A gap junction is a specialised channel of transport and communication for small molecules and ions between the cytoplasm of two closely apposed cells.
Electron microscopy reveals the two cell membranes to be separated by only 1-2 nanometres at the point where the gap junctions are sited. Each membrane has a large number of small protein channels termed connexons. Individual connexons of both cells are joined to each other to create a communicating tunnel which spans the membrane gap. The internal diameter of the passage is about 2 nanometres.
Each connexon on each membrane is made up of 6 polypeptide chain subunits which span the membrane.
Electrophysiological studies have shown low electrical resistance for the cell when there are abundant gap junctions. Injection of low molecular weight dyes results in rapid spread between cells. Therefore, gap junctions provide a pathway by which small molecules and ions can rapidly move down concentration gradients between cells.
Gap junctions are evident in epithelia and smooth muscle. They permit all of the cells to act as a synctium. However, if continued passive transport is detrimental to the synctium, individual gap junctions can be rapidly close.
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