Final answer:
The P wave in an ECG indicates atrial depolarization leading to atrial systole, while the QRS complex indicates ventricular depolarization preceding ventricular systole. The ST segment shows when the heart muscle is fully depolarized and ventricular contraction occurs, and the T wave represents ventricular repolarization.
Step-by-step explanation:
The P wave represents atrial depolarization, followed immediately by atrial systole. The QRS represents ventricular depolarization, followed immediately by ventricular systole. The ST segment corresponds to phase 2 of the action potential, during which time the heart muscle is completely depolarized and contraction normally occurs.
An electrocardiogram (ECG) provides a graphical representation of these events. The P wave is associated with the depolarization of the atria, leading to atrial contraction or systole. This occurs before the QRS complex. Shortly after, the QRS complex signifies the depolarization of the ventricles, preceding ventricular systole or contraction. The T wave on an ECG indicates the repolarization of the ventricles, marking the onset of ventricular relaxation. It is worth noting that repolarization of the atria happens during the QRS complex, but it is not visible on the ECG because it is masked by the larger electrical activity of ventricular depolarization.