Sleep influences on diaphragmatic motor unit discharge

The discharge of motor units in the costal and crural regions of the diaphragm was studied in unrestrained cats during defined sleep and waking states. Changes in motor unit recruitment and pattern of activity were observed for different sleep-waking states. During periods of active waking, many motor units were recruited and discrimination of single-unit waveforms was difficult. However, in the transition from active to quiet waking, many diaphragmatic motor units ceased discharging, thus making it possible to discern single-unit waveforms. This cessation of motor unit activity continued during the transition from quiet waking (AW) to quiet sleep (QS) and from QS to rapid eye movement (REM) sleep. In some diaphragmatic motor units, discharge stopped completely during REM sleep and resumed only upon arousal. Thus, it was apparent that the recruitment of diaphragmatic motor units was markedly affected by the sleep-waking state. Changes in the patterns of discharge of diaphragmatic motor units also occurred during different sleep-waking states. Motor unit firing rate was typically slower in QS than AW. The rate of motor unit discharge during REM sleep varied considerably. Autocorrelation histograms demonstrated periodic patterns of motor unit discharge within inspiratory bursts. Intervals of repetitive peaks in the autocorrelation histograms ranged from 40 to 110 ms. These periodic patterns were strongest during AW and REM sleep and generally attenuated during QS. Cross-correlation histograms revealed the presence of synchronous activity between motor units in the diaphragm. This coupling of diaphragmatic motor unit discharge was also attenuated during QS compared with AW and REM sleep. Together, these results demonstrated a marked influence of the sleep-waking state on the neuromotor control of the diaphragm.