Final answer:
The QRS complex on an ECG represents the plateau in the myocardial action potential when the ventricles contract and pump blood, with the T wave indicating ventricular repolarization.
Step-by-step explanation:
The plateau in the myocardial action potential occurs during ventricular systole, when the ventricles are contracting and pumping blood. This phase corresponds to the QRS complex on an electrocardiogram (ECG) which represents ventricular depolarization. It is during the R wave's peak that the ventricles begin to contract. The T wave follows after the QRS complex, indicating the repolarization of the ventricles and marking the beginning of ventricular relaxation. Atrial repolarization occurs during the QRS complex but is not visible on an ECG as it is masked by the more dominant ventricular depolarization.