Impact of upper cervical spinal cord hemisection on diaphragm neuromotor control

Neural drive to the diaphragm muscle (DIAm) for breathing is generated in the medulla and descends primarily ipsilaterally to phrenic motor neurons (PhMNs) in the cervical spinal cord. Neuromotor control of the DIAm during breathing involves motor unit (MU) recruitment, sustained activity, and derecruitment, which may be differentially altered on ipsilateral and contralateral sides after C₂ spinal hemisection (C₂SH). In awake rats, bilateral DIAm electromyographic (EMG) activity was recorded via chronically implanted electrodes during eupnea in nine Sprague-Dawley rats before and 14 days (day 14) after C₂SH. The durations of MU recruitment and derecruitment were estimated by evaluating EMG signal stationarity, i.e., changes in the mean square average of onset- and offset-aligned 10-ms bins within a longer (e.g., 80 ms) sampling period reflect changes in EMG due to MU recruitment. The amplitudes of DIAm EMG at the end of the recruitment and beginning of the derecruitment phase were measured. On the ipsilateral side, C₂SH decreased DIAm EMG recruitment amplitude by ∼30% and peak amplitude by ∼35% by day 14, likely reflecting a decrease in the number of MUs recruited. On the contralateral side, C₂SH increased recruitment amplitude by ∼70% and peak amplitude by ∼80% by day 14, likely reflecting an increase in the number of MUs recruited. The derecruitment amplitude decreased ipsilaterally and increased contralaterally, consistent with the recruitment of higher threshold MUs contralaterally. The present study reveals that the impact of C₂SH on ipsilateral and contralateral DIAm neuromotor control varies depending on the loss of ipsilateral neural drive.NEW & NOTEWORTHY Neuromotor control of the diaphragm muscle (DIAm) after C₂ spinal hemisection (C₂SH) is impacted differentially between ipsilateral and contralateral sides. We show neural drive to ipsilateral phrenic motor neurons decreases after C₂SH, leading to a reduction in the number of DIAm motor units (MUs) recruited. Contralateral neural drive increases after C₂SH, leading to an increase in the number of MU recruited. These results provide novel information about the impact of C₂SH on DIAm neuromotor control.