Editing Chemical synapse

{{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.

==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.

{{-}}

== Signaling in chemical synapses ==

===Overview===
Here is a summary of the sequence of events that take place in synaptic transmission from a presynaptic neuron to a postsynaptic cell.  Each step is explained in more detail below.  Note that with the exception of the final step, the entire process may run only a few hundred microseconds, in the fastest synapses.<ref name=":0">{{Cite book |title=Neuroscience: exploring the brain |url=https://archive.org/details/neuroscienceexpl00bear_099 |url-access=limited |author1=Bear, Mark F |author2=Connors, Barry W |author3=Paradiso, Michael A |publisher=Lippincott Williams & Wilkins |year=2007 |location=Philadelphia, PA|pages=[https://archive.org/details/neuroscienceexpl00bear_099/page/n146 113]–118}}</ref>

[[File:Synapse.theora.ogv|right|200px]]
# The process begins with a wave of electrochemical excitation called an [[action potential]] traveling along the membrane of the presynaptic cell, until it reaches the synapse.
# The electrical [[depolarization]] of the membrane at the synapse causes channels to open that are permeable to calcium ions.
# Calcium ions flow through the presynaptic membrane, rapidly increasing the calcium concentration in the interior.
# The high calcium concentration activates a set of calcium-sensitive proteins attached to [[Synaptic vesicle|vesicles]] that contain a [[neurotransmitter]] chemical.
# These proteins change shape, causing the membranes of some "docked" vesicles to fuse with the membrane of the presynaptic cell, thereby opening the vesicles and dumping their neurotransmitter contents into the synaptic cleft, the narrow space between the membranes of the pre- and postsynaptic cells.
# The neurotransmitter diffuses within the cleft.  Some of it escapes, but some of it binds to [[chemical receptor]] molecules located on the membrane of the postsynaptic cell.
# The binding of neurotransmitter causes the receptor molecule to be ''activated'' in some way.  Several types of activation are possible, as described in more detail below.  In any case, this is the key step by which the synaptic process affects the behavior of the postsynaptic cell.
# Due to [[Johnson–Nyquist noise|thermal vibration]], the motion of atoms, vibrating about their equilibrium positions in a crystalline solid, neurotransmitter molecules eventually break loose from the receptors and drift away.
# The neurotransmitter is either reabsorbed by the presynaptic cell, and then repackaged for future release, or else it is broken down metabolically.

===Neurotransmitter release===
[[File:Neuro Muscular Junction.png|thumb|Release of neurotransmitter occurs at the end of axonal branches.]]
The release of a neurotransmitter is triggered by the arrival of a nerve impulse (or [[action potential]]) and occurs through an unusually rapid process of cellular secretion ([[exocytosis]]). Within the presynaptic nerve terminal, [[vesicle (biology)|vesicle]]s containing neurotransmitter are localized near the synaptic membrane. The arriving action potential produces an influx of [[second messenger|calcium ions]] through [[Voltage-dependent calcium channel|voltage-dependent, calcium-selective ion channels]] at the down stroke of the action potential (tail current).<ref name="Llinás81">
{{cite journal |vauthors=Llinás R, Steinberg IZ, Walton K |title=Relationship between presynaptic calcium current and postsynaptic potential in squid giant synapse |journal=Biophysical Journal |volume=33 |issue=3 |pages=323–351 |year=1981 |pmid=6261850 |doi=10.1016/S0006-3495(81)84899-0 |pmc=1327434 |bibcode=1981BpJ....33..323L }}</ref> Calcium ions then bind to [[synaptotagmin]] proteins found within the membranes of the synaptic vesicles, allowing the vesicles to fuse with the presynaptic membrane.<ref>{{Cite journal|last=Chapman|first=Edwin R.|date=2002|title=Synaptotagmin: A Ca2+ sensor that triggers exocytosis?|journal=Nature Reviews Molecular Cell Biology|language=En|volume=3|issue=7|pages=498–508|doi=10.1038/nrm855|pmid=12094216|s2cid=12384262|issn=1471-0080}}</ref> The fusion of a vesicle is a [[stochastic]] process, leading to frequent failure of synaptic transmission at the very small synapses that are typical for the [[central nervous system]]. Large chemical synapses (e.g. the [[neuromuscular junction]]), on the other hand, have a synaptic release probability, in effect, of 1. [[Vesicle fusion]] is driven by the action of a set of proteins in the presynaptic terminal known as [[SNARE (protein)|SNAREs]]. As a whole, the protein complex or structure that mediates the docking and fusion of presynaptic vesicles is called the active zone.<ref>Craig C. Garner and Kang Shen. Structure and Function of Vertebrate and Invertebrate Active Zones. Structure and Functional Organization of the Synapse. Ed: Johannes Hell and Michael Ehlers. Springer, 2008.</ref> The membrane added by the fusion process is later retrieved by [[endocytosis]] and [[Endocytic cycle|recycled]] for the formation of fresh neurotransmitter-filled vesicles.

