Contribution of hypercapnic stimuli and of vagal afferents to the timing of breathing in anesthetized cats.

In tracheotomized, anesthetized cats, the authors studied the role of phasic and tonic vagal discharge and of hypercapnic stimuli on the timing of breathing. The effect of tonic vagal discharge was separated from that of the other two parameters by comparing the duration of inspiration and of expiration of control breaths with those obtained during occlusions of the airways at FRC (bulbo-pontine activity). The effect of tonic vagal discharge was then separated from that of hypercapnic stimuli by comparing the bulbo-pontine activity before and after vagotomy. Hypercapnia caused shortening of inspiratory and expiratory duration set by the bulbo-pontine pacemaker and increased sensitivity of the respiratory centers for a given phasic vagal input (displacement to the left of the tidal volume vs inspiratory duration relationship). Vagotomy did not modify the bulbo-pontine duration of inspiration nor its possibility to shorten in hypercapnia; by contrast it caused a lengthening of the expiratory time which did not shorten any more in hypercapnia, suggesting that tonic vagal discharge mainly influences the expiratory duration.