Phrenic motoneuron firing rates during brief inspiratory resistive
loads.
Road, Jeremy, Sally Osborne, and Andrew Cairns.
Department of Medicine, University of British Columbia, Vancouver,
B.C., V6T 2B5, Canada
APStracts 2:0273A, 1995.
The neural activation of the diaphragm during quiet and vigorously
stimulated breathing has been hypothesized to be submaximal. In this
study, we measured phrenic motoneuron firing rates during brief,
progressively increasing inspiratory resistive loads in anesthetized
rabbits. We recorded activity in 68 phrenic motoneurons in 17
rabbits. We found that 40 of these axons were active during quiet
breathing. Twenty-seven axons were silent during quiet breathing, but
began to fire as inspiratory loading progressed. The level of drive
reflected by Pdi where silent phrenic motoneurons were recruited
ranged from 5-45 cm H20. Silent motoneurons showed significantly
higher average rates of firing and significantly greater increases in
firing rate as loading progressed (p<0.01). The firing rate of
both active and silent axons tended to plateau as rates approached
70-80 Hz. All motoneurons except for one, which may have been an
afferent, were activated by inspiratory resistive loading.
Inspiratory resistive loading activated phrenic motoneurons at high
rates and our results did not support the presence of significant
numbers of unrecruited motoneurons.
Received 20 May 1994; accepted in final form 14 June 1995.
APS Manuscript Number A490-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 July 1995.