An exception to the general trend of neurotransmitter release by vesicular fusion is found in the type II receptor cells of  mammalian [[taste bud]]s. Here the neurotransmitter [[Adenosine triphosphate|ATP]] is released directly from the cytoplasm into the synaptic cleft via voltage gated channels.<ref name="RomanovLasher2018">{{cite journal|last1=Romanov|first1=Roman A.|last2=Lasher|first2=Robert S.|last3=High|first3=Brigit|last4=Savidge|first4=Logan E.|last5=Lawson|first5=Adam|last6=Rogachevskaja|first6=Olga A.|last7=Zhao|first7=Haitian|last8=Rogachevsky|first8=Vadim V.|last9=Bystrova|first9=Marina F.|last10=Churbanov|first10=Gleb D.|last11=Adameyko|first11=Igor|last12=Harkany|first12=Tibor|last13=Yang|first13=Ruibiao|last14=Kidd|first14=Grahame J.|last15=Marambaud|first15=Philippe|last16=Kinnamon|first16=John C.|last17=Kolesnikov|first17=Stanislav S.|last18=Finger|first18=Thomas E.|title=Chemical synapses without synaptic vesicles: Purinergic neurotransmission through a CALHM1 channel-mitochondrial signaling complex|journal=Science Signaling|volume=11|issue=529|year=2018|pages=eaao1815|issn=1945-0877|doi=10.1126/scisignal.aao1815|pmid=29739879|pmc=5966022}}</ref>

===Receptor binding===
Receptors on the opposite side of the synaptic gap bind neurotransmitter molecules. Receptors can respond in either of two general ways. First, the receptors may directly open [[ligand-gated ion channel]]s in the postsynaptic cell membrane, causing ions to enter or exit the cell and changing the local [[transmembrane potential]].<ref name=":0" /> The resulting change in [[voltage]] is called a [[postsynaptic potential]]. In general, the result is ''excitatory'' in the case of [[Depolarization|depolarizing]] currents, and ''inhibitory'' in the case of [[Hyperpolarization (biology)|hyperpolarizing]] currents. Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel conduct the postsynaptic current(s), which in turn is a function of the type of receptors and neurotransmitter employed at the synapse. The second way a receptor can affect membrane potential is by modulating the production of [[second messenger system|chemical messengers]] inside the postsynaptic neuron. These second messengers can then amplify the inhibitory or excitatory response to neurotransmitters.<ref name=":0" />

===Termination===
After a neurotransmitter molecule binds to a receptor molecule, it must be removed to allow for the postsynaptic membrane to continue to relay subsequent [[Excitatory postsynaptic potential|EPSP]]s and/or [[IPSP]]s. This removal can happen through one or more processes:
* The neurotransmitter may diffuse away due to thermally-induced oscillations of both it and the receptor, making it available to be broken down metabolically outside the neuron or to be reabsorbed.<ref name="Sherwood">Sherwood L., stikawy (2007).  Human Physiology 6e: From Cells to Systems</ref>
* Enzymes within the subsynaptic membrane may inactivate/metabolize the neurotransmitter.
* [[Reuptake]] pumps may actively pump the neurotransmitter back into the presynaptic [[axon terminal]] for reprocessing and re-release following a later action potential.<ref name="Sherwood"/>

