Which component initiates the electrical impulse causing the heart to contract?

• A. The atrioventricular node
• B. The Purkinje fibers
• C. The ventricular myocardium
• D. The sino-atrial node (SA node) situated in the wall of the right atrium

Answer: (D)

The heart’s rhythm begins with the natural pacemaker, the sinoatrial node, located in the wall of the right atrium. Its pacemaker cells spontaneously generate electrical impulses that trigger atrial contraction and set the pace for the whole heart. The impulse then travels through the conduction system to the atrioventricular node, onward to the His-Purkinje network, and finally to the ventricles to produce ventricular contraction. The AV node and Purkinje fibers are crucial for conduction and coordination, but they do not initiate the heartbeat under normal conditions; if the SA node fails, a secondary pacemaker can take over, though at a slower rate.