Fig. 3.

Phase resetting in the sinoatrial node (SAN). A Application of subthreshold square-wave pulse in the early (1), middle (2), and late (3) phase of the cardiac cycle in the rabbit isolated SAN (lower tracings in each section are action potentials from the SAN region close to the atrium to show time of stimulus artefacts) and B the relationship between cycle length and time of stimulation in the cardiac cycle, showing that subthreshold depolarising current pulses result in an increase or a decrease in cycle length, depending on the timing of the stimulation within the cardiac cycle. From Sano et al. (1978)