How two cells share a cytoplasm
A gap junction forms when a connexon (a ring of six connexin proteins) in one cell's membrane docks with a matching connexon in a neighboring cell. The result is a continuous aqueous channel built from twelve connexin subunits in total, linking the two cytoplasms directly without crossing the extracellular space.
These channels are large enough to pass ions and small molecules under roughly one kilodalton, including calcium ions, cAMP, and metabolites, but they exclude proteins and nucleic acids. Connexins also exist as undocked hemichannels in the membrane, which have their own signaling roles.
Coordinating tissues that must act together
Direct coupling lets groups of cells behave as a unit. In heart muscle, gap junctions concentrated at intercalated discs pass the electrical wave that keeps contraction synchronized. In smooth muscle, the lens of the eye, and many epithelia, the same coupling spreads signals and nutrients quickly.
Calcium waves traveling cell to cell through these junctions are a common way tissues propagate a local signal across a wider field. Mutations in connexin genes are linked to conditions affecting hearing and skin, which illustrates how dependent normal tissue function is on this coupling.