Which structure acts as the natural pacemaker of the heart?

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

Answer: (B)

The sinoatrial node acts as the heart’s natural pacemaker because its cells have the highest intrinsic rate of spontaneous firing and set the pace for the entire heart. It continuously generates electrical impulses that automatically reach threshold and trigger rhythmic heartbeats, driving atrial contraction and the overall heart rhythm. This pace dominates under normal conditions, so it determines how fast the heart beats.

If the SA node’s pacing falls away, the atrioventricular node can take over briefly, but at a slower rate. The Bundle of His and the Purkinje fibers are crucial conduction pathways that rapidly transmit impulses to the ventricles; they don’t initiate the rhythm themselves, and their intrinsic pacing is slower.