DECREASE IN CARDIAC ACTIVITY BY CAROTID SINUS BAROCEPTOR REFLEX.

Electrical stimulation of the sinus nerve or excitation of the carotid baroceptors by increasing the intrasinus pressure significantly decreased the right ventricular contractile force, heart rate, and blood pressure in 19 anesthetized cats. The decreases in force and rate, but not blood pressure, were abolished by stellate ganglionectomy and by 3–6 mg/kg of dichloroisoproterenol (DCI). The decreases in cardiac contractility and rate were not secondary to the hypotension for they were still obtained when aortic pressure was kept constant. The reductions in contractile force and heart rate are the results of an inhibition of sympathetic control of the heart activity. The data indicates that 40 per cent of contractile force measured in the anesthetixed vagotomized preparation is dependent upon tonic sympathetic impulses. In experiments in which the stellate ganglia were decentralized, the force and rate were increased to predenervation levels by stimulating the isolated right stellate ganglion at a frequency of 1/sec. Seventy-five per cent of the maximum response in force and rate were obtained at stimulus frequencies of 3/sec. The sinus baroceptor reflex exercises its major effects on the heart and blood vessels within a relatively limited range of 40 mm Hg on either side of normal blood pressure for the cat.