What process begins when the impulse reaches the heart apex?

When the impulse reaches the heart apex, it primarily results in ventricular excitation. The heart's electrical conduction system begins in the sinoatrial (SA) node, transmitting impulses through the atria, leading to atrial contraction. The impulse travels down to the atrioventricular (AV) node and then to the bundle of His, which bifurcates into the right and left bundle branches, extending down to the apex of the heart.

At the apex, the Purkinje fibers distribute the electrical impulse throughout the ventricular myocardium. This timely conduction leads to ventricular excitation, culminating in ventricular contraction. The contraction is essential for pumping blood effectively out of the ventricles into the aorta and pulmonary artery. Thus, identifying ventricular excitation as the key process that begins when the impulse reaches the apex captures the central role of electrical signaling in cardiac function.

The other processes mentioned either precede or follow the ventricular excitation and contraction, making them less relevant in this specific context.