Gap Junction

A gap junction is a specialised channel between 1.5 to 2 nm in diameter, that connects adjacent cells to allow direct communication with each other.  Gap junctions allow ions, metabolites such as amino acids, and water soluble molecules, but not macromolecules like proteins, to pass through the channel due to it's narrow diameter. The molecules that are able to manoeuvre through the channel play a significant role in intracellular communication.  The key feature of a gap junction is that it enables direct cell to cell communication without needing signalling molecules to travel to the extracellular space or to other organelles.  Another key feature that sets gap junctions apart from the other cell signalling types, is that a gap junction allows communication to flow in both directions between the adjacent cells.  The main purpose of this bi-directional communication is to normalise the conditions between communicating cells.  In addition, the bi-directional communication can assist in spreading the message between the cells caused by any extracellular signals, therefore the cells are able to ellicit a response and coordinate their actions rapidly.

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. 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.

Gap junctions are also present in an electrical synapse, particularly in hormone-secretory neurons, as they allow action potientials to be triggered in multiple cells at once promoting a burst of hormone to be secreted into the circulatory system.

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 with a hole in the middle; a cell-to-cell hydrophilic channel is formed when two connexons are aligned and joined together, 1 from each adjacent cell. The cytoplasm of the two cells is joined together via this channel and this is how materials are able to pass between them.

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. This mechanism is vital, for example if one cell is damaged and the plasma membrane becomes 'leaky' there will be an influx of Ca²⁺ ions into the cell, due to it's concentration gradient. This triggers the gap junction to close, effectively sealing it from the other cell, preventing further damage.