Which node initiates the normal heartbeat?

• A. SA node
• B. AV node
• C. Bundle of His
• D. Purkinje fibers

Answer: (A)

In a normal heartbeat, the sinoatrial node acts as the natural pacemaker. Its cells have the fastest intrinsic rate of spontaneous depolarization, so it sets the rhythm by generating the impulse that begins each heartbeat. This impulse travels through the atria, causing atrial contraction, then to the atrioventricular node where there’s a brief delay to allow ventricular filling, and onward through the His-Purkinje system to depolarize the ventricles. The AV node or the His-Purkinje system can pace only if the SA node fails, and they do so at slower rates, but normal initiation comes from the SA node.