Respiratory resetting induced by activation of inspiratory bulbo-spinal neurons.

The respiratory effects elicited by spinal (C2-C3) stimulation at the level of descending inspiratory axons were studied in paralysed, non-vagotomized and artificially ventilated cats anaesthetized with urethane-chloralose. The activation of inspiratory bulbospinal axons in the ventrolateral quadrant was confirmed by recording the ipsilateral phrenic excitation following a single pulse. Brief stimulus trains delivered at the same locus during expiration elicited short- and long-term phrenic activations. The short-term activation consisted of a tetanic orthodromic response. The long-term activation, of central origin, exhibited the same pattern as a spontaneous inspiration and consisted of an inspiratory resetting which necessitated weak anaesthesia and light hypocapnia. Control experiments (restricted lesions of the medulla and the cervical cord, recording of afferent activity in thalamic sensory nuclei, medullary stimulation) revealed that this inspiratory resetting could not be related to appreciable activation of either non-respiratory efferents or spinal afferent pathways studied but was likely to depend on the activation of the descending inspiratory axons. We conclude that the respiratory resetting obtained by spinal stimulation resulted from mass antidromic activation of the inspiratory bulbospinal neurons which thus appear to be involved in the generation of the respiratory rhythm.