Relative Roles of Sympathetic and Parasympathetic Nervous Systems in the Carotid Sinus Reflex in Dogs

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Abstract
The effects of electrical stimulation of the carotid sinus on sinus rate and atrioventricular (A-V) conduction were studied before and after alternate interruption of the vagi and sympathetic nerves to the heart in dogs. In group I, carotid sinus stimulation caused a similar absolute decrease in sinus rate before and after vagotomy, although after vagotomy the response was more latent. Subsequent administration of propranolol blocked the effects of carotid sinus stimulation on sinus rate. In group II (dogs with intact vagi), the effects of carotid sinus stimulation and bilateral occlusion of the common carotid artery on sinus rate were abolished by bilateral stellatectomy and upper thoracic ganglionectomy. In group III, the effects of carotid sinus stimulation on A-V conduction during atrial pacing at a fixed rate were not significantly altered by vagotomy and were blocked by subsequent administration of propranolol. In group IV, carotid sinus stimulation had no effect on A-V conduction after sympathetic denervation. It is concluded that the effects of the carotid sinus reflex on the heart are mediated primarily by the sympathetic nervous system efferents. In addition, evidence suggesting that sympathetic tone is a major determinant of vagal tone has been presented.