The upper panel of this figure illustrates a non-premature or late atrial premature beat. It is able to enter both the fast and slow pathways because both will have recovered their excitability after the preceding beat and re-entry will not occur. The lower panel illustrates an earlier premature beat. It is able to enter the slow but not the fast pathway because only the slow pathway will have recovered its excitability after the preceding beat. However, by the time the impulse travels the slow pathway to reach the lower portion of the AV node, the fast pathway will have regained its excitability and will then be able to conduct the impulse in a retrograde direction to return to the atria and by so doing, to establish a re-entry circuit. This is one mechanism for paroxysmal supraventricular tachycardias that occur in the absence of an AV nodal bypass tract