Inspiratory inhibition and rebound activation elicited by intermittent electrical bulbar stimulation in various states of pulmonary afferent vagal excitation.

In anesthetized rabbits spirogram and diaphragmatic activity were examined during electrical stimulation of regions of the medulla oblongata. The volleys were triggered by the animal's own respiration. 1. One volley of 120 msec duration at 100 pulses p.s., applied during inspiratory, caused an immediate and transient inhibition of the diaphragmatic activity. After the end of the volley and inspiratory rebound appeared: the tidal volume was increased and the inspiration was prolonged by some 150 msec. The respiratory rate decreased. 2. Continuous low or high frequency electrical stimulation of pulmonary stretch afferents caused an inspiratory or an expiratory effect respectively. In both conditions the effects of additional intermittent bulbar stimulation remained essentially unaltered. 3. By means of specially designed spirometer both pulmonary collapse and marked lung distension were produced. At low lung volume collapse afferents were excited. Inspiratory inhibition and rebound activation upon central stimulation persisted throughout the whole range of lung volume investigated; the rebound increase in tidal volume consecutive to the stimulus volley, however, was minimal in extreme lung distension. 4. Results suggest that the stimulation effects were the consequence of manipulation on intrinsic mechansims of the bulbar respiratory centre.