<P> The two bundle branches taper out to produce numerous Purkinje fibers, which stimulate individual groups of myocardial cells to contract . </P> <P> The spread of electrical activity through the ventricular myocardium produces the QRS complex on the ECG . </P> <P> Atrial repolarization occurs and is masked during the QRS complex by ventricular depolarization on the ECG . </P> <P> The last event of the cycle is the repolarization of the ventricles . It is the restoring of the resting state . In the ECG, repolarization includes the J point, ST segment, and T and U waves . </P>

Where do electrical impulses in the heart come from