Which structure initiates the electrical impulse that sets the heart's pace?

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

Answer: (D)

The heart’s rhythm is set by the structure that acts as the natural pacemaker. The sinoatrial node, located in the right atrium near the superior vena cava, has automaticity—its cells depolarize spontaneously and regularly. This intrinsic pacing activity generates the electrical impulse that sets the heart rate (about 60–100 bpm in a healthy adult) and starts the sequence that leads to atrial contraction.

From there, the impulse spreads through the atria to the atrioventricular node, which provides a brief delay to allow ventricular filling, and then travels down the His-Purkinje system to cause ventricular contraction. The AV node and the conduction pathways are essential for coordinating timing, but they don’t normally initiate the pace; they can take over if the SA node fails, but at a slower rate. Purkinje fibers simply conduct the impulse rapidly to the ventricles.

So, the structure that initiates the electrical impulse that sets the heart’s pace is the sinoatrial node.