gap junction (nexus) A passage through the lipid bilayers of adjacent plasma membranes that mediates the transfer of small molecules or ions between interacting cells. Gap junctions are abundant in epithelial tissues and cardiac muscle. They consist of hexagonally packed tubes, approximately 7 nm in diameter, through which small molecules or ions may directly pass from the interior of one cell to the interior of the other. Gap junctions, together with chemical synapses (which function through neurotransmitters), are communicating junctions and comprise one of several types of cell junction.

gap junction A communicating junction between cells, which provides a rapid passage for ions and small molecules such that they need not cross the bilipid layer of the opposed membranes. At the junction these membranes are only 2–3 nm apart, and within the intervening space there are arrays of closely packed cylinders, approximately 7 nm in diameter, each of which is pierced by a central channel. These junctions are a particular feature of cells that are in direct electrical coupling with one another (e.g. nerve and heart muscle cells, and perhaps embryonic cells).