Which structure is the primary pacemaker of the heart?

• A. Sinoatrial node
• B. Atrioventricular node
• C. Bundle of His
• D. Purkinje fibers

Answer: (A)

The heart’s rhythm starts where automatic cardiac tissue fires the quickest. The sinoatrial node sits in the right atrium and has the fastest rate of spontaneous depolarization, so it sets the pace for the entire heart. Its impulses start the heartbeat and spread through the atria, then to the ventricles via the AV node, which provides a normal delay to allow the atria to contract first. The AV node can take over if the SA node fails, but its intrinsic rate is slower, so the heart would beat more slowly. The bundle of His and the Purkinje fibers are conduction pathways to the ventricles and don’t generate impulses themselves. So the sinoatrial node is the primary pacemaker.