Most rapidly depolarizing fibers in the ventricle are:

Purkinje fibers and ventricular fibers are basically rapidly depolarizing fibers. Yet, the rates of their depolarization are different. Fiber at the apex depolarizes relatively slow. As one moves toward base, the rates of depolarization become more and more rapid. Fiber at the base depolarizes most rapidly. It immediately repolarizes as well.Then, the repolarization spreads in the reverse direction. When repolarization reaches the apex, the apical fiber has just now finished its depolarization; it now begins to repolarize.