Final answer:
The P wave on an ECG depicts the depolarization of the atria, leading to their contraction, and precedes the QRS complex and T wave, which represent ventricular depolarization and repolarization, respectively.
Step-by-step explanation:
The P wave on an electrocardiogram (ECG) represents the depolarization of cells in the atria. During a cardiac cycle, depolarization is the process that leads to the contraction of the heart's chambers.
Initially, when both the atria and ventricles are relaxed in diastole, the P wave occurs, signaling the depolarization of the atria and followed by atrial contraction, known as atrial systole. This is before the large QRS complex, which represents the more intense electrical activity needed for depolarizing the larger ventricular muscle, leading to ventricular contraction. The final step depicted in an ECG is the T wave, illustrating the repolarization of the ventricles indicating the beginning of ventricular relaxation.