Which structure is the heart's natural pacemaker that sets the heart rate under normal conditions?

• A. Sinoatrial node
• B. AV node
• C. Atrioventricular junction
• D. Purkinje fibers

Answer: (D)

The heart’s natural pacemaker is the sinoatrial node, a small cluster of pacemaker cells in the right atrium near the opening of the superior vena cava. These cells have automaticity, meaning they spontaneously depolarize and generate an impulse at a steady rate—about 60 to 100 beats per minute in a resting adult. This impulse starts there and then propagates through the atrioventricular node and down the His-Purkinje system to coordinate the timing of atrial and then ventricular contractions. Because it sets the pace under normal circumstances, the sinoatrial node is the primary pacemaker.

Purkinje fibers are responsible for rapidly distributing the impulse through the ventricles. They are excellent for quick conduction but do not initiate the heartbeat under normal conditions. Their intrinsic rhythm is slower, and they act as a backup pacemaker only if the primary pacemaker fails, which leads to a slower, less reliable rhythm.