Cardiac muscle cells

These form the bulk of the myocardium.
They are discrete, multinucleate cells which function as a single unit in response to physiological stimulation.
They are connected, end to end by intercalated discs which contain gap junctions.

Here the membranes of opposing cells are only 3nm apart. The cells are held close together by desmosomes which also serve as attachments for the myofibrils containing the contractile proteins.

Cells are thus effectively coupled, both electrically and mechanically, meaning that the muscle mass of both atria and ventricles functions as a syncitium. This gives rise to smooth, co-ordinated contractions and relaxations. Cells also contain abundant mitochondria and myoglobin and have a rich blood supply.

Cardiac muscle cells are stimulated to produce action potentials by a wave of depolarization originating in the sino-atrial node. This excitation then spreads from cell to cell by electrotonic conduction because of the low resistance pathway at the intercalated disc.