What is the primary pacemaker site that initiates the cardiac impulse in normal rhythm?

• A. SA Node
• B. AV Node
• C. Purkinje network
• D. Bundle of His

Answer: (A)

The heart’s natural pace is set by the sinoatrial node because its pacemaker cells have the fastest automatic, or spontaneous, depolarization rate. This means the SA node reaches the threshold for firing first each cycle and initiates the cardiac impulse, setting the rhythm at about 60–100 beats per minute in a healthy adult. After the impulse starts here, it spreads through the atria to the atrioventricular node and onward through the His-Purkinje system to coordinate ventricular contraction. If the SA node fails, slower backup pacemakers take over—the AV node at roughly 40–60 bpm, and the Purkinje/ventricular system even slower—so under normal conditions the SA node drives the rhythm. The other structures—the AV node, Purkinje network, and Bundle of His—are crucial conduction pathways, but they do not normally generate the heartbeat rate because their cells have lower automaticity.