Final answer:
The P wave on the electrocardiogram represents the depolarization wave received by the atria from the SA node. It is followed by the QRS complex for ventricular depolarization and then by the T wave for ventricular repolarization.
Step-by-step explanation:
The portion of the electrocardiogram (ECG) that represents the depolarization wave received by the atria from the sinoatrial (SA) node is the P wave. The P wave indicates the depolarization of the atria, which precedes the atrial contraction. The contraction of the atria starts shortly after the onset of the P wave.
The QRS complex represents the depolarization of the ventricles and also includes the repolarization of the atria, which is not visible on the ECG because it is masked by the larger electrical activity of the ventricles. Following the depolarization of the ventricles, the ventricular muscles begin to contract as the QRS reaches the peak of the R wave. Lastly, the T wave represents the repolarization of the ventricles, which leads to their relaxation.