Final answer:
The ECG tracing that occurs when the atria depolarize is called the P wave. The P wave represents depolarization and contraction of the atria.
Step-by-step explanation:
The ECG tracing that occurs when the atria depolarize is called the P wave. The P wave is represented on the ECG as a small upward deflection and it indicates the depolarization and contraction of the atria. It is followed by atrial contraction, or atrial systole.