Final answer:
The PR interval on an ECG represents atrial depolarization and ventricular depolarization. It starts with the P wave and ends at the beginning of the QRS complex, with atrial repolarization being masked by the QRS complex.
Step-by-step explanation:
The activities underlying the PR interval on an ECG (electrocardiogram) are atrial depolarization and ventricular depolarization. The PR interval begins with the onset of the P wave, which denotes atrial depolarization, and ends at the beginning of the QRS complex, representing ventricular depolarization. The atrial repolarization occurs during the QRS complex but is masked by it, making it invisible on the ECG. The PR interval covers the spread of electrical activity that results in the preparation of the ventricles for systole following atrial contraction.