Where does ventricular contraction begin?

Ventricular contraction begins in the ventricles of the heart, specifically after the electrical activity has moved through the conduction system. The correct answer is associated with the Q wave, which is part of the QRS complex in an electrocardiogram (ECG).

The Q wave represents the initial depolarization of the ventricles, signaling the start of ventricular contraction. This depolarization triggers the ventricles to contract and pump blood out of the heart. The significance of the Q wave lies in its timing; it indicates that the ventricles have received the electrical impulse necessary for contraction following the atrial depolarization represented by the P wave.

The P wave itself corresponds to atrial depolarization, and while it is a critical part of the cardiac cycle, it does not initiate ventricular contraction. The S wave, also part of the QRS complex, follows the Q wave and is associated with having already begun ventricular contraction. Thus, understanding the relationship between these waves on an ECG is crucial for recognizing the sequence of electrical events leading to effective heart function.