Which structure coordinates atrial and ventricular activation and delays impulses by about 0.12 seconds to allow atrial contraction?

• A. Atrioventricular node
• B. Sinoatrial node
• C. Purkinje fibers
• D. Purkinje network

Answer: (A)

The key idea is the way the heart times atrial and ventricular activation by a deliberate pause in conduction at the AV node. When the electrical impulse travels from the atria toward the ventricles, the AV node slows it enough to produce about a 0.12-second delay. This pause lets the atria finish their contraction and pushes any remaining blood into the ventricles, ensuring they fill properly before they contract. After this delay, the impulse travels quickly through the His-Purkinje system to activate the ventricles almost together. The sinoatrial node starts the heartbeat as the natural pacemaker, not the delay mechanism, and Purkinje fibers and the Purkinje network mainly serve to rapidly spread the impulse through the ventricles rather than to create the atrial-ventricular delay.