Effect of synchronous activation of medullary inspiratory bulbo‐spinal neurones on phrenic nerve discharge in cat.

Abstract

The effects on phrenic nerve discharge elicited by intraspinal stimulation which produced synchronous activation of bulbo-spinal inspiratory neurons were investigated in chloralose-urethane anesthetized, paralyzed, vagotomized and artificially ventilated cats. Descending respiratory axons were activated in the ventrolateral spinal cord at the 2nd cervical level using either monopolar or bipolar stimulation (25-200 .mu.A, 100 .mu.s, 1-300 Hz). Activation of bulbo-spinal axons was confirmed by recording both orthodromic phrenic nerve excitation and antidromic spike invasion of single, inspiratory modulated units in either the dorsal respiratory group (DRG) or ventral respiratory group (VRG). Antidromic activation of inspiratory bulbo-spinal neurons was confirmed by the criteria of high frequency following and collison tests. Spinal cord stimulation at intensities of 100 .mu.A antidromically activated approximately half of the inspiratory bulbo-spinal neurons in the DRG and VRG. Stimulation pulses delivered to the spinal cord elicited an orthodromic excitation of the ipsilateral phrenic nerve lasting 2-12 ms during inspiration. The onset latency of excitation was 2-4 ms, decreasing as inspiration progressed. Following the initial excitation there was a 4-30 ms period of reduced phrenic nerve discharge. Continuous trains of stimuli (< 100 .mu.A, 100 .mu.s, 1-300 Hz) or phrenic gated trains delivered during every 4th inspiratory or expiratory cycle had little or no effect on the duration of inspiration or expiration. Brief trains (400 ms, 50 Hz, 100 .mu.A) of bilateral spinal cord stimulation delivered at various delays from the onset of inspiration had only a transient effect on the pattern of phrenic nerve discharge, with no noticeable effect 60 ms after termination of stimulation. If synchronous activation of a portion of the central pattern generator for respiration phase shifts or resets the rhythm, then bulbo-spinal inspiratory neurons are not responsible for generation of respiratory timing signals; at most they play a limited role in the generation of the augmenting central inspiratory activity.