The labile respiratory activity of ribcage muscles of the rat during sleep.

1. Sleep-waking states of chronically implanted rats were identified polygraphically while recording the integrated electromyogram (e.m.g.) of extrinsic (scalenus medius and levator costae) and intrinsic (external and internal interosseous intercostal and parasternal) muscles of the thoracic cage. Rats breathed air, air enriched in CO2 (5%) or air deficient in O2 (10% O2 in N2) and were free to adopt any desired posture. 2. In non-rapid eye movement (non-r.e.m.) sleep, the scalenus medius and intercostal muscles of the cephalic spaces were always inspiratory; intercostal muscles of the mid-thoracic spaces were commonly expiratory while the more caudal ones were only occasionally expiratory. Expiratory activity, when present in quiet wakefulness, extended for a variable period of time into non-r.e.m. sleep and always disappeared in r.e.m. sleep regardless of the ribcage muscle under study. 3. Inspiratory activity, when present in non-r.e.m. sleep, was unaffected, partially attenuated or abolished at entry into r.e.m. sleep. The peak integrated e.m.g. activity of ribcage muscles was measured as a function of posture, gas mixture breathed and ribcage site: (a) the greater the degree of curled-up posture, the greater the respiratory activity of scalenus medius, an effect augmented by CO2 but depressed by hypoxia, and (b) the more caudally placed ribcage muscles exhibited respiratory activity which was essentially unaffected by posture and gas mixture inspired. 4. The presence or absence of tonic activity in ribcage respiratory muscles during non-r.e.m. sleep was unrelated to posture. When tonic activity was present, it always disappeared in r.e.m. sleep. When expiratory activity was present in non-r.e.m. sleep, it too always disappeared in r.e.m. sleep. Inspiratory activity present in non-r.e.m. sleep was variably affected at entry into r.e.m. sleep; it was unchanged, partially attenuated or abolished. 5. It is concluded that thoracic cage muscles exhibit marked variability in their respiratory activity depending on posture, sleep-waking states and gas mixture breathed. It is postulated that the presence of tonic and/or expiratory activity in ribcage muscles during non-r.e.m. sleep reflects an increase in functional residual capacity (F.R.C.).