zecca L, Gallorini M, Schünemann V, Trautwein AX, Gerlach M, Riederer P, Vezzoni P, Tampellini D | title = Iron, neuromelanin and ferritin content in the substantia nigra of normal subjects at different ages: consequences for iron storage and neurodegenerative processes | journal = Journal of Neurochemistry | volume = 76 | issue = 6 | pages = 1766–73 | date = March 2001 | pmid = 11259494 | doi = 10.1046/j.1471-4159.2001.00186.x | s2cid = 31301135 }}</ref><ref>{{cite journal | vauthors = Herrero MT, Hirsch EC, Kastner A, Luquin MR, Javoy-Agid F, Gonzalo LM, Obeso JA, Agid Y | title = Neuromelanin accumulation with age in catecholaminergic neurons from Macaca fascicularis brainstem | journal = Developmental Neuroscience | volume = 15 | issue = 1 | pages = 37–48 | date = 1993 | pmid = 7505739 | doi = 10.1159/000111315 }}</ref><ref>{{cite journal | vauthors = Brunk UT, Terman A | title = Lipofuscin: mechanisms of age-related accumulation and influence on cell function | journal = Free Radical Biology & Medicine | volume = 33 | issue = 5 | pages = 611–9 | date = September 2002 | pmid = 12208347 | doi = 10.1016/s0891-5849(02)00959-0 }}</ref> Other structural proteins that are important for neuronal function are [[actin]] and the [[tubulin]] of [[microtubule]]s. [[Class III β-tubulin]] is found almost exclusively in neurons. Actin is predominately found at the tips of axons and dendrites during neuronal development. There the actin dynamics can be modulated via an interplay with microtubule.<ref>{{cite journal | vauthors = Zhao B, Meka DP, Schellenberg R, König T, Schwanke B, Kobler O, Windhorst S, Kreutz MR, Mikhaylova M, Calderon de Anda F | title = Microtubules Modulate F-actin Dynamics during Neuronal Polarization | journal = Scientific Reports | volume = 7 | issue = 1 | pages = 9583 | date = August 2017 | pmid = 28851982 | pmc = 5575062 | doi = 10.1038/s41598-017-09832-8 | bibcode = 2017NatSR...7.9583Z }}</ref>

There are different internal structural characteristics between axons and dendrites. Typical axons seldom contain [[ribosomes]], except some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, in diminishing amounts as the distance from the cell body increases.