Two types of cardiac muscle cells work together to achieve a normal heartbeat. First, contractile cells, which make up most of the myocardium, contract to reduce chamber volumes and propel blood out of the chambers. Second, conducting cells, which are specialized cardiac muscle cells, initiate and distribute the electrical stimulus to contract.
The sinoatrial node contains conducting cells that are autorhythmic, meaning they generate action potentials spontaneously. In an adult at rest, an action potential is generated in the sinoatrial node roughly 70 times each minute. This electrical impulse is distributed to atrial cells, to stimulate their contraction, and it is propagated toward the ventricles. Before reaching the contractile cells of the ventricles, the signal will travel through the atrioventricular node, the atrioventricular bundle, the bundle branches, and the Purkinje fibers. The distribution of the impulse over this system takes time, and by the time the ventricular muscle cells are stimulated to contract, the atria have finished contracting.