Which part of the conduction system mediates the contraction of the ventricles?

The Purkinje fibers play a crucial role in mediating the contraction of the ventricles. Located within the ventricular wall, these fibers are specialized conductive pathways that rapidly propagate electrical impulses throughout the ventricles. When the impulses travel from the atrioventricular node through the Bundle of His and then into the bundle branches, they eventually reach the Purkinje fibers. This system ensures that the ventricles contract in a coordinated manner, allowing for an efficient and effective ejection of blood into the systemic and pulmonary circulation.

The sinoatrial node, while essential as the primary pacemaker of the heart, primarily influences atrial contraction rather than ventricular contraction. The bundle branches, which conduct impulses down the interventricular septum to the apex of the heart, are important, but they do not directly trigger the contraction of the ventricular muscle itself. The internodal pathways serve to conduct electrical signals through the atria and towards the atrioventricular node, playing a key role in coordinating atrial contraction, but again, they do not directly mediate ventricular contraction. Thus, the Purkinje fibers are the primary structure responsible for initiating the contraction of the ventricles effectively.