The SA node initiates the depolarization impulse, which in turn generates an action potential that spreads throughout the atria to the AV node. Here, the impulse is delayed briefly before continuing on to the ventricles through the AV bundle, bundle branches and Purkinje fibers. Action potentials, which spread from the autorhythmic cells to the contractile cells, are electrical events. The subsequent contraction of the contractile cells is a mechanical event that causes a heartbeat. Here we see heart electrical activity reflected in an ECG wave tracing. The P wave represents atrial depolarization. The QRS complex represents ventricular depolarization and the T wave represents ventricular repolarization. Please note that in a normal ECG wave tracing atrial repolorization is hidden by the QRS complex. These diagrams show the correlation between heart electrical activity and an ECG wave tracing. The P wave represents atrial depolarization, which is followed by atrial contraction. The QRS complex represents ventricular depolarization, which is followed by ventricular contraction and the T wave represents ventricular repolarization, which is followed by ventricular relaxation.