Nervous control of heart rate: activity in the cardiac vagus of the dogfish.

In the absence of any sympathetic innervation to the heart, nervous control of heart rate in the dogfish is solely attributable to inhibitory parasympathetic input from the vagus nerve. Action potentials can be recorded from the cardiac vagus of the dogfish following its exposure in the anterior cardinal sinus. The rates of heartbeat and ventilation, blood pressure, hematocrit, and responses to external stimuli such as hypoxia, which include a bradycardia, remained typical of fish with their nervous and circulatory systems virtually intact. The recordings included sporadically active units that accelerated during hypoxia, possibly inducing the bradycardia, and regular bursts of action potentials synchronous with ventilatory movements that appeared to arise reflexly from stimulation of pharyngeal proprioceptors. These bursts may loosely couple the respiratory and cardiac pumps, increasing the probability of concurrence between periods of maximum water and blood flow. The preparation enables detailed study of the temporal relationships between the pumps and its neurophysiological basis.