==Synaptic strength==<!-- This section is linked from [[Post-synaptic potential]]. See [[WP:MOS#Section management]] -->
The strength of a synapse has been defined by [[Bernard Katz]] as the product of (presynaptic) release probability ''pr'', quantal size ''q'' (the postsynaptic response to the release of a single neurotransmitter vesicle, a 'quantum'), and ''n'', the number of release sites. "Unitary connection" usually refers to an unknown number of individual synapses connecting a presynaptic neuron to a postsynaptic neuron. 
The amplitude of postsynaptic potentials (PSPs) can be as low as 0.4 mV to as high as 20 mV.<ref>{{cite journal |doi=10.2307/4134559 |vauthors=Díaz-Ríos M, Miller MW |title=Target-specific regulation of synaptic efficacy in the feeding central pattern generator of Aplysia: potential substrates for behavioral plasticity? |journal=Biol. Bull. |volume=210 |issue=3 |pages=215–29 |date=June 2006 |pmid=16801496 |url=http://www.biolbull.org/cgi/pmidlookup?view=long&pmid=16801496|jstor=4134559 |s2cid=34154835 |url-access=subscription }}</ref>  The amplitude of a PSP can be modulated by [[neuromodulators]] or can change as a result of previous activity.  Changes in the synaptic strength can be short-term, lasting seconds to minutes, or long-term ([[long-term potentiation]], or LTP), lasting hours.  Learning and memory are believed to result from long-term changes in synaptic strength, via a mechanism known as [[synaptic plasticity]].

==Receptor desensitization==

Desensitization of the postsynaptic receptors is a decrease in response to the same neurotransmitter stimulus. It means that the strength of a synapse may in effect diminish as a train of action potentials arrive in rapid succession – a phenomenon that gives rise to the so-called frequency dependence of synapses. The nervous system exploits this property for computational purposes, and can tune its synapses through such means as [[phosphorylation]] of the proteins involved.

== Synaptic plasticity ==
{{Main|Synaptic plasticity}}
Synaptic transmission can be changed by previous activity. These changes are called synaptic plasticity and may result in either a decrease in the efficacy of the synapse, called depression,  or an increase in efficacy, called potentiation. These changes can either be long-term or short-term. Forms of [[Synaptic plasticity#Short-term plasticity|short-term plasticity]] include [[synaptic fatigue]] or depression and [[Synaptic plasticity#Synaptic enhancement|synaptic augmentation]]. Forms of [[Synaptic plasticity#Long-term plasticity|long-term plasticity]] include [[long-term depression]] and [[long-term potentiation]]. Synaptic plasticity can be either homosynaptic (occurring at a single synapse) or heterosynaptic (occurring at multiple synapses).

===Homosynaptic plasticity=== <!--Homotropic modulation redirects here-->
{{Main|Homosynaptic plasticity}}
[[Homosynaptic plasticity]] (or also homotropic modulation) is a change in the synaptic strength that results from the history of activity at a particular synapse.  This can result from changes in presynaptic calcium as well as feedback onto presynaptic receptors, i.e. a form of [[autocrine signaling]]. Homosynaptic plasticity can affect the number and replenishment rate of vesicles or it can affect the relationship between calcium and vesicle release. Homosynaptic plasticity can also be postsynaptic in nature. It can result in either an increase or decrease in synaptic strength.

One example is neurons of the [[sympathetic nervous system]] (SNS), which release [[noradrenaline]], which, besides affecting postsynaptic receptors, also affects presynaptic [[α2-adrenergic receptors]], inhibiting further release of noradrenaline.<ref name=Rang>{{cite book |author1=Rang, H.P. |author2=Dale, M.M. |author3=Ritter, J.M. |title=Pharmacology |publisher=Churchill Livingstone |location=Edinburgh |year=2003 |page=129 |isbn=978-0-443-07145-4 |edition=5th}}</ref> This effect is utilized with [[clonidine]] to perform inhibitory effects on the SNS.

