Gap junctions
Gap junctions, also referred to as cell-to-cell channels, are present in most animal tissues and form aqueous channels between adjoining cells. Inorganic molecules as well as numerous metabolites, such as amino acids and sugars, can pass through these channels. However, macromolecules, such as proteins and nucleic acids, cannot move across. Cell-to-cell channels allow communication between cells, including excitable cells. Gap junctions allow communication between cells in both directions. They also allow the spread of an extracellular signal; this is achieved by intracellular intermediaries, such as cyclic AMP and calcium ions, moving between channels. [1] One example of communication occurs in heart muscle cells, whereby a rapid flow of ions through these cells enables a speedy and coordinated response to a stimulus, ultimately leading to contraction of the muscle. [2]
Specialised forms of transmembrane proteins, called connexins, form gap junctions. Twelve connexin molecules assemble to form a gap junction. A single connexin molecule has four membrane spanning proteins. Connexons, also called hemichannels, are formed when six connexin molecules are arranged in a hexagonal structure; a cell-to-cell channel is formed when two connexons are aligned and joined together.
Gap junctions differ from other membrane channels in a number of different ways. This includes cell-to-cell channels spanning across two membranes, instead of one, and channels forming between the cytoplasm of two cells, rather than to the extracellular matrix.
An important point to note is that cell-to-cell channels are closed by high calcium and hydrogen ion concentrations; this mechanism protects healthy cells from damaged ones. [3]