Gap junction

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Gap junction
Vertebrate gap junction
Identifiers
MeSH	D017629
TH	H1.00.01.1.02024
FMA	67423
Anatomical terminology [edit on Wikidata]

Gap junctions are membrane channels between cells that allow the exchange of substances from the cytoplasm of one cell directly to the cytoplasm of an adjacent cell.^[1] Substances exchanged include small molecules, substrates, and metabolites.^[1]

They were first imaged using an electron microscope circa 1952.^[2][3] They were named in 1969^[4] after the 2-4 nm gap they bridge between cell membranes.^[5]

Gap junctions use protein complexes known as connexons to connect one cell to another. The proteins are called connexins. Gap junction proteins include the more than 26 types of connexin, and at least 12 non-connexin components that make up the gap junction complex.^[6] These components include the tight junction protein 1 which holds the membranes together,^[7] sodium channels,^[8] and aquaporin.^[9][10]

More gap junction proteins have become known due to the development of next-generation sequencing. Connexins were found to be structurally homologous between vertebrates and invertebrates but different in sequence.^[11] As a result, the term innexin is used to differentiate invertebrate connexins.^[12] There are more than 20 known innexins,^[13] along with unnexins in parasites and vinnexins in viruses.

An electrical synapse is a gap junction that can transmit action potentials between neurons. Connexon pairs act as generalized regulated gates for ions and smaller molecules between cells. Hemichannel connexons form channels to the extracellular environment.^{[14][15][16][17]}

A gap junction may also be called a nexus or macula communicans. It should not be confused with an ephapse. While an ephapse, like a gap junction, involves the transmission of electrical signals, the two are distinct from each other because ephaptic coupling involves electrical signals external to the cells. Ephapses are often studied in the context of electrically induced potentials propagated among groups of nerve cell membranes, even in the absence of gap junction communication, with no discrete subcellular structures known.^[18][19] Unlike gap junctions, no specific structure related to an ephapse has yet been described, so the process is often referred to as ephaptic coupling rather than as an ephapse.