Phrenic and recurrent laryngeal motoneuron activities during hyperoxia and hypoxia in piglets

We hypothesized that synchronization of inspiratory motoneurons may involve inputs from two central pattern generators (CPG): one characterized by medium-frequency (< 50 Hz) and the other by high-frequency oscillations (> or = 50 Hz). We studied phrenic and recurrent laryngeal nerve activities recorded during hyperoxia and hypoxia in Saffan anesthetized, paralyzed, and artificially ventilated piglets. Spectral analyses, derived from the full as well as partitioned halves of inspiration, showed that phrenic and recurrent laryngeal discharges contained peaks in the medium-frequency band, which were indicative of common inputs. The phrenic spectra of many animals had peaks in the high-frequency band; such peaks were uncommon in recurrent laryngeal spectra; consequently, correlated activities corresponding to high-frequency oscillations were not usually observed. Thus, it is likely that acquisition of modulating inputs from a high-frequency CPG may emerge in an age-dependent manner in different motoneuron pools. During hypoxia, both phrenic and recurrent laryngeal discharges were facilitated as shown by increases in both the amplitudes of signal-averaged histograms and the magnitudes of their respective power spectral activities. Also, there was a significant increase in the values of phrenic-recurrent laryngeal coherence estimates in the medium-frequency region. Hence, medium-frequency oscillations are more apparent in early development, perhaps to facilitate synchronization of inspiratory motoneuron activities, especially under conditions of increased chemical drive.