Mechanisms controlling inspiration studied by electrical vagal stimulations in rabbits

Electrical stimulations of either constant (step) or linearly increasing (ramp) stimulus frequency (F) were applied with different timing during inspiration to the central ends of cut vagi of anaesthetized rabbits. Integrated diaphragmatic activity (Ad) time course during stimulation, after a period of common path (Tu) with control, increased at reduced rate for a substantial period before end-inspiration (Ti). With stimulations maintained into early expiration, the higher F, the shorter Tu and Ti and the smaller end-inspiratory Ad. Delaying the onset of stimulation or using ramp instead of step stimulations, shifted rightwards both F vs. Tu and F vs. Ti relationships which became steeper. With increasing delay, Ad at Ti or Tu became eventually independent of F. Short stimulations ineffective to shorten Ti, prolonged Ti beyond control, end-inspiratory Ad being smaller, the smaller Ti prolongation.

(a) vagal input, beyond a time dependent threshold, exerts a graded inhibitory effect on central inspiratory output (CIO); (b) vagal input undergoes a certain degree of integration; (c) vagal inspiratory inhibitory effects are independent of CIO; (d) vagal input prevents some centrally generated activity inhibiting CIO.