Gap junctions as electrical synapses

Gap junctions form between many cell types. Between neurons, they constitute one class of electrical synapses. Gap junctions are aggregates of membrane channels between the conjoined cells and in mammals they are comprised of connexins, which are encoded by a gene family that has 21 members in humans. Each of the coupled cells contributes a hemichannel to each cell-cell channel. Channel turnover can occur within hours, or channels may last a lifetime. Not all connexins will form channels with every other connexin, and connexin compatibility is one limit on junction formation. Other mechanisms including cell attachment and recognition molecules contribute to specificity of gap junction formation. Electrical synapses are characterized by specificity, but mistakes, i.e., inappropriate connections, are sometimes made. Pannexins/innexins form gap junctions in invertebrates, but apparently only hemichannels in mammals.