Editing Chemical synapse (section)

==Structure==
[[Image:Synapse figure.png|right|thumb|400px|Diagram of a chemical synaptic connection]]
{{further|topic=formation of synapses|Synaptogenesis}}
Synapses are functional connections between neurons, or between neurons and other types of cells.<ref>

{{cite book
|last=Rapport
|first=Richard L.
|title=Nerve Endings: The Discovery of the Synapse
|url=https://books.google.com/books?id=fWBBIUhoLzMC&q=Synapse
|year=2005
|publisher=W. W. Norton & Company
|isbn=978-0-393-06019-5
|pages=1–37
}}

</ref><ref>
{{cite book
|last=Squire
|first=Larry R.
|author2=Floyd Bloom |author3=Nicholas Spitzer
 |title=Fundamental Neuroscience
|url=https://books.google.com/books?id=GOxrtYzmixcC
|year=2008
|publisher=Academic Press
|isbn=978-0-12-374019-9
|pages=425–6
}}

</ref>  A typical neuron gives rise to several thousand synapses, although there are some types that make far fewer.<ref>

{{cite book
|last=Hyman
|first=Steven E.
|author2=Eric Jonathan Nestler
|title=The Molecular Foundations of Psychiatry
|url=https://books.google.com/books?id=pI4ayEWvcQkC&q=neuron+gives+rise+to+several+thousand+synapses&pg=PA24
|year=1993
|publisher= American Psychiatric Pub
|isbn=978-0-88048-353-7
|pages=425–6
}}

</ref>  Most synapses connect [[axon]]s to [[dendrite]]s,<ref>

{{cite book
|last=Smilkstein
|first=Rita
|title=We're Born to Learn: Using the Brain's Natural Learning Process to Create Today's Curriculum
|url=https://books.google.com/books?id=6ZHELyI9XEIC&q=synapses+connect+axons+to+dendrites&pg=PA56
|year=2003
|publisher= Corwin Press
|isbn=978-0-7619-4642-7
|page=56
}}

</ref><ref name="Lytton"/> but there are also other types of connections, including axon-to-cell-body,<ref name="Garber"/><ref name="Weiss"/> axon-to-axon,<ref name="Garber">

{{cite book
|last=Garber
|first=Steven D.
|title=Biology: A Self-Teaching Guide
|url=https://archive.org/details/biologyselfteach00garb
|url-access=registration
|quote=synapses connect axons to cell body.
|year=2002
|publisher= John Wiley and Sons
|isbn=978-0-471-22330-6
|page=[https://archive.org/details/biologyselfteach00garb/page/175 175]
}}

</ref><ref name="Weiss">

{{cite book
|last=Weiss
|first=Mirin
|author2=Dr Steven M. Mirin |author3=Dr Roxanne Bartel
 |title=Cocaine
|url=https://books.google.com/books?id=efqxJ5jhOaEC&q=synapses+connect+axons+to+cell+body&pg=PA52
|access-date=2008-12-26
|year=1994
|publisher=American Psychiatric Pub
|isbn=978-1-58562-138-5
|page=52
}} Axons terminating on the postsynaptic cell body are axosomatic synapses.  Axons that terminate on axons are axoaxonic synapses

</ref> and [[Dendrodendritic synapse|dendrite-to-dendrite]].<ref name="Lytton">{{cite book
|last= Lytton
|first=William W.
|title=From Computer to Brain: Foundations of Computational Neuroscience|url=https://books.google.com/books?id=vfL4uxiWZ9YC&q=synapses+connect+dendrites+to+dendrites&pg=PA28
|year=2002
|publisher=Springer
|isbn=978-0-387-95526-1
|page=28
}} Axons connecting dendrite to dendrite are dendrodendritic synapses.  Axons which connect axon to dendrite are called axodendritic synapses

</ref> Synapses are generally too small to be recognizable using a [[light microscope]] except as points where the membranes of two cells appear to touch, but their cellular elements can be visualized clearly using an [[electron microscope]].

Chemical synapses pass information directionally from a presynaptic cell to a postsynaptic cell and are therefore asymmetric in structure and function. The presynaptic [[axon terminal]], or synaptic {{anchor|bouton}}bouton, is a specialized area within the axon of the presynaptic cell that contains [[neurotransmitter]]s enclosed in small membrane-bound spheres called [[synaptic vesicle]]s (as well as a number of other supporting structures and organelles, such as [[mitochondria]] and [[endoplasmic reticulum]]). Synaptic vesicles are docked at the presynaptic [[plasma membrane]] at regions called [[active zone]]s.

Immediately opposite is a region of the postsynaptic cell containing neurotransmitter [[receptor (biochemistry)|receptor]]s; for synapses between two neurons the postsynaptic region may be found on the dendrites or cell body. Immediately behind the postsynaptic membrane is an elaborate complex of interlinked proteins called the [[postsynaptic density]] (PSD).

Proteins in the PSD are involved in anchoring and trafficking neurotransmitter receptors and modulating the activity of these receptors. The receptors and PSDs are often found in specialized protrusions from the main dendritic shaft called [[dendritic spine]]s.

Synapses may be described as symmetric or asymmetric. When examined under an electron microscope, asymmetric synapses are characterized by rounded vesicles in the presynaptic cell, and a prominent postsynaptic density. Asymmetric synapses are typically excitatory. Symmetric synapses in contrast have flattened or elongated vesicles, and do not contain a prominent postsynaptic density. Symmetric synapses are typically inhibitory.

The <!--Synaptic cleft redirects here, so this term is bolded per MOS:BOLD.-->'''synaptic cleft'''—also called '''synaptic gap'''—is a gap between the pre- and postsynaptic cells that is about 20&nbsp;nm (0.02 μ) wide.<ref name="Widrow Kim Park Perin 2019 pp. 1–30">{{cite book |last1=Widrow |first1=Bernard |last2=Kim |first2=Youngsik |last3=Park |first3=Dookun |last4=Perin |first4=Jose Krause |title=Artificial Intelligence in the Age of Neural Networks and Brain Computing |chapter=Nature's Learning Rule |publisher=Elsevier |year=2019 |isbn=978-0-12-815480-9 |doi=10.1016/b978-0-12-815480-9.00001-3 |pages=1–30|s2cid=125516633 }}</ref> The small volume of the cleft allows neurotransmitter concentration to be raised and lowered rapidly.<ref name="KandelPrin">{{harvnb|Kandel|Schwartz|Jessell|2000|p=182}}</ref>

An [[autapse]] is a chemical (or electrical) synapse formed when the axon of one neuron synapses with its own dendrites.

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