What directly causes the contraction of the myocardium in the ventricular walls?

The contraction of the myocardium in the ventricular walls is primarily driven by the Purkinje fibers, which play a crucial role in the heart's conduction system. Once the electrical impulse from the sinoatrial (SA) node spreads through the heart, it reaches the atrioventricular (AV) node, where it is briefly delayed before continuing through the bundle of His and into the Purkinje fibers.

These fibers are specialized muscle cells that rapidly distribute the electrical signal throughout the ventricular myocardium. This distribution ensures a coordinated contraction of the ventricular walls, allowing for effective pumping of blood into the aorta and pulmonary arteries. Thus, it is the Purkinje fibers that directly stimulate the myocardial cells in the ventricles to contract.

In contrast, the SA node initiates the heartbeat and sets the pace for the heart, while the AV node serves primarily as a relay point that regulates the timing of impulses. The aorta, though essential for blood circulation, does not play a role in the contraction of the myocardium.