Cardiac Conduction System

Each effective heartbeat depends on two linked events: an electrical impulse must reach the myocardium in the correct sequence, and the resulting contraction must generate adequate perfusion. The normal impulse begins in pacemaker cells of the sinoatrial (SA) node in the right atrium. Their spontaneous depolarization establishes the sinus rate, which is modified by sympathetic and parasympathetic tone. The impulse spreads across the atrial myocardium, producing atrial depolarization and contraction. It then reaches the atrioventricular (AV) node. The AV node conducts slowly on purpose. This brief delay allows the atria to empty into the ventricles before ventricular depolarization begins; without it, atrial and ventricular contraction would occur too close together to support efficient filling. From the AV node, the impulse enters the bundle of His, travels through the right and left bundle branches, and reaches the Purkinje network. Rapid Purkinje conduction distributes activation through the ventricular myocardium, producing a coordinated ventricular contraction. The ventricles therefore contract after the atria, not simultaneously. The AV node and His-Purkinje system have latent escape automaticity. If the SA node fails or conduction from the...