Sequence of Cardiac Activation
Depolarisation is initiated by an impulse form the SA node. Impulse spreads through both atria. Inter-atrial conduction and atrial myocardial depolarization creates the "P" wave in the cardiac cycle. The normal time taken is 0.08 to 0.12 seconds. The impulse reaches the AV node which delays the impulse about 0.04 to 0.01 seconds. The impulse is transmitted along the branches of the bundle of His to the Purkinje fibers, activating both ventricles almost simultaneously. Activation of ventricular muscle proceeds from apex towards base of heart. The time for a normal ventricular depolarization to occur is 0.06 to 0.10 seconds. PR interval represents the amount of time the electrical impulse takes form the beginning of atrial depolarization to the beginning of ventricular excitation. Normal time is 0.12 o 0.20 seconds. After depolarization, there is ventricular depolarization indicated by the "T" wave in the ECG.
The heart's electrical physiology serves to maintain a coordinated cycle for the electrical depolarization of myocardial fibers that produce chamber filling and emptying.