Which electrical event in the heart is primarily responsible for ventricular contraction?

The electrical event primarily responsible for ventricular contraction is the activation of the Purkinje fibers. The conduction system of the heart involves a sequence of electrical impulses that enable coordinated heart contractions. When the SA node fires, it initiates the heartbeat by causing atrial contraction. This impulse subsequently reaches the AV node, which serves as a critical relay point to delay the signal slightly, allowing the ventricles to fill with blood.

However, for the actual contraction of the ventricles to occur, it is the Purkinje fibers that play the essential role. These fibers rapidly distribute the electrical signal throughout the ventricular myocardium, triggering ventricular contraction almost simultaneously. This ensures that the ventricles contract efficiently and effectively pump blood out of the heart.

The other options do not directly result in ventricular contraction. While the SA node and AV node are crucial for coordinating the heart's rhythm, they do not directly stimulate the ventricles to contract. The quiescent period refers to a phase of heart activity where the heart muscle is relaxed and not contracting.