EXCITABILITY OF PHRENIC MOTONEURONES TO AFFERENT INPUT FROM LOWER INTERCOSTAL NERVES IN SPINAL CAT

The reflex connections between the lower intercostal segments and the phrenic motor pool, described in a preceding paper, has been studied further. It was found that the diaphragm on contraction could exert enough mechanical force on the lower part of the rib cage to provoke a reflex discharge in the phrenic motoneurones. The intercostal‐to‐phrenic reflex arch thus constitutes a link in a loop for self‐excitation of the phrenic nucleus which may make up for its lack of autogenetic facilitation. Up to five synchronized reverberating phrenic volleys could be recorded in response to a single diaphragmatic jerk. These were abolished by paralyzing doses of gallamine. Conduction times and excitability cycles for the loop have been studied. Examination of reflex firing probality for phrenic motoneurones has revealed considerable individual differencies in their responsiveness to each of the three afferent input parameters: a) nerve stimulated, b) strength and c) frequency of stimulation. The order of reflex recruitment of phrenic motoneurones followed the same order as found for their respiratory engagement: small spikes always appeared earlier than bigger spikes. © 1969 Scandinavian Physiological Society