Effect of hypoxia on abdominal motor unit activities in spontaneously breathing cats

Mateika, J. H., E. Essif, and R. F. Fregosi. Effect ofhypoxia on abdominal motor unit activities in spontaneously breathingcats. J. Appl. Physiol. 81(6):2428-2435, 1996.These experiments were designed to examine thebehavior of external oblique motor units in spontaneously breathingcats during hypoxia and to estimate the contribution of recruitment andrate coding to changes in the integrated external obliqueelectromyogram (iEMG). Motor unit activities in the external obliquemuscle were identified while the cats expired against a positiveend-expiratory pressure (PEEP) of 1-2.5cmH₂O. After localization of unitactivity, PEEP was removed, and recordings were made continuously for3-4 min during hyperoxia, normoxia, and hypoxia. A total of 35 single motor unit activities were recorded from 10 cats. At each level of fractional concentration of end-tidalO₂, the motor unit activity wascharacterized by an abrupt increase in mean discharge frequency, at~30% of expiratory time, which then continued to increase gradually or remained constant before declining abruptly at the end ofexpiration. The transition from hyperoxia to normoxia and hypoxia wasaccompanied by an increase in the number of active motor units (16 of35, 20 of 35, and 29 of 35, respectively) and by an increase in the mean discharge frequency of those units active during hyperoxia. Thechanges in motor unit activity recorded during hypoxia were accompaniedby a significant increase in the average peak amplitude of theabdominal iEMG. Linear regression analysis revealed that motor unitrate coding was responsible for close to 60% of the increase in peakiEMG amplitude. The changes in abdominal motor unit activity and theexternal oblique iEMG that occurred during hypoxia were abolished ifthe arterial PCO₂ was allowed tofall. We conclude that external oblique motor units are activated during the latter two-thirds of expiration and that rate coding andrecruitment contribute almost equally to the increase in expiratory muscle activity that occurs with hypoxia. In addition, the excitation of abdominal motor units during hypoxia is critically dependent onchanges in CO₂ and/ortidal volume.