Nonlinearities of the human carotid baroreceptor-cardiac reflex

Carotid baroreceptors of nine healthy young men and women were stretched or compressed with neck suction or pressure, before and after beta-adrenergic and cholinergic blockade, to evaluate several nonlinearities of sinus node baroreflex responses. Sinus node inhibition was related linearly to the intensity of brief baroreceptor stimuli over a range extending from carotid-distending pressures of about 101 +/- 5 (mean +/- SE) to 160 +/- 6 mm Hg (the subject's average systolic pressure was 108 +/- 2 mm Hg). Sinus node response to sustained (5 seconds) neck suction or pressure were strikingly asymmetrical. Responses were abolished by atropine, or by atropine and propranolol. Propranolol alone augmented sinus node responses to both neck suction and pressure. These results suggest that, in normal human subjects, sinus node responses to abrupt alterations of afferent baroreceptor traffic are nonlinear and are mediated by fluctuations of efferent cholinergic activity. Most of the observed nonlinear behavior of the integrated reflex can be explained on the basis of known properties of afferent and central portions of the baroreflex arc.