Gap junction

A gap junction may also be called a nexus or macula communicans. When found in nerves they are also referred to as an electrical synapse. While an ephapse has some similarities to a gap junction, by modern definition these two entities are different.

Gap junctions are a specialized intercellular connection between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulated gate between cells.

One gap junction channel is composed of two connexons (or hemichannels), which connect across the intercellular space. Gap junctions are analogous to the plasmodesmata that join plant cells.

Gap junctions are expressed in virtually all tissues of the body, with the exception of adult fully developed skeletal muscle and mobile cell types such as sperm or erythrocytes. Gap Junctions, however, are not found in simpler organisms such as sponges and slime molds.

Structure

In vertebrates, gap junction hemichannels are primarily homo- or hetero-hexamers of connexin proteins. Invertebrate gap junctions comprise proteins from the innexin family. Innexins have no significant sequence homology with connexins. Though differing in sequence to connexins innexins are similar enough to connexins for people to consider innexins form gap junctions in vivo in the same way connexins do. The recently characterized pannexin family, which was originally thought to form intercellular channels (based on similar amino acid sequence similarity to innexins), in fact functions as single-membrane channels that communicate with the extracellular environment, and have been shown to pass calcium and ATP.