Sleep, wakefulness and the monosynaptic reflex in fetal and newborn lambs

1. Electrocorticogram (ECoG), electromyogram (EMG) of the lateral rectus and antigravity muscles (neck and masseter) and breathing activity (FB) were monitored in chronically prepared fetal sheep of 125-140 days gestation and in newborn lambs up to 11 days postnatal age. 2. Awake state (AW), non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM) were defined using standard criteria for ECoG, eye movements and postