| Electrical synapse | |
|---|---|
 Diagram of a gap junction | |
| Identifiers | |
| MeSH | D054351 |
| TH | H1.00.01.1.02024 |
| FMA | 67130 |
| Anatomical terminology | |
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.
Compared to chemical synapses, electrical synapses conductnerve impulses faster 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 theconnexons 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.[9][10]
Each gap junction (sometimes called anexus) contains numerous gap junctionchannels that cross theplasma membranes of 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 potential of one cell changes,ions 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 calledconnexons in vertebrates, one contributed by each cell at thesynapse.[2][12][14] Connexons are formed by six 7.5 nm long, four-pass membrane-spanningprotein subunits calledconnexins, which may be identical or slightly different from one another.[12]
Anautapse is an electrical (or chemical) synapse formed when the axon of one neuron synapses with its own dendrites.
They are found in manyregions 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.[15]
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 channels that open in response todepolarization of 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 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.[18][19]
Electrical synapses are present throughout thecentral nervous system and 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 cord ofvertebrates.[20] Other examples of functional gap junctions detectedin vivo are in thestriatum,cerebellum, andsuprachiasmatic nucleus.[21][22]
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 hypothesis was 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 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 theNobel 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 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 incrayfish in the late 1950s,[23] and were later found in vertebrates.[9]