===Heterosynaptic plasticity=== <!--Heterotropic modulation redirects here-->
{{Main|Heterosynaptic plasticity}}
[[Heterosynaptic plasticity]] (or also heterotropic modulation) is a change in synaptic strength that results from the activity of other neurons. Again, the plasticity can alter the number of vesicles or their replenishment rate or the relationship between calcium and vesicle release.  Additionally, it could directly affect calcium influx.  Heterosynaptic plasticity can also be postsynaptic in nature, affecting receptor sensitivity.

One example is again neurons of the [[sympathetic nervous system]], which release [[noradrenaline]], which, in addition, generates an inhibitory effect on presynaptic terminals of neurons of the [[parasympathetic nervous system]].<ref name=Rang/>

== Integration of synaptic inputs ==<!-- This section is linked from [[Summation]] -->
{{Main|Summation (neurophysiology)}}
In general, if an [[excitatory synapse]] is strong enough, an [[action potential]] in the presynaptic neuron will trigger an action potential in the postsynaptic cell. In many cases the [[excitatory postsynaptic potential]] (EPSP) will not reach the [[action potential|threshold]] for eliciting an action potential. When action potentials from multiple presynaptic neurons fire simultaneously, or if a single presynaptic neuron fires at a high enough frequency, the EPSPs can overlap and summate. If enough EPSPs overlap, the summated EPSP can reach the threshold for initiating an action potential. This process is known as summation, and can serve as a high pass filter for neurons.<ref>{{cite book |editor1=Bruce Alberts |editor2=Alexander Johnson |editor3=Julian Lewis |editor4=Martin Raff |editor5=Keith Roberts |editor6=Peter Walter |chapter=Ch. 11. Section: Single Neurons Are Complex Computation Devices
 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK26910/#A2067
 |title=Molecular Biology of the Cell
 |publisher=Garland Science |isbn=978-0-8153-3218-3 |year=2002 |edition=4th
 |url=https://www.ncbi.nlm.nih.gov/books/NBK21054/
}} 
</ref>

On the other hand, a presynaptic neuron releasing an inhibitory neurotransmitter, such as [[GABA]], can cause an [[inhibitory postsynaptic potential]] (IPSP) in the postsynaptic neuron, bringing the [[membrane potential]] farther away from the threshold, decreasing its excitability and making it more difficult for the neuron to initiate an action potential. If an IPSP overlaps with an EPSP, the IPSP can in many cases prevent the neuron from firing an action potential. In this way, the output of a neuron may depend on the input of many different neurons, each of which may have a different degree of influence, depending on the strength and type of synapse with that neuron. [[John Carew Eccles]] performed some of the important early experiments on synaptic integration, for which he received the [[Nobel Prize for Physiology or Medicine]] in 1963.

==Volume transmission==
When a neurotransmitter is released at a synapse, it reaches its highest concentration inside the narrow space of the synaptic cleft, but some of it is certain to diffuse away before being reabsorbed or broken down.  If it diffuses away, it has the potential to activate receptors that are located either at other synapses or on the membrane away from any synapse.  The extrasynaptic activity of a neurotransmitter is known as ''volume transmission''.<ref>{{cite journal |vauthors=Zoli M, Torri C, Ferrari R |title=The emergence of the volume transmission concept |journal=Brain Res. Brain Res. Rev. |volume=26 |pages=136–47 |year=1998 |pmid=9651506 |doi=10.1016/S0165-0173(97)00048-9 |issue=2–3|s2cid=20495134 |display-authors=etal}}</ref>  It is well established that such effects occur to some degree, but their functional importance has long been a matter of controversy.<ref>{{cite journal |vauthors=Fuxe K, Dahlström A, Höistad M |title=From the Golgi-Cajal mapping to the transmitter-based characterization of the neuronal networks leading to two modes of brain communication: wiring and volume transmission |journal=Brain Res Rev |volume=55 |pages=17–54 |year=2007 |pmid=17433836 |doi=10.1016/j.brainresrev.2007.02.009 |issue=1|display-authors=etal|url=https://iris.unipa.it/bitstream/10447/9980/1/2007%20Brain%20Res%20rev.pdf |hdl=10447/9980 |s2cid=1323780 |hdl-access=free }}</ref>

