Cardiac Conduction System

The cardiac conduction system consists of specialized autorhythmic cells that generate and propagate electrical impulses. The sinoatrial (SA) node in the right atrium serves as the primary pacemaker (60-100 bpm), generating action potentials through spontaneous Phase 4 depolarization via funny channels (If) and T-type calcium channels. Impulses travel through internodal pathways to the atrioventricular (AV) node, which introduces a critical 0.1-second delay allowing atrial contraction to complete before ventricular activation. The AV node is the only electrical connection between atria and ventricles. From the AV node, conduction passes through the Bundle of His, splits into right and left bundle branches (left further dividing into anterior and posterior fascicles), and terminates in Purkinje fibers that rapidly depolarize ventricular myocardium from endocardium to epicardium. Each level has intrinsic automaticity: AV junction 40-60 bpm, ventricular escape 20-40 bpm. Understanding conduction velocity, refractory periods, and automaticity hierarchy is essential for diagnosing and managing arrhythmias and conduction blocks.