Which structure initiates electrical impulses that set the heart rate?

• A. Atrioventricular node
• B. Sinoatrial node
• C. Bundle of His
• D. Purkinje fibers

Answer: (B)

The sinoatrial node acts as the natural pacemaker of the heart, generating electrical impulses that set the heart rate. Its cells spontaneously depolarize at a regular rate, producing an impulse that starts in the right atrium near the superior vena cava and triggers atrial contraction. From there, the signal travels to the atrioventricular node, which provides a brief delay to allow the atria to finish contracting, then moves through the Bundle of His and Purkinje fibers to stimulate the ventricles. The AV node, Bundles, and Purkinje fibers are important conduits, but they do not initiate the beat. If the SA node is out of commission, another pacemaker can take over at a slower rate, but under normal conditions the SA node sets the pace.