View source for Electrical synapse

{{short description|Type of connection between neurons}}
{{Infobox anatomy
| Name        = Electrical synapse
| Latin       =
| Image       = Gap_cell_junction-en.svg
| Caption     = Diagram of a gap junction
| Image2      =
| Caption2    =
| Precursor   =
| System      =
| Artery      =
| Vein        =
| Nerve       =
| Lymph       =
}}
An '''electrical synapse''', or '''gap junction''', is a mechanical and electrically [[conductor (material)|conductive]] [[synapse]], a functional junction between two neighboring [[neuron]]s. The synapse is formed at a narrow gap between the pre- and postsynaptic neurons known as a [[gap junction]].  At gap junctions, such cells approach within about 3.8&nbsp;nm of each other,<ref name=Kandel00>{{cite book |last1=Kandel |first1=ER |last2=Schwartz |first2=JH |last3=Jessell |first3=TM |title=Principles of Neural Science |publisher=McGraw-Hill |location=New York |year=2000 |isbn=978-0-8385-7701-1 |edition=4th |author-link=Eric R. Kandel|title-link=Principles of Neural Science }}</ref> a much shorter distance than the 20- to 40-nanometer distance that separates cells at a [[chemical synapse]].<ref name=Hormuzdi04>{{cite journal |vauthors=Hormuzdi SG, Filippov MA, Mitropoulou G, Monyer H, Bruzzone R |title=Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks |journal=Biochim. Biophys. Acta |volume=1662 |issue=1–2 |pages=113–37 |date=March 2004 |pmid=15033583 |doi=10.1016/j.bbamem.2003.10.023 |doi-access=free }}</ref> In many{{specify|date=September 2024}} [[animal]]s, electrical synapse-based systems co-exist with chemical synapses.

Compared to chemical synapses, electrical synapses conduct [[nerve impulse]]s faster and provide continuous-time bidirectional coupling via linked cytoplasm.<ref name="Kandel00" /><ref>{{Cite book |title=Neuroscience |date=2004 |publisher=Sinauer Associates |isbn=978-0-87893-915-2 |editor-last=Purves |editor-first=Dale |edition=3rd |location=Sunderland, Mass |editor-last2=Williams |editor-first2=Stephen Mark}}</ref><ref>{{Cite journal |last=Bennett |first=M. V. L. |date=1966 |title=PHYSIOLOGY OF ELECTROTONIC JUNCTIONS* |url=https://nyaspubs.onlinelibrary.wiley.com/doi/10.1111/j.1749-6632.1966.tb50178.x |journal=Annals of the New York Academy of Sciences |language=en |volume=137 |issue=2 |pages=509–539 |doi=10.1111/j.1749-6632.1966.tb50178.x |issn=0077-8923|url-access=subscription }}</ref><ref name=":0">{{Cite journal |last=Connors |first=Barry W. |last2=Long |first2=Michael A. |date=2004-07-21 |title=ELECTRICAL SYNAPSES IN THE MAMMALIAN BRAIN |url=https://www.annualreviews.org/doi/10.1146/annurev.neuro.26.041002.131128 |journal=Annual Review of Neuroscience |language=en |volume=27 |issue=1 |pages=393–418 |doi=10.1146/annurev.neuro.26.041002.131128 |issn=0147-006X|url-access=subscription }}</ref> As such, the notion of signal directionality across these synapses is not always defined.<ref name=":0" /> They are known to produce synchronization of network activity in the brain<ref>{{Cite journal |last=Bennett |first=Michael V.L |last2=Zukin |first2=R.Suzanne |date=2004 |title=Electrical Coupling and Neuronal Synchronization in the Mammalian Brain |url=https://linkinghub.elsevier.com/retrieve/pii/S0896627304000431 |journal=Neuron |language=en |volume=41 |issue=4 |pages=495–511 |doi=10.1016/S0896-6273(04)00043-1|doi-access=free }}</ref> and can create chaotic network level dynamics.<ref>{{Cite journal |last=Makarenko |first=Vladimir |last2=Llinás |first2=Rodolfo |date=1998-12-22 |title=Experimentally determined chaotic phase synchronization in a neuronal system |url=https://pnas.org/doi/full/10.1073/pnas.95.26.15747 |journal=Proceedings of the National Academy of Sciences |language=en |volume=95 |issue=26 |pages=15747–15752 |doi=10.1073/pnas.95.26.15747 |issn=0027-8424 |pmc=28115 |pmid=9861041}}</ref><ref>{{Cite journal |last=Korn |first=Henri |last2=Faure |first2=Philippe |date=2003-09-01 |title=Is there chaos in the brain? II. Experimental evidence and related models |url=https://comptes-rendus.academie-sciences.fr/biologies/articles/10.1016/j.crvi.2003.09.011/ |journal=Comptes Rendus. Biologies |language=en |volume=326 |issue=9 |pages=787–840 |doi=10.1016/j.crvi.2003.09.011 |issn=1768-3238|url-access=subscription }}</ref> In situations where a signal direction can be defined, they lack [[Gain (electronics)|gain]] (unlike chemical synapses)—the signal in the postsynaptic neuron is the same or smaller than that of the originating neuron.{{citation needed|date=September 2024}} The fundamental bases for perceiving electrical synapses comes down to the [[connexons]] that are located in the gap junction between two neurons. Electrical synapses are often found in neural systems that require the fastest possible response, such as defensive reflexes. An important characteristic of electrical synapses is that they are mostly bidirectional, allowing impulse transmission in either direction.<ref name="Purves">{{cite book | author = Purves, Dale | author2 = George J. Augustine | author3 = David Fitzpatrick | author4 = William C. Hall | author5 = Anthony-Samuel LaMantia | author6 = James O. McNamara | author7 = Leonard E. White | name-list-style = amp | title = Neuroscience | edition = 4th | publisher = Sinauer Associates | pages = 85–88 | year = 2008 | isbn = 978-0-87893-697-7}}</ref><ref>{{cite book | author = Purves, Dale | author2 = George J. Augustine | author3 = David Fitzpatrick | author4 = William C. Hall | author5 = Anthony-Samuel LaMantia | author6 = Richard D. Mooney | author7 = Leonard E. White | author8 = Michael L. Platt | name-list-style = amp | title = Neuroscience | edition = 6th | publisher = Oxford University Press | pages = 86–87 | year = 2018 | isbn = 978-1605353807}}</ref>

==Structure==
Each gap junction (sometimes called a ''nexus'') contains numerous gap junction [[ion channel|channel]]s that cross the [[cell membrane|plasma membranes]] of both cells.<ref name=Gibson05>{{cite journal |vauthors=Gibson JR, Beierlein M, Connors BW |title=Functional properties of electrical synapses between inhibitory interneurons of neocortical layer 4 |journal=J. Neurophysiol. |volume=93 |issue=1 |pages=467–80 |date=January 2005 |pmid=15317837 |doi=10.1152/jn.00520.2004 }}</ref>  With a lumen diameter of about 1.2 to 2.0&nbsp;nm,<ref name=Hormuzdi04/><ref name=Bennet04>{{cite journal |doi=10.1016/S0896-6273(04)00043-1 |vauthors=Bennett MV, Zukin RS |title=Electrical coupling and neuronal synchronization in the Mammalian brain |journal=Neuron |volume=41 |issue=4 |pages=495–511 |date=February 2004 |pmid=14980200 |s2cid=18566176 |doi-access=free }}</ref> the pore of a gap junction channel is wide enough to allow ions and even medium-size molecules like signaling molecules to flow from one cell to the next,<ref name=Hormuzdi04/><ref>{{harvnb|Kandel|Schwartz|Jessell|2000|pp=178–180}}</ref> thereby connecting the two cells' [[cytoplasm]].  Thus when the [[membrane potential]] of one cell changes, [[ion]]s may move through from one cell to the next, carrying positive charge with them and depolarizing the postsynaptic cell.

Gap junction channels are composed of two hemichannels called [[connexon]]s in vertebrates, one contributed by each cell at the [[synapse]].<ref name=Hormuzdi04/><ref name=Bennet04/><ref>{{harvnb|Kandel|Schwartz|Jessell|2000|p=178}}</ref>  Connexons are formed by six 7.5&nbsp;nm long, four-pass membrane-spanning [[protein]] subunits called [[connexin]]s, which may be identical or slightly different from one another.<ref name=Bennet04/>

