Introduction to Gap Junctions
A gap junction Figure 2 is a specialized cell junction that directly connects the cytoplasm of two cells. In this way, gap junctions allow various molecules and ions to pass freely between cells. A single gap junction channel is composed of two connexons, also known as hemichannels, which line up across the intercellular space.
In vertebrates, most gap junction hemichannels are composed of a complex of six connexin proteins, each characterized by four transmembrane domains. Six connexin sub-units assemble to create one connexon, or hemichannel. Hemichannels of uniform connexin composition are called homomeric, while those with differing connexins are heteromeric. Furthermore, gap junction channels formed from two identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic. Channel composition, both at the level of the individual connexin proteins and the hemichannel, influences the function of the gap junction.
Functions of Gap Junctions
Gap junctions serve a number of critical cellular functions. Most notably, gap junctions allow for direct electrical communication between cells. However, gap junctions also permit forms of chemical communication between cells through the diffusion of small second messengers. Although different connexin subunits can impart different pore sizes to a gap junction channel, large biomolecules, such as nucleic acid and proteins, are too large to pass through gap junctions and are precluded from cytoplasmic transfer between cells.
Gap junctions are expressed in virtually all tissues of the body, but the role of gap junctions is most obvious for cell types that benefit from direct electrical communication, such as neurons and cardiac muscle Figure 1. Gap junctions in cardiac muscle allow the cells of the heart to contract in tandem, for example.
A gap junction located in neurons is often referred to as an electrical synapse. However, electrical synapses in the human brain are thought to be relatively small in number compared to chemical synapses and few brain regions have been discovered with significant gap junction coupling between neurons. However, the glial cells known as astrocytes appear to be strongly coupled by gap junctions.