A Cardiocardiac Sympathovagal Reflex in the Cat

The reflex changes in single cardiac vagal efferent fibers elicited by excitation of afferent cardiac sympathetic fibers were studied in cats anesthetized with chloralose and urethane. Efferent vagal and sympathetic units were dissected from the end of a cardiac nerve cut at its junction with the right atrium. In some cases, efferent vagal units were dissected from the cervical vagus. Excitation always evoked a clear reduction in the discharge of cardiac vagal units and a clear increase in the discharge of cardiac sympathetic fibers. The effects on cervical vagal efferents were variable. Hence, excitation of afferent cardiac sympathetic fibers could simultaneously elicit inhibition of the vagal outflow to the heart and excitation of the sympathetic outflow. By contrast, stimulation of the cut central end of the contralateral vagus produced the opposite effects. Convergence of afferent fibers in the vagi and the cardiac sympathetic nerves on the same cardiac vagal and cardiac sympathetic postganglionic neurons was demonstrated. Convergence from carotid sinus baroreceptors was also observed.