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

==Effects==
They are found in many [[Gap junction#Occurrence and distribution|regions]] in animal and human body. The simplicity of electrical synapses results in synapses that are fast, but more importantly the bidirectional coupling can produce very complex behaviors at the network level.<ref>{{Cite journal |last=Rydin Gorjão |first=Leonardo |last2=Saha |first2=Arindam |last3=Ansmann |first3=Gerrit |last4=Feudel |first4=Ulrike |last5=Lehnertz |first5=Klaus |date=2018-10-01 |title=Complexity and irreducibility of dynamics on networks of networks |url=https://pubs.aip.org/aip/cha/article-abstract/28/10/106306/1051711/Complexity-and-irreducibility-of-dynamics-on?redirectedFrom=fulltext |journal=Chaos: An Interdisciplinary Journal of Nonlinear Science |volume=28 |issue=10 |doi=10.1063/1.5039483 |issn=1054-1500|arxiv=1808.00305 }}</ref>
*Without the need for receptors to recognize chemical messengers, signal transmission at electrical synapses is more rapid than that which occurs across chemical synapses, the predominant kind of junctions between neurons.  Chemical transmission exhibits synaptic delay—recordings from squid synapses and neuromuscular junctions of the frog reveal a delay of 0.5 to 4.0 milliseconds—whereas electrical transmission takes place with almost no delay.  However, the difference in speed between chemical and electrical synapses is not as marked in [[Mammal|mammals]] as it is in cold-blooded animals.<ref name=Bennet04/>
*Because electrical synapses do not involve neurotransmitters, electrical neurotransmission is less modifiable than chemical neurotransmission.
*The response always has the same sign as the source. For example, [[depolarization]] of the pre-synaptic membrane will always induce a depolarization in the post-synaptic membrane, and vice versa for [[Hyperpolarization (biology)|hyperpolarization]].
*The response in the postsynaptic neuron is in general smaller in amplitude than the source. The amount of attenuation of the signal is due to the membrane [[Electrical resistance and conductance|resistance]] of the presynaptic and postsynaptic neurons.
*Long-term changes can be seen in electrical synapses.  For example, changes in electrical synapses in the [[retina]] are seen during light and dark adaptations of the retina.<ref>[https://med.uth.edu/ibp/faculty/john-obrien/ Dr. John O'Brien || Faculty Biography || The Department of Ophthalmology and Visual Science at the University of Texas Medical School at Houston<!-- Bot generated title -->]</ref>

The relative speed of electrical synapses also allows for many neurons to fire synchronously.<ref name=Gibson05/><ref name=Bennet04/><ref name=Kandel00_180>{{harvnb|Kandel|Schwartz|Jessell|2000|p=180}}</ref> Because of the speed of transmission, electrical synapses are found in escape mechanisms and other processes that require quick responses, such as the response to danger of the [[sea hare]] ''[[Aplysia]]'', which quickly releases large quantities of ink to obscure enemies' vision.<ref name=Kandel00/>

Normally, current carried by ions could travel in either direction through this type of synapse.<ref name=Hormuzdi04/> However, sometimes the junctions are [[rectifying synapse]]s,<ref name=Hormuzdi04/> containing [[voltage-gated ion channel]]s that open in response to [[depolarization]] of an axon's plasma membrane, and prevent current from traveling in one of the two directions.<ref name=Kandel00_180/>  Some channels may also close in response to increased [[calcium in biology|calcium]] ({{chem|Ca|2+}}) or [[hydrogen]] ({{chem|H|+}}) ion concentration, so as not to spread damage from one cell to another.<ref name=Kandel00_180/>

There is also evidence of [[synaptic plasticity]] where the electrical connection established can either be strengthened or weakened as a result of activity, or during changes in the intracellular concentration of magnesium.<ref>{{cite journal | last1 = Palacios-Prado | first1 = Nicolas | display-authors = etal  | date = Mar 2013 | title = Intracellular magnesium-dependent modulation of gap junction channels formed by neuronal connexin36 | journal = Journal of Neuroscience | volume = 33 | issue = 11| pages = 4741–53 | doi = 10.1523/JNEUROSCI.2825-12.2013 | pmid = 23486946 | pmc = 3635812}}</ref><ref>{{cite journal | last1 = Activity-Dependent | last2 = Synapses | first2 = Electrical | last3 = Haas | first3 = Julie S. | display-authors = etal  | year = 2011 | title =  Activity-dependent long-term depression of electrical synapses| journal = Science | volume = 334 | issue = 6054| pages = 389–93 | doi = 10.1126/science.1207502 | pmid=22021860| bibcode = 2011Sci...334..389H | s2cid = 35398480 | pmc = 10921920 }}</ref>

