Which part of the heart's conduction system directly stimulates the ventricular myocardium to contract?

Choice A rationale The bundle branches are responsible for conducting the electrical impulse from the Bundle of His down through the interventricular septum toward the apex of the heart. While they are a vital link in the conduction pathway, they do not directly interface with the ventricular myocytes to trigger contraction. Their role is to rapidly transport the signal so that the ventricles can eventually be stimulated in a coordinated fashion from the bottom up by the terminal fibers. Choice B rationale The Atrioventricular (AV) node acts as a gatekeeper and electrical relay station between the atria and the ventricles. Its primary function is to delay the electrical impulse for about 0.1 seconds, allowing the atria to contract fully and fill the ventricles before ventricular systole begins. It does not directly touch or stimulate the ventricular myocardium; instead, it passes the signal into the Bundle of His to continue the conduction process down toward the ventricles. Choice C rationale The Sinoatrial (SA) node is the natural pacemaker of the heart, located in the right atrium. It initiates the electrical impulse that starts the cardiac cycle, typically at a rate of 60 to 100 beats per minute. The impulse then spreads across the atria, causing them to contract. The SA node is far removed from the ventricular muscle and relies on the rest of the conduction system to deliver the signal to the lower chambers. Choice D rationale Purkinje fibers are the terminal part of the heart's conduction system. They spread throughout the ventricular myocardium and are responsible for the direct electrical stimulation of the cardiac muscle cells. Once the impulse reaches these fibers, it triggers the synchronized contraction of the ventricles, allowing blood to be ejected into the pulmonary and systemic circulation. This rapid conduction ensures that the entire ventricular mass contracts almost simultaneously for maximum efficiency in pumping.