As discussed in chapter 2, the rate of spontaneous diastolic depolarization in the sino-atrial (sinus) node is usually more rapid than the rate of spontaneous diastolic depolarization in subsidiary pacemakers such as those located in the AV node and those in the His-Purkinje system. For this reason, the sinus node is the dominant pacemaker of the heart and impulses arising in the sinus node depolarize, or overdrive, the other potential pacemaker regions.