What initiates the electrical impulses that regulate heart rate?

The sinoatrial (SA) node is known as the natural pacemaker of the heart, responsible for initiating electrical impulses that regulate heart rate. Located in the right atrium, the SA node generates spontaneous action potentials due to its unique pacemaker cells, which have the ability to reach threshold potential without external stimulation. This results in the rhythmic contractions of the heart, orchestrating the heart's pumping action by sending impulses through the cardiac conduction system.

Once the electrical impulse originates from the SA node, it propagates through the atria, leading to their contraction and subsequently reaching the atrioventricular (AV) node. The AV node serves as a critical pathway that connects the atria to the ventricles but does not generate impulses itself. It briefly pauses the transmission to ensure the atria have emptied before the ventricles contract.

Other components of the cardiac conduction system, like the Bundle of His and Purkinje fibers, play roles in the conduction and distribution of impulses throughout the ventricles but are not the ones that initiate the electrical activity. Therefore, the SA node is identified as the primary site of impulse generation, effectively setting the heart rate under normal physiological conditions.