Which part of the heart initiates contraction?

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

Answer: (D)

The heartbeat starts with automatic pacemaker cells in the Sinoatrial node. These cells spontaneously generate electrical impulses that depolarize to threshold, creating an action potential. Because the SA node has the fastest inherent rate, it sets the pace for the entire heart, earning the role of natural pacemaker. This impulse quickly spreads through the atria to trigger atrial contraction, then moves to the atrioventricular node, which delays the signal slightly to allow the ventricles time to fill, and then onward through the conduction system to coordinate ventricular contraction. Purkinje fibers and the bundle of His are essential for rapidly propagating the impulse to the ventricles, but they don’t initiate the heartbeat. The AV node also helps coordinate timing by delaying the signal, not by starting the rhythm.