Electrical synapses are present throughout the [[central nervous system]] and have been studied specifically in the [[neocortex]], [[hippocampus]], [[thalamic reticular nucleus]], [[locus coeruleus]], [[inferior olivary nucleus]], [[mesencephalic nucleus of the trigeminal nerve]], [[olfactory bulb]], [[retina]], and [[spinal cord]] of [[vertebrate]]s.<ref>Electrical synapses in the mammalian brain, Connors & Long, "Annu Rev Neurosci" 2004;27:393-418</ref> Other examples of functional gap junctions detected ''in vivo'' are in the [[striatum]], [[cerebellum]], and [[suprachiasmatic nucleus]].<ref>{{Cite journal|last1=Eugenin|first1=Eliseo A.|last2=Basilio|first2=Daniel|last3=Sáez|first3=Juan C.|last4=Orellana|first4=Juan A.|last5=Raine|first5=Cedric S.|last6=Bukauskas|first6=Feliksas|last7=Bennett|first7=Michael V. L.|last8=Berman|first8=Joan W.|date=2012-09-01|title=The role of gap junction channels during physiologic and pathologic conditions of the human central nervous system|journal=Journal of Neuroimmune Pharmacology|volume=7|issue=3|pages=499–518|doi=10.1007/s11481-012-9352-5|issn=1557-1904|pmc=3638201|pmid=22438035}}</ref><ref>{{Cite journal|last1=Pereda|first1=Alberto E.|last2=Curti|first2=Sebastian|last3=Hoge|first3=Gregory|last4=Cachope|first4=Roger|last5=Flores|first5=Carmen E.|last6=Rash|first6=John E.|date=2013-01-01|title=Gap junction-mediated electrical transmission: regulatory mechanisms and plasticity|journal=Biochimica et Biophysica Acta (BBA) - Biomembranes|volume=1828|issue=1|pages=134–146|doi=10.1016/j.bbamem.2012.05.026|issn=0006-3002|pmc=3437247|pmid=22659675}}</ref>

== History ==
The model of a reticular network of directly interconnected cells was one of the early hypotheses for the organization of the nervous system at the beginning of the 20th century.  This [[Reticular theory|reticular hypothesis]] was considered to conflict directly with the now predominant ''[[neuron doctrine]]'', a model in which isolated, individual neurons signal to each other chemically across synaptic gaps.  These two models came into sharp contrast at the award ceremony for the 1906 [[Nobel Prize in Physiology or Medicine]], in which the award went jointly to [[Camillo Golgi]], a reticularist and widely recognized cell biologist, and [[Santiago Ramón y Cajal]], the champion of the [[neuron doctrine]] and the father of modern neuroscience. Golgi delivered his Nobel lecture first, in part detailing evidence for a reticular model of the nervous system. Ramón y Cajal then took the podium and refuted Golgi's conclusions in his lecture. Modern understanding of the coexistence of chemical and electrical synapses, however, suggests that both models are physiologically significant; it could be said that the [[Nobel Prize#Selection|Nobel committee]] acted with great foresight in awarding the Prize jointly.

There was substantial debate on whether the transmission of information between neurons was chemical or electrical in the first decades of the twentieth century, but chemical synaptic transmission was seen as the only answer after [[Otto Loewi]]'s demonstration of chemical communication between neurons and heart muscle. Thus, the discovery of electrical communication was surprising.

Electrical synapses were first demonstrated between escape-related giant neurons in [[crayfish]] in the late 1950s,<ref>{{Cite journal |last=Pappas |first=George D. |last2=Bennett |first2=Michael V. L. |date=1966 |title=Specialized junctions involved in electrical transmission between neurons |url=https://nyaspubs.onlinelibrary.wiley.com/doi/10.1111/j.1749-6632.1966.tb50177.x |journal=Annals of the New York Academy of Sciences |language=en |volume=137 |issue=2 |pages=495–508 |doi=10.1111/j.1749-6632.1966.tb50177.x |issn=0077-8923|url-access=subscription }}</ref> and were later found in vertebrates.<ref name="Purves" />

==See also==
* [[Junctional complex]]
* [[Cardiac muscle]]

== References ==
{{Reflist|30em}}

==Further reading==
* {{cite book
|author1=Andrew L. Harris |author2=Darren Locke | year      = 2009
| title     = Connexins, a guide
| pages     = 574
| publisher = Springer
| location  = New York
| url       = https://www.springer.com/978-1-934115-46-6
| isbn      = 978-1-934115-46-6}}
* {{Cite journal
| last       = Haas
| first      = Julie S.
| year       = 2011
| author2    = Baltazar Zavala |author3=Carole E. Landisman
| title      = Activity-dependent long-term depression of electrical synapses
| journal    = Science
| volume     = 334
| issue      = 6054
| pages      = 389–393
| doi        = 10.1126/science.1207502
| pmid       = 22021860
| bibcode = 2011Sci...334..389H
| s2cid = 35398480
| pmc= 10921920
}}
* {{Cite journal
| last       = Hestrin
| first      = Shaul
| year       = 2011
| title      = The strength of electrical synapses
| journal    = Science
| volume     = 334
| issue      = 6054
| pages      = 315–316
| doi        = 10.1126/science.1213894
| pmid       = 22021844
| pmc= 4458844
| bibcode = 2011Sci...334..315H
}}

{{Nervous tissue}}
{{Authority control}}

{{DEFAULTSORT:Electrical Synapse}}
[[Category:Cell communication]]
[[Category:Electrophysiology]]
[[Category:Neural synapse]]