Synchronization of the heart

The behaviour of cardiac pacemaker cells bears a resemblance to that of relaxation oscillators. A characteristic property of relaxation oscillators is that they may be synchronized by an external signal, if the latter has a periodicity not differing too much from the spontaneous frequency of the oscillator. In the present study, it is shown that the isolated, perfused, and submerged rat heart can be synchronized by a sinusoidal electrical current through the heart. This synchronization is not to be confused with stimulation, which occurs at higher current densities. Synchronization was also found when the frequency of the synchronizing signal was in the neighborhood of a multiple of the spontaneous frequency of the heart. Synchronization was demonstrated for the SA node as well as for a lower pacemaker, presumably the AV node. The possible role which this type of synchronization may play in spontaneous impulse formation in pacemaker tissue is discussed