Which component is typically the natural pacemaker of the heart?

Which component is typically the natural pacemaker of the heart?

• A. SA node
• B. AV node
• C. Purkinje fibers
• D. AV bundle

Answer: (A)

The heart’s rhythm is set by the fastest automaticity among its pacemaker tissues. The sinoatrial node, located in the right atrium near the superior vena cava, contains pacemaker cells that fire spontaneously at about 60–100 times per minute. Because its intrinsic rate is highest, it automatically generates the impulse that starts each heartbeat and thus sets the pace for the entire heart. From there, the impulse travels to the AV node and onward through the His-Purkinje system to coordinate contraction of the ventricles. The AV node can pace the heart if the SA node fails, but its intrinsic rate is slower (roughly 40–60 bpm), so it would slow the heartbeat. Purkinje fibers and the AV bundle are parts of the ventricular conduction pathway and are not the normal starting pacemakers.

The heart’s rhythm is set by the fastest automaticity among its pacemaker tissues. The sinoatrial node, located in the right atrium near the superior vena cava, contains pacemaker cells that fire spontaneously at about 60–100 times per minute. Because its intrinsic rate is highest, it automatically generates the impulse that starts each heartbeat and thus sets the pace for the entire heart. From there, the impulse travels to the AV node and onward through the His-Purkinje system to coordinate contraction of the ventricles.

The AV node can pace the heart if the SA node fails, but its intrinsic rate is slower (roughly 40–60 bpm), so it would slow the heartbeat. Purkinje fibers and the AV bundle are parts of the ventricular conduction pathway and are not the normal starting pacemakers.