Discharge of group IV phrenic afferent fibers increases during diaphragmatic fatigue.

The discharge of single unit group III (n=7) and group IV (n=8) phrenic afferent fibers was recorded during rhythmic diaphragmatic contractions before and after the onset of fatigue. Compared to pre-fatigue impulse activity, group IV, but not group III, phrenic afferent fibers discharged more (p<0.05) during rhythmic diaphragmatic contractions when the diaphragm was fatigued. This increase in group IV fiber discharge during diaphragmatic fatigue provides electrophysiological evidence consistent with the notion that group IV phrenic afferent fibers comprise the afferent arm of a fatigue-induced inhibitory reflex originating in the diaphragm.