Gap junction

Gap junctions are one of four broad categories of intercellular connections that form between a multitude of animal cell types. First photographed around 1952, it wasn't until 1969 that gap junctions were referred to as "gap junctions". Named after the 2-4 nm gap they bridged between cell membranes, they had been characterised in more detail by 1967.

Within a gap junction reside protein complexes, referred initially to as "globules", observed to connect one cell to another, and also vesicles within a cell to the outer cell membrane. By 1974 one of the major gap junction proteins was dubbed a "connexin", and six connexins were observed to form a channels called a "connexon", due to the connections connexon pairs made between cells. The initial discovery of gap junctions in nerve cells lent credence to their function in transmission of electrical impulses. Experimental confirmation followed with molecules, ions and electrical impulses shown to pass through the connexons which proved to be a generalized regulated gate between cells in gap junctions. A type of hemichannel connexons also form channels to the extracellular regions as well.

More than 26 different connexins are known to comprise gap junctions in various different tissues, and there are at least 12 other components that form the specialized area of membrane called the gap junction complex. These components include the tight junction protein ZO-1 that holds the membranes close together, sodium channels, and aquaporin.

The increasing ability to sequence the DNA of organisms also increased the complexity of the gap junction family of proteins. The term connexin was used to describe the gap junction proteins connecting two cells with pores. Sequencing of these pore proteins showed them to be structurally similar between vertebrates and invertebrates but different in sequence. As a result the term "innexin" was used to differentiate invertebrate from vertebrate connexins. While sequencing of invertebrate species is far less complete than for vertebrates, more than 20 innexins have already been uncovered, along with unnexins in parasites and vinnexins in viruses.

A gap junction is less frequently called a nexus or macula communicans. There has been some confusion with gap junctions being related to ephapses in the past. While an ephapse, like a gap junction, involves the transmission of electrical signals, the two are different. 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. Unlike gap junctions, no specific structure related to an ephapse has yet been described so the process of "ephaptic coupling" is often referred to rather than an ephapse.