Which structure initiates the cardiac electrical impulse?

• A. AV node
• B. Purkinje fibers
• C. SA node
• D. Bundle of His

Answer: (C)

The sinoatrial node is the heart’s natural pacemaker, and it initiates the cardiac electrical impulse. Its cells have automaticity, meaning they spontaneously depolarize to generate an action potential at a normal rate of about 60–100 beats per minute in a healthy adult. This impulse starts in the right atrium and sets the pace for the entire heart.

From there, the impulse travels across the atria to the atrioventricular node, which provides a brief delay to allow for ventricular filling, and then moves through the Bundle of His and the Purkinje fibers to coordinate ventricular contraction. The AV node, Purkinje fibers, and Bundle of His are parts of the conduction pathway and help propagate the signal, but none of them initiates the impulse. The AV node can take over if the SA node fails, but typically at a slower rate.