Editing Neuron (section)

==Connectivity==
{{Main|Synapse|Chemical synapse}}
[[File:Chemical synapse schema cropped.jpg|thumb|right|350px|A signal propagating down an axon to the cell body and dendrites of the next cell]]
[[File:Neuro Muscular Junction.png|thumb|Chemical synapse|left]]
Neurons communicate with each other via [[synapses]], where either the [[axon terminal]] of one cell contacts another neuron's dendrite, soma, or, less commonly, 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, either increasing or decreasing activity in the target neuron, respectively. Some neurons also communicate via electrical synapses, which are direct, electrically conductive [[gap junction|junctions]] between cells.<ref>{{cite book |last1=Macpherson |first1=Gordon |title=Black's Medical Dictionary |date=2002 |publisher=Scarecrow Press |location=Lanham, MD |isbn=0810849844 |pages=431–434 |edition=40 }}</ref>

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 receptors on the postsynaptic neuron. High cytosolic calcium in the [[axon terminal]] triggers mitochondrial calcium uptake, which, in turn, activates mitochondrial [[energy metabolism]] to produce [[Adenosine triphosphate|ATP]] to support continuous neurotransmission.<ref name="pmid23746507">{{cite journal | vauthors = Ivannikov MV, Macleod GT | title = Mitochondrial free Ca²⁺ levels and their effects on energy metabolism in Drosophila motor nerve terminals | journal = Biophysical Journal | volume = 104 | issue = 11 | pages = 2353–61 | date = June 2013 | pmid = 23746507 | pmc = 3672877 | doi = 10.1016/j.bpj.2013.03.064 | bibcode = 2013BpJ...104.2353I }}</ref>

An [[autapse]] is a synapse in which a neuron's axon connects to its dendrites.

The [[human brain]] has some 8.6 x 10<sup>10</sup> (eighty six billion) neurons.<ref>{{ cite journal | vauthors = Herculano-Houzel S | title = The human brain in numbers: a linearly scaled-up primate brain | journal = Frontiers in Human Neuroscience | volume = 3 | pages = 31 | date = November 2009 | pmid = 19915731 | doi = 10.3389/neuro.09.031.2009 | pmc = 2776484 | doi-access = free }}</ref><ref name="Allen2022">{{cite web |title=Why is the human brain so difficult to understand? We asked 4 neuroscientists. |url=https://alleninstitute.org/news/why-is-the-human-brain-so-difficult-to-understand-we-asked-4-neuroscientists/ |website=Allen Institute |access-date=17 October 2023}}</ref> Each neuron has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 10<sup>15</sup> synapses (1 quadrillion). [[Synaptic pruning|This number declines with age]], stabilizing by adulthood. Estimates vary for an adult, ranging from 10<sup>14</sup> to 5 x 10<sup>14</sup> synapses (100 to 500 trillion).<ref>{{cite journal | vauthors = Drachman DA | title = Do we have brain to spare? | journal = Neurology | volume = 64 | issue = 12 | pages = 2004–5 | date = June 2005 | pmid = 15985565 | doi = 10.1212/01.WNL.0000166914.38327.BB | s2cid = 38482114 }}</ref>

