Gap junction

Gap junctions are one of three 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 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 channel 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 being shown to pass through the connexons; this proved them to be a generalized regulated gate between cells in gap junctions. A type of hemichannel connexon also forms channels to the extracellular regions as well.

More than 26 different connexins are known to comprise gap junctions in various different tissues; 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 led to an increase in 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 connexins from vertebrate ones. While the 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. While an ephapse, like a gap junction, also involves the transmission of electrical signals, the two are distinct from each other. 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 is often referred to as ephaptic coupling rather than as an ephapse.