Editing Chemical synapse (section)

{{Short description|Biological junctions through which neurons' signals can be sent}}
{{About|chemical synapses of the nervous system|general information|synapse|other uses|synapse (disambiguation)}}
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[[File:Chemical synapse schema cropped.jpg|thumb|Artistic interpretation of the major elements in chemical synaptic transmission. An electrochemical wave called an [[action potential]] travels along the [[axon]] of a [[neuron]]. When the action potential reaches the presynaptic terminal, it provokes the release of a synaptic vesicle, secreting its quanta of [[neurotransmitter]] molecules. The neurotransmitter binds to chemical receptor molecules located in the membrane of another neuron, the postsynaptic neuron, on the opposite side of the synaptic cleft.]]

'''Chemical synapses''' are biological junctions through which [[neuron]]s' signals can be sent to each other and to non-neuronal cells such as those in [[neuromuscular junction|muscle]]s or [[gland]]s. Chemical synapses allow neurons to form [[biological neural network|circuits]] within the [[central nervous system]]. They are crucial to the [[biological computation]]s that underlie [[perception]] and [[thought]]. They allow the [[nervous system]] to connect to and control other systems of the body.

At a chemical synapse, one neuron releases [[neurotransmitter]] [[molecule]]s into a small space (the [[#Structure|synaptic cleft]]) that is adjacent to another neuron. The neurotransmitters are contained within small sacs called [[synaptic vesicle]]s, and are released into the synaptic cleft by [[exocytosis]]. These molecules then bind to [[neurotransmitter receptor]]s on the postsynaptic cell. Finally, the neurotransmitters are cleared from the synapse through one of several [[Action potential|potential mechanism]]s including enzymatic degradation or [[Reuptake|re-uptake]] by [[Neurotransmitter transporter|specific transporters]] either on the presynaptic cell or on some other [[neuroglia]] to terminate the action of the neurotransmitter.

The [[adult]] [[human]] [[brain]] is estimated to contain from 10<sup>14</sup> to 5 × 10<sup>14</sup> (100–500 trillion) synapses.<ref>{{cite journal | author = Drachman D | title = Do we have brain to spare? | journal = Neurology | volume = 64 | issue = 12 | pages = 2004–5 | year = 2005 | pmid = 15985565 | doi = 10.1212/01.WNL.0000166914.38327.BB| s2cid = 38482114 }}</ref> Every cubic millimeter of [[cerebral cortex]] contains roughly a billion ([[Long and short scales|short scale]], i.e. 10<sup>9</sup>) of them.<ref>{{cite journal |vauthors=Alonso-Nanclares L, Gonzalez-Soriano J, Rodriguez JR, DeFelipe J |title=Gender differences in human cortical synaptic density |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=38 |pages=14615–9 |date=September 2008 |pmid=18779570 |pmc=2567215 |doi=10.1073/pnas.0803652105 |bibcode=2008PNAS..10514615A|doi-access=free }}</ref> The number of synapses in the  human [[cerebral cortex]] has separately been estimated at 0.15 quadrillion (150 trillion)<ref>[https://faculty.washington.edu/chudler/facts.html Brain Facts and Figures] Washington University.</ref>

The word "synapse" was introduced by Sir [[Charles Scott Sherrington]] in 1897.<ref name="Cowan">{{cite book |last1=Cowan |first1=W. Maxwell |last2=Südhof |first2=Thomas C. |last3=Stevens |first3=Charles F. |title=Synapses |date=2003 |publisher=JHU Press |page=11 |isbn=9780801871184 |url=https://books.google.com/books?id=FO5efrKGVQoC&q=Sherrington |access-date=9 June 2020}}</ref> Chemical synapses are not the only type of biological [[synapse]]: [[electrical synapse|electrical]] and [[immunological synapse]]s also exist. Without a qualifier, however, "synapse" commonly refers to chemical synapses.