=== Nonelectrochemical signaling ===
Beyond electrical and chemical signaling, studies suggest neurons in healthy human brains can also communicate through:
* force generated by the enlargement of dendritic spines<ref>{{cite journal |last1=Ucar |first1=Hasan |last2=Watanabe |first2=Satoshi |last3=Noguchi |first3=Jun |last4=Morimoto |first4=Yuichi |last5=Iino |first5=Yusuke |last6=Yagishita |first6=Sho |last7=Takahashi |first7=Noriko |last8=Kasai |first8=Haruo |title=Mechanical actions of dendritic-spine enlargement on presynaptic exocytosis |journal=Nature |date=December 2021 |volume=600 |issue=7890 |pages=686–689 |doi=10.1038/s41586-021-04125-7 |pmid=34819666 |bibcode=2021Natur.600..686U |s2cid=244648506 |language=en |issn=1476-4687}}<br />Lay summary:<br />{{cite news |title=Forceful synapses reveal mechanical interactions in the brain |url=https://www.nature.com/articles/d41586-021-03516-0 |access-date=21 February 2022 |journal=Nature |date=24 November 2021 |language=en |doi=10.1038/d41586-021-03516-0}}</ref>
* the transfer of [[protein]]s – transneuronally transported proteins (TNTPs)<!--e.g. between [[Retinal ganglion cell|RGC]] and [[Excitatory synapse|excitatory]] [[lateral geniculate nucleus|LGN]] neurons--><ref>{{cite news |title=Researchers discover new type of cellular communication in the brain |url=https://medicalxpress.com/news/2022-01-cellular-brain.html |access-date=12 February 2022 |work=The Scripps Research Institute |language=en}}</ref><ref>{{cite journal |last1=Schiapparelli |first1=Lucio M. |last2=Sharma |first2=Pranav |last3=He |first3=Hai-Yan |last4=Li |first4=Jianli |last5=Shah |first5=Sahil H. |last6=McClatchy |first6=Daniel B. |last7=Ma |first7=Yuanhui |last8=Liu |first8=Han-Hsuan |last9=Goldberg |first9=Jeffrey L. |last10=Yates |first10=John R. |last11=Cline |first11=Hollis T. |title=Proteomic screen reveals diverse protein transport between connected neurons in the visual system |journal=Cell Reports |date=25 January 2022 |volume=38 |issue=4 |page=110287 |doi=10.1016/j.celrep.2021.110287 |pmid=35081342 |pmc=8906846 |language=English |issn=2211-1247}}</ref>

They can also get modulated by input from the environment and [[hormone]]s released from other parts of the organism,<ref>{{cite book |last1=Levitan |first1=Irwin B. |last2=Kaczmarek |first2=Leonard K. |title=The Neuron |chapter=Electrical Signaling in Neurons |year=2015 |pages=41–62 |doi=10.1093/med/9780199773893.003.0003 |publisher=Oxford University Press |isbn=978-0-19-977389-3 }}</ref> which could be influenced more or less directly by neurons. This also applies to [[neurotrophin]]s such as [[BDNF]]. The [[gut microbiome]] is also connected with the brain.<ref>{{cite journal |last1=O’Leary |first1=Olivia F. |last2=Ogbonnaya |first2=Ebere S. |last3=Felice |first3=Daniela |last4=Levone |first4=Brunno R. |last5=C. Conroy |first5=Lorraine |last6=Fitzgerald |first6=Patrick |last7=Bravo |first7=Javier A. |last8=Forsythe |first8=Paul |last9=Bienenstock |first9=John |last10=Dinan |first10=Timothy G. |last11=Cryan |first11=John F. |title=The vagus nerve modulates BDNF expression and neurogenesis in the hippocampus |journal=European Neuropsychopharmacology |date=1 February 2018 |volume=28 |issue=2 |pages=307–316 |doi=10.1016/j.euroneuro.2017.12.004 |pmid=29426666 |s2cid=46819013 |language=en |issn=0924-977X|doi-access=free }}</ref>
Neurons also communicate with [[microglia]], the brain's main immune cells via specialized contact sites, called "somatic junctions". These connections enable microglia to constantly monitor and regulate neuronal functions, and exert neuroprotection when needed.<ref name="Science">{{cite journal |vauthors=Cserép C, Pósfai B, Lénárt N, Fekete R, László ZI, Lele Z |date=January 2020 |title=Microglia monitor and protect neuronal function through specialized somatic purinergic junctions |url= https://epub.ub.uni-muenchen.de/76442/|journal=Science |volume=367 |issue=6477 |pages=528–537 |doi=10.1126/science.aax6752 |pmid=31831638|bibcode=2020Sci...367..528C |s2cid=209343260 }}</ref>