Which structure initiates the heartbeat?

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

Answer: (A)

The heartbeat is started by the heart’s natural pacemaker, the sinoatrial node. Its pacemaker cells spontaneously depolarize, giving rise to regular electrical impulses that set the pace for the entire heart. This impulse then travels through the atria to the atrioventricular node, which delays the signal to allow the ventricles to fill, and then moves down the conduction system via the bundle of His and Purkinje fibers to trigger ventricular contraction. The reason this structure is the initiator is it has the fastest intrinsic firing rate, so it normally dictates the rhythm. The atrioventricular node can take over only if the SA node fails and does so more slowly, and the Purkinje system mainly distributes the impulse to the ventricles rather than initiating it.