The SA node is the normal pacemaker for the entire heart. Its depolarization normally initiates the cardiac excitation sequence, generating the action potential that leads to depolarization of all other cardiac muscle cells, and so its discharge rate determines the heart rate, the number of times the heart contracts per minute. The action potential initiated in the SA node spreads throughout the myocardium, passing from cell to cell by way of gap junctions. The spread throughout the right atrium and from the right atrium to the left atrium does not depend on fibers of the conducting system. The conduction through atrial muscle cells is rapid enough that the two atria are depolarized and contract at essentially the same time. However, the spread of the action potential to the ventricles is more complicated and involves the rest of the conducting system. The link between atrial depolarization and ventricular depolarization is a portion of the conducting system called the atrioventricular (AV) node, which is located at the base of the right atrium. The action potential spreading through the muscle cells of the right atrium causes depolarization of the AV node. This node has a particularly important characteristic: the propagation of action potentials through the AV node is relatively slow (requiring approximately 0.1 s). This results in a delay that allows atrial contraction to be completed before ventricular excitation occurs.
After leaving the AV node, the impulse enters the wall—the interventricular septum—between the two ventricles. This pathway has conducting-system fibers termed the bundle of His (or atrioventricular bundle) after its discoverer (pronounced Hiss). It should be emphasized that the AV node and the bundle of His constitute the only electrical link between the atria and the ventricles. Except for this pathway, the atria are completely separated from the ventricles by a layer of nonconducting connective tissue. Within the interventricular septum the bundle of His divides into right and left bundle branches, which eventually leave the septum to enter the walls of both ventricles. These fibers in turn make contact with Purkinje fibers, large conducting cells that rapidly distribute the impulse throughout much of the ventricles. Finally, the Purkinje fibers make contact with ventricular myocardial cells, by which the impulse spreads through the rest of the ventricles. The rapid conduction along the Purkinje fibers and the diffuse distribution of these fibers cause depolarization of all right and left ventricular cells more or less simultaneously and ensure a single coordinated contraction. Actually, though, depolarization and contraction begin slightly earlier in the bottom (apex) of the ventricles and spread upward. The result is a more efficient contraction, like squeezing a tube of toothpaste from the bottom up.