Recent work indicates that volume transmission may be the predominant mode of interaction for some special types of neurons.  In the mammalian cerebral cortex, a class of neurons called [[neurogliaform cells]] can inhibit other nearby cortical neurons by releasing the neurotransmitter GABA into the extracellular space.<ref name="Olah2009">{{cite journal |vauthors=Oláh S, Füle M, Komlósi G |title=Regulation of cortical microcircuits by unitary GABA-mediated volume transmission |journal=Nature |volume=461 |pages=1278–81 |year=2009 |pmid=19865171 |doi=10.1038/nature08503 |issue=7268 |pmc=2771344|bibcode = 2009Natur.461.1278O |display-authors=etal}}</ref> Along the same vein, GABA released from neurogliaform cells into the extracellular space also acts on surrounding [[astrocyte]]s, assigning a role for volume transmission in the control of ionic and neurotransmitter homeostasis.<ref>{{cite journal |vauthors=Rózsa M, Baka J, Bordé S, Rózsa B, Katona G, Tamás G |title=Unitary GABAergic volume transmission from individual interneurons to astrocytes in the cerebral cortex |journal=Brain Structure and Function |volume=222 |issue=1 |pages=651–659 |year=2015|pmid=26683686 |doi=10.1007/s00429-015-1166-9  |s2cid=30728927 |display-authors=etal|url=http://publicatio.bibl.u-szeged.hu/7829/1/2015%20-%20R%C3%B3zsa%20et%20al.%20-%20Unitary%20GABAergic%20volume%20transmission%20from%20individual%20interneurons%20to%20astrocytes%20in%20the%20cerebral%20cortex%20-%20Brain%20St.pdf }}</ref> Approximately 78% of neurogliaform cell boutons do not form classical synapses. This may be the first definitive example of neurons communicating chemically where classical synapses are not present.<ref name="Olah2009" />

==Relationship to electrical synapses==
An [[electrical synapse]] is an electrically [[electrical conductor|conductive]] link between two abutting [[neuron]]s that is formed at a narrow gap between the pre- and postsynaptic [[cell (biology)|cell]]s, known as a [[gap junction]]. At gap junctions, cells approach within about 3.5&nbsp;[[Nanometre|nm]] of each other, rather than the 20 to 40&nbsp;nm distance that separates cells at chemical synapses.<ref>{{harvnb|Kandel|Schwartz|Jessell|2000|p=176}}</ref><ref>{{harvnb|Hormuzdi|Filippov|Mitropoulou|Monyer|Bruzzone|2004}}</ref> As opposed to chemical synapses, the postsynaptic potential in electrical synapses is not caused by the opening of ion channels by chemical transmitters, but rather by direct electrical coupling between both neurons. Electrical synapses are faster than chemical synapses.<ref name="KandelPrin"/> Electrical synapses are found throughout the nervous system, including in the [[retina]], the [[reticular nucleus of the thalamus]], the [[neocortex]], and in the [[hippocampus]].<ref>{{cite journal |vauthors=Connors BW, Long MA |title=Electrical synapses in the mammalian brain |journal=Annu. Rev. Neurosci. |volume=27 |issue= 1|pages=393–418 |year=2004 |pmid=15217338 |doi=10.1146/annurev.neuro.26.041002.131128 |url=https://zenodo.org/record/894386 }}</ref> While chemical synapses are found between both excitatory and inhibitory neurons, electrical synapses are most commonly found between smaller local inhibitory neurons.  Electrical synapses can exist between two axons, two dendrites, or between an axon and a dendrite.<ref>{{cite journal |vauthors=Veruki ML, Hartveit E |title=Electrical synapses mediate signal transmission in the rod pathway of the mammalian retina |journal=J. Neurosci. |volume=22 |issue=24 |pages=10558–66 |date=December 2002 |pmid=12486148|pmc=6758447 |doi=10.1523/JNEUROSCI.22-24-10558.2002 }}
</ref><ref>{{cite journal |vauthors=Bennett MV, Pappas GD, Aljure E, Nakajima Y |title=Physiology and ultrastructure of electrotonic junctions. II. Spinal and medullary electromotor nuclei in mormyrid fish |journal=J. Neurophysiol. |volume=30 |issue=2 |pages=180–208 |date=March 1967 |pmid=4167209 |doi=10.1152/jn.1967.30.2.180 }}</ref>  In some [[fish]] and [[amphibian]]s, electrical synapses can be found within the same terminal of a chemical synapse, as in [[Mauthner cells]].<ref>{{cite journal |vauthors=Pereda AE, Rash JE, Nagy JI, Bennett MV |title=Dynamics of electrical transmission at club endings on the Mauthner cells |journal=Brain Res. Brain Res. Rev. |volume=47 |issue=1–3 |pages=227–44 |date=December 2004 |pmid=15572174 |doi=10.1016/j.brainresrev.2004.06.010 |citeseerx=10.1.1.662.9352 |s2cid=9527518 }}</ref>

