Wikipedia

Electrical synapse

Anelectrical synapse,orgap junction,is a mechanical and electricallyconductivesynapse,a functional junction between two neighboringneurons.The synapse is formed at a narrow gap between the pre- and postsynaptic neurons known as agap junction.At gap junctions, such cells approach within about 3.8 nm of each other,[1]a much shorter distance than the 20- to 40-nanometer distance that separates cells at achemical synapse.[2]In many[specify]animals,electrical synapse-based systems co-exist with chemical synapses.

Electrical synapse
Diagram of a gap junction
Identifiers
MeSHD054351
THH1.00.01.1.02024
FMA67130
Anatomical terminology

Compared to chemical synapses, electrical synapses conductnerve impulsesfaster and provide continuous-time bidirectional coupling via linked cytoplasm.[1][3][4][5]As such, the notion of signal directionality across these synapses is not always defined.[5]They are known to produce synchronization of network activity in the brain[6]and can create chaotic network level dynamics.[7][8]In situations where a signal direction can be defined, they lackgain(unlike chemical synapses)—the signal in the postsynaptic neuron is the same or smaller than that of the originating neuron[citation needed].The fundamental bases for perceiving electrical synapses comes down to theconnexonsthat 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.[9][10]

Structure

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Each gap junction (sometimes called anexus) contains numerous gap junctionchannelsthat cross theplasma membranesof both cells.[11]With a lumen diameter of about 1.2 to 2.0 nm,[2][12]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,[2][13]thereby connecting the two cells'cytoplasm.Thus when themembrane potentialof one cell changes,ionsmay 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 calledconnexonsin vertebrates, one contributed by each cell at thesynapse.[2][12][14]Connexons are formed by six 7.5 nm long, four-pass membrane-spanningproteinsubunits calledconnexins,which may be identical or slightly different from one another.[12]

Anautapseis an electrical (or chemical) synapse formed when the axon of one neuron synapses with its own dendrites.

Effects

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They are found in manyregionsin 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.[15]

  • 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 mammals as it is in cold-blooded animals.[12]
  • 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,depolarizationof the pre-synaptic membrane will always induce a depolarization in the post-synaptic membrane, and vice versa forhyperpolarization.
  • 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 membraneresistanceof the presynaptic and postsynaptic neurons.
  • Long-term changes can be seen in electrical synapses. For example, changes in electrical synapses in theretinaare seen during light and dark adaptations of the retina.[16]

The relative speed of electrical synapses also allows for many neurons to fire synchronously.[11][12][17]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 thesea hareAplysia,which quickly releases large quantities of ink to obscure enemies' vision.[1]

Normally, current carried by ions could travel in either direction through this type of synapse.[2]However, sometimes the junctions arerectifying synapses,[2]containingvoltage-gated ion channelsthat open in response todepolarizationof an axon's plasma membrane, and prevent current from traveling in one of the two directions.[17]Some channels may also close in response to increasedcalcium(Ca2+
) orhydrogen(H+
) ion concentration, so as not to spread damage from one cell to another.[17]

There is also evidence ofsynaptic plasticitywhere the electrical connection established can either be strengthened or weakened as a result of activity, or during changes in the intracellular concentration of magnesium.[18][19]

Electrical synapses are present throughout thecentral nervous systemand have been studied specifically in theneocortex,hippocampus,thalamic reticular nucleus,locus coeruleus,inferior olivary nucleus,mesencephalic nucleus of the trigeminal nerve,olfactory bulb,retina,andspinal cordofvertebrates.[20]Other examples of functional gap junctions detectedin vivoare in thestriatum,cerebellum,andsuprachiasmatic nucleus.[21][22]

History

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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. Thisreticular hypothesiswas considered to conflict directly with the now predominantneuron 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 1906Nobel Prize in Physiology or Medicine,in which the award went jointly toCamillo Golgi,a reticularist and widely recognized cell biologist, andSantiago Ramón y Cajal,the champion of theneuron doctrineand 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 theNobel committeeacted 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 afterOtto 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 incrayfishin the late 1950s,[23]and were later found in vertebrates.[9]

See also

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References

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  1. ^abcKandel, ER;Schwartz, JH; Jessell, TM (2000).Principles of Neural Science(4th ed.). New York: McGraw-Hill.ISBN978-0-8385-7701-1.
  2. ^abcdefHormuzdi SG, Filippov MA, Mitropoulou G, Monyer H, Bruzzone R (March 2004)."Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks".Biochim. Biophys. Acta.1662(1–2): 113–37.doi:10.1016/j.bbamem.2003.10.023.PMID15033583.
  3. ^Purves, Dale; Williams, Stephen Mark, eds. (2004).Neuroscience(3rd ed.). Sunderland, Mass: Sinauer Associates.ISBN978-0-87893-915-2.
  4. ^Bennett, M. V. L. (1966)."PHYSIOLOGY OF ELECTROTONIC JUNCTIONS*".Annals of the New York Academy of Sciences.137(2): 509–539.doi:10.1111/j.1749-6632.1966.tb50178.x.ISSN0077-8923.
  5. ^abConnors, Barry W.; Long, Michael A. (2004-07-21)."ELECTRICAL SYNAPSES IN THE MAMMALIAN BRAIN".Annual Review of Neuroscience.27(1): 393–418.doi:10.1146/annurev.neuro.26.041002.131128.ISSN0147-006X.
  6. ^Bennett, Michael V.L; Zukin, R.Suzanne (2004)."Electrical Coupling and Neuronal Synchronization in the Mammalian Brain".Neuron.41(4): 495–511.doi:10.1016/S0896-6273(04)00043-1.
  7. ^Makarenko, Vladimir; Llinás, Rodolfo (1998-12-22)."Experimentally determined chaotic phase synchronization in a neuronal system".Proceedings of the National Academy of Sciences.95(26): 15747–15752.doi:10.1073/pnas.95.26.15747.ISSN0027-8424.PMC28115.PMID9861041.
  8. ^Korn, Henri; Faure, Philippe (2003-09-01)."Is there chaos in the brain? II. Experimental evidence and related models".Comptes Rendus. Biologies.326(9): 787–840.doi:10.1016/j.crvi.2003.09.011.ISSN1768-3238.
  9. ^abPurves, Dale; George J. Augustine; David Fitzpatrick; William C. Hall; Anthony-Samuel LaMantia; James O. McNamara & Leonard E. White (2008).Neuroscience(4th ed.). Sinauer Associates. pp. 85–88.ISBN978-0-87893-697-7.
  10. ^Purves, Dale; George J. Augustine; David Fitzpatrick; William C. Hall; Anthony-Samuel LaMantia; Richard D. Mooney; Leonard E. White & Michael L. Platt (2018).Neuroscience(6th ed.). Oxford University Press. pp. 86–87.ISBN978-1605353807.
  11. ^abGibson JR, Beierlein M, Connors BW (January 2005). "Functional properties of electrical synapses between inhibitory interneurons of neocortical layer 4".J. Neurophysiol.93(1): 467–80.doi:10.1152/jn.00520.2004.PMID15317837.
  12. ^abcdeBennett MV, Zukin RS (February 2004)."Electrical coupling and neuronal synchronization in the Mammalian brain".Neuron.41(4): 495–511.doi:10.1016/S0896-6273(04)00043-1.PMID14980200.S2CID18566176.
  13. ^Kandel, Schwartz & Jessell 2000,pp. 178–180
  14. ^Kandel, Schwartz & Jessell 2000,p. 178
  15. ^Rydin Gorjão, Leonardo; Saha, Arindam; Ansmann, Gerrit; Feudel, Ulrike; Lehnertz, Klaus (2018-10-01)."Complexity and irreducibility of dynamics on networks of networks".Chaos: An Interdisciplinary Journal of Nonlinear Science.28(10).arXiv:1808.00305.doi:10.1063/1.5039483.ISSN1054-1500.
  16. ^Dr. John O'Brien || Faculty Biography || The Department of Ophthalmology and Visual Science at the University of Texas Medical School at Houston
  17. ^abcKandel, Schwartz & Jessell 2000,p. 180
  18. ^Palacios-Prado, Nicolas; et al. (Mar 2013)."Intracellular magnesium-dependent modulation of gap junction channels formed by neuronal connexin36".Journal of Neuroscience.33(11): 4741–53.doi:10.1523/JNEUROSCI.2825-12.2013.PMC3635812.PMID23486946.
  19. ^Activity-Dependent; Synapses, Electrical; Haas, Julie S.; et al. (2011)."Activity-dependent long-term depression of electrical synapses".Science.334(6054): 389–93.Bibcode:2011Sci...334..389H.doi:10.1126/science.1207502.PMC10921920.PMID22021860.S2CID35398480.
  20. ^Electrical synapses in the mammalian brain, Connors & Long, "Annu Rev Neurosci" 2004;27:393-418
  21. ^Eugenin, Eliseo A.; Basilio, Daniel; Sáez, Juan C.; Orellana, Juan A.; Raine, Cedric S.; Bukauskas, Feliksas; Bennett, Michael V. L.; Berman, Joan W. (2012-09-01)."The role of gap junction channels during physiologic and pathologic conditions of the human central nervous system".Journal of Neuroimmune Pharmacology.7(3): 499–518.doi:10.1007/s11481-012-9352-5.ISSN1557-1904.PMC3638201.PMID22438035.
  22. ^Pereda, Alberto E.; Curti, Sebastian; Hoge, Gregory; Cachope, Roger; Flores, Carmen E.; Rash, John E. (2013-01-01)."Gap junction-mediated electrical transmission: regulatory mechanisms and plasticity".Biochimica et Biophysica Acta (BBA) - Biomembranes.1828(1): 134–146.doi:10.1016/j.bbamem.2012.05.026.ISSN0006-3002.PMC3437247.PMID22659675.
  23. ^Pappas, George D.; Bennett, Michael V. L. (1966)."Specialized junctions involved in electrical transmission between neurons".Annals of the New York Academy of Sciences.137(2): 495–508.doi:10.1111/j.1749-6632.1966.tb50177.x.ISSN0077-8923.

Further reading

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