Cardiovascular responses to electrocardiogram-coupled stimulation of rabbit aortic nerve

Electrical stimulation of the rabbit's aortic nerve during one or more cardiac cycles resulted in a reflex fall in heart rate and mean arterial blood pressure (MAP). The onset of bradycardia and of fall in MAP were independent of the number of beats stimulated. The initial slope of the heart rate and MAP responses increased as the number of beats stimulated increased, reaching a maximum at five beats of stimulation. Bradycardia peaked 8 and 10 beats after the end of one and two cycles of stimulation, respectively, while the peak response occurred at, or prior to, the end of stimulation when 12 or more beats were involved. Onset and recovery of both responses were consistent, and seldom did MAP indicate a return toward control during stimulation. Thus, central nervous system modulation of sympathetic activity to the peripheral vasculature was sustained as long as the aortic nerve input was maintained. However, reflex control of heart rate was more complex, involving simultaneous alteration in both vagal and sympathetic efferent activity.