Cardiac vagal and sympathetic nerve responses to baroreceptor stimulation in the dog

The effects of ascending stepwise pressure changes in the isolated carotid sinuses on cardiac vagal and sympathetic nerve activities were studied in anesthetized, open chest dogs. The steady state responses of the cardiac vagal and the sympathetic nerve activity and arterial blood pressure were plotted against the sinus pressure and the relations were approximated by the normal distribution function (response curve). The sinus pressure- vs. "reflex gain" relations (reflex gain curve) were approximated by the normal density function. The maximum gain and the "range of change" were found to be greater for the vagal than for the sympathetic and arterial pressure responses. The sinus pressure values derived from "response curves" and "reflex gain curves" for vagal and sympathetic nerve responses were close to each other, while these values and those obtained from arterial pressure responses were considerably apart. It was concluded that: (1) The cardiac vagal neurons are more sensitive to the baroreceptor input than the sympathetic neurons; (2) The similar type of baroreceptor afferent inputs reach the cardiac vagal and the sympathetic structures which are controlling the autonomic outflows.