are arrays of small hexagonal tubes called connexons (composed of monomer transmembrane protein - connexin) which form aqueous channels roughly between the two cells. Alignment of connexons from each cell across the gap results in the formation of the pores which functionally define the gap junction. By watching the fluorescent dyes of increasing sizes spread from the cell into which it was injected to neighboring cells the channels were shown to be about 2 nanometers in diameter. This channel is wide enough to allow the exchange of small intracellular molecules (1000 daltons Ca & other signaling ions, cAMP, G-6-P, amino acids & nucleotides).
Gap junctions allow electrical and metabolic coupling among cells (smooth muscle, cardiac & nerve cells) because signals inititated in one cell can readily propagate to neighboring cells. The action potential’s in heart tissue flows from cell to cell providing the rhythmic contraction of the heartbeat.
Gap junctions may play a role in embryonic development. When cells begin to differentiate they start to lose their gap junctions and only maintain connections with cells of the same tissue type maintaining the ability to behave as a cooperative assembly.