Which small mass of specialized cardiac muscle in the wall of the right atrium acts as the natural pacemaker by producing regular impulses?

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

Answer: (A)

The heart’s rhythm starts with automatic, self-depolarizing tissue that acts as the natural pacemaker. In the wall of the right atrium, a small mass of specialized cardiac muscle—the Sinoatrial node—generates regular electrical impulses without needing neural input. This automatic firing sets the pace for the entire heart because it tends to have the fastest intrinsic rate under normal conditions, so its impulses initiate each heartbeat.

From the Sinoatrial node, the impulse spreads through the atrial muscle to the Atrioventricular node, which provides a brief delay to allow the atria to finish contracting and filling the ventricles. The signal then travels down through the AV bundle and Purkinje fibers to depolarize the ventricles and cause a coordinated contraction.

The Atrioventricular node, Purkinje fibers, and the AV bundle are crucial for conduction, but they do not set the heart rate themselves. They transmit and distribute the impulse started by the Sinoatrial node. Autonomic input can modulate the SA node’s rate, speeding it up or slowing it down as needed.