==Effects of drugs==
{{Main|Neuropharmacology}}

One of the most important features of chemical synapses is that they are the site of action for the majority of [[psychoactive drugs]].  Synapses are affected by drugs, such as curare, strychnine, cocaine, morphine, alcohol, LSD, risperidone, and countless others. These drugs have different effects on synaptic function, and often are restricted to synapses that use a specific neurotransmitter. For example, [[curare]] is a poison that stops [[acetylcholine]] from depolarizing the postsynaptic membrane, causing [[paralysis]]. [[Strychnine]] blocks the inhibitory effects of the neurotransmitter [[glycine]], which causes the body to pick up and react to weaker and previously ignored stimuli, resulting in uncontrollable [[muscle spasms]]. [[Morphine]] acts on synapses that use [[endorphin]] neurotransmitters, and [[alcohol (drug)|alcohol]] increases the inhibitory effects of the neurotransmitter [[GABA]]. [[LSD]] interferes with synapses that use the neurotransmitter [[serotonin]]. [[Risperidone]] is a blocker of various receptors including several dopamine and serotonin receptors, and it can bind with high affinity  to some types of serotonin receptors. [[Cocaine]] blocks reuptake of [[dopamine]] and therefore increases its effects.

==History and etymology==

During the 1950s, [[Bernard Katz]] and [[Paul Fatt]] observed spontaneous miniature synaptic currents at the frog [[neuromuscular junction]].<ref>{{Cite journal|last1=Augustine|first1=George J.|last2=Kasai|first2=Haruo|date=2007-02-01|title=Bernard Katz, quantal transmitter release and the foundations of presynaptic physiology|journal=The Journal of Physiology|language=en|volume=578|issue=Pt 3|pages=623–625|doi=10.1113/jphysiol.2006.123224|pmid=17068096|pmc=2151334}}</ref> Based on these observations, they developed the 'quantal hypothesis' that is the basis for our current understanding of neurotransmitter release as [[exocytosis]] and for which Katz received the [[Nobel Prize in Physiology or Medicine]] in 1970.<ref>{{Cite journal|date=1970-10-24|title=Nobel prize.|journal=British Medical Journal|language=en|volume=4|issue=5729|pages=190|doi=10.1136/bmj.4.5729.190|pmid=4320287|pmc=1819734}}</ref> In the late 1960s, [[Ricardo Miledi]] and Katz advanced the hypothesis that depolarization-induced influx of calcium ions triggers [[exocytosis]].

[[Charles Scott Sherrington|Sir Charles Scott Sherringtonin]] coined the word 'synapse' and the history of the word was given by Sherrington in a letter he wrote to John Fulton:

{{blockquote|'I felt the need of some name to call the junction between nerve-cell and nerve-cell... I suggested using "syndesm"... He [ [[Michael Foster (physiologist)|Sir Michael Foster]] ] consulted his Trinity friend [[A. W. Verrall|Verrall]], the [[Euripides|Euripidean]] scholar, about it, and Verrall suggested "synapse" (from the Greek "clasp").'–Charles Scott Sherrington<ref name="Cowan"/>}}

==See also==
* [[Acclimatisation (neurons)]]
* [[Neuroscience]]
* [[Neurexin]]
* [[Ribbon synapse]]

==Notes==
{{reflist|30em}}

==References==
* {{cite book |last=Carlson |first=Neil R. |author-link=Neil R. Carlson |title=Physiology of Behavior |publisher=Pearson Education |location=Boston, MA |year=2007 |edition=9th |isbn=978-0-205-59389-7 |title-link=Physiology of Behavior }}
* {{cite book |last1=Kandel |first1=Eric R. |author1-link=Eric R. Kandel |last2=Schwartz |first2=James H. |last3=Jessell |first3=Thomas M. |title=Principles of Neural Science |publisher=McGraw-Hill |location=New York |year=2000 |edition=4th |isbn=978-0-8385-7701-1 |title-link=Principles of Neural Science }}
* {{cite journal |doi=10.1073/pnas.79.7.2415 |vauthors=Llinás R, Sugimori M, Simon SM|title=Transmission by presynaptic spike-like depolarization in the squid giant synapse |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=79 |issue=7 |pages=2415–9 |date=April 1982 |pmid=6954549 |pmc=346205 |bibcode=1982PNAS...79.2415L|doi-access=free}}
* {{cite journal |vauthors=Llinás R, Steinberg IZ, Walton K|year=1981 |journal=Biophysical Journal |volume=33 |issue=3 |pages=323–352 |doi=10.1016/S0006-3495(81)84899-0 |pmc=1327434 |title=Relationship between presynaptic calcium current and postsynaptic potential in squid giant synapse |pmid=6261850 |bibcode=1981BpJ....33..323L}}
* {{cite book |last1=Bear |first1=Mark F. |last2=Connors |first2=Barry W. |last3=Paradiso |first3=Michael A. |title=Neuroscience: Exploring the Brain |publisher=Lippincott Williams & Wilkins |year=2001 |location=Hagerstown, MD |isbn=978-0-7817-3944-3}}
* {{cite journal |last1=Hormuzdi |first1=SG |last2=Filippov |first2=MA |last3=Mitropoulou |first3=G |last4=Monyer |first4=H |last5=Bruzzone |first5=R |date=March 2004 |title=Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks |journal=Biochim Biophys Acta |volume=1662 |issue=1–2 |pages=113–137 |doi=10.1016/j.bbamem.2003.10.023 |pmid=15033583 |doi-access= }}
* {{cite book |last=Karp |first=Gerald |title=Cell and Molecular Biology: concepts and experiments |url=https://archive.org/details/cellmolecularbio04edkarp |url-access=registration |edition=4th |publisher=John Wiley & Sons |location=Hoboken, NJ |year=2005 |isbn=978-0-471-46580-5}}
* {{cite book |last1=Nicholls |first1=J.G. |last2=Martin |first2=A.R. |last3=Wallace |first3=B.G. |last4=Fuchs |first4=P.A. |year=2001 |title=From Neuron to Brain |edition=4th |publisher=Sinauer Associates |location=Sunderland, MA |isbn=978-0-87893-439-3 |url=https://archive.org/details/fromneurontobrai00arob }}

==External links==
{{Spoken Wikipedia|Synapse.ogg|date=2005-06-19}}
* [http://faculty.washington.edu/chudler/synapse.html Synapse Review for Kids]
* [http://ccdb.ucsd.edu/sand/main?stype=lite&keyword=synapse&Submit=Go&event=display&start=1 Synapse – Cell Centered Database]
* [https://synapseweb.clm.utexas.edu/atlas/ Atlas of Ultrastructure Neurocytology] An electron microscope picture gallery assembled by Kristen Harris' lab of synapses and other neuronal structures.
{{Nervous tissue}}

{{DEFAULTSORT:Chemical Synapse}}
[[Category:Cell signaling]]
[[Category:Signal transduction]]
[[Category:Articles containing video clips]]
[[Category:Neural synapse]]