A Gap junction or nexus is a specialized intercellular connection between multitudes of animal cell types. Gap junctions are also known as the cylindrical channels between animal cells that allow small molecules and ions to pass from the inside of one cell to the inside of the adjacent cell. It directly connects the cytoplasm (the thick liquid residing between the cell membrane) of 2 cells, which allows various molecules and ions to pass freely between cells. One gap junction channel is composed of two connexons ot hemichannels (an assembly of 6 proteins called connexins that can be a part of a gap junction channel between the cytoplasm of 2 adjacent You do not have access to view this node) which connect across the intercellular space. Gap junctions are analogus to the plasmodesmata (channels which traverse the plant cell walls) that join the plant You do not have access to view this node.
In vertebrates (animals with back bone), gap junction hemichannels are primarily homo or hetero-hexamers (6 sub units) of connexin proteins. Invertebrate (animals without a back-bone) gap junctions comprise of proteins from the innexin (a member of a class of proteins which is used to create gap junctions in invertebrates) family. At gap junctions, the intercellular space is 4nm and unit connexons in the membrane of each cell are lined up with one another. Gap junction channels formed from 2 identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic.
The three principle functions of gap junctions are as follows.
In Heart: Gap junctions are abundant in cardiac and smooth muscles. De polarization (cell’s charge becoming more positive) of one group of muscle cells quickly spreads to adjacent cells, leading to well-coordinated contractions of those muscles.
In Neurons: Electrical coupling also take place between adjacent neurons at specialized gap junctions called the electrical synapses. These synapses also convey neurons across the intracellular space to create a depolarization, or a more positive charge in the adjacent neuron. Electrical signaling is much faster than chemical signaling and can often send out signals in both directions.
In Retina: Neurons within the retina show extensive coupling, both within populations of one cell type, and between different cell types.