Phrenic motoneuron firing rates during brief inspiratory resistive loads.

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 transdiaphragmatic pressure where silent phrenic motoneurons were recruited ranged from 5 to 45 cmH2O. 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.