On a normal ECG tracing, which wave represents depolarization of the atria?

1. A normal ECG shows several prominent, labeled points. 2. The small P wave represents depolarization of the atria, and the atria begin contracting about 25 ms after it starts. 3. The larger QRS complex represents depolarization of the ventricles, and the T wave represents their repolarization. 4. The PR segment is the interval between two waves rather than a wave itself, and the QRS complex and T wave represent different electrical events from atrial depolarization. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 19 "The Cardiovascular System: The Heart", section 19.2, p. 854_