Vagus nerves and baroreceptor control of ventricular performance.

• Several recent studies have shown that stimulation of the carotid sinus baroreceptors exerts a reflex, negative inotropic effect upon ventricular performance." Since it was generally accepted that the vagus nerves do not directly influence the ventricular myocardium," it was assumed that the efferent fibers of the carotido-ventricular reflex traversed sympathetic pathways exclusively. In many" or all of the experiments in these previous studies, the cervical vagus nerves were transected to preclude alterations in cardiac frequency, atrial performance, and atrio-ventricular conduction. According to recent studies," however, the assumption that the parasympathetic division of the autonomic nervous system does not directly affect the ventricles is unwarranted. Efferent vagal stimulation has been shown to depress ventricular performance." Furthermore, the vagi have been shown to mediate a depressant influence upon the left ventricle in response to carotid chemoreceptor stimulation.' Since it is well established that the cardiac vagal centers are also activated by carotid sinus baroreceptor stimulation, it was decided to determine whether the baroreceptor reflex affected myocardial performance via parasympathetic pathways to