[Biocybernetic studies on reflectory heart modification in the rabbit]

In rabbits the depressor nerves and cardiac vagal branches were stimulated. Their actions on heart rate, atrio-ventricular conduction time, myocardial action potential and mean central blood pressure were recorded. The frequency-effect characteristics of the chronotropic, dromotropic and electrotropic actions on the heart, resulting from afferent and efferent nerve stimulation, are compared. The participation of each of the depressor nerves in their total effects on heart rate and blood pressure is studied. Time courses of heart rate and blood pressure decrease by afferent and efferent nerve stimulation with sinusoidally modulated pulse rates are presented. The results are discussed with respect to the different dynamics of blood pressure and heart rate control. It is concluded that at least two mechanisms are involved in blood pressure control by the depressor nerves: 1. Decrease of vascular resistance by lowering the sympathetic tone. 2. Decrease of heart rate by enhancing the cardiac vagal activity. It is suggested that the parasympathetic control unit compensates rapid disturbances, whereas the slow-acting sympathetic vascular mechanism exerts a long-time pressure control of high efficiency.