Stimulation parameters for cardiovascular responses from medulla and hypothalamus

¹ Department of Physiology, Stritch School of Medicine and The Graduate School, Loyola University, Chicago, Illinois

Electrical stimulation was carried out in the medulla and hypothalamus of 33 vagotomized cats anesthetized with alpha-chloralose. A wide range of stimulation frequencies was used at pulse durations of 2.0, 0.2, and 0.02 msec. The lowest frequency at which the majority of animals reach a maximal response in heart rate or blood pressure was 100 cycles/sec at all three pulse durations. At 2 msec pulse duration, the response declined by 30% at 225 cycles/sec. At pulse durations of 0.2 and 0.02 msec, 30% decline in response occurred, respectively, at 1,040 and 1,250 cycles/sec. The response curves were essentially similar for both heart rate and blood pressure. Strength-duration curves, using a 3% change in heart rate as a threshold response, showed a similarity in the curves for fibers mediating sympathetic acceleration and those mediating sympathetic inhibition of heart rate. The decline in response at high frequencies does not appear to be due to transmitter depletion. The basis for optimal frequency of 100 cycles/sec and for declining responses at higher frequencies is discussed.