The cardiac muscle action potential is prolonged compared with that of skeletal muscle fibers (it has a long plateau) to match the slow contraction of cardiac muscle fibers. During the action potential, another action potential (and cardiac muscle contraction) cannot take place. This is the refractory period, which is much longer than in skeletal muscle.
Cardiac muscle action potentials spread throughout the myocardium, from muscle cell to muscle cell. This ensures that the myocardium contracts as a unit. Electrical signals pass through the gap junctions of the intercalated discs between cardiac muscle cells. The branching of cardiac myocytes ensures the spread of the electrical signal. In addition, spot junctions (macula adherens or desmosomes) in the intercalated disks provide support for contractile stress.
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