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 (connexons), approximately 10 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.
http://www.chemistry.emory.edu/justice/seminar/morphology%20of%20syn.htm Morphology of Synapses: includes a stylized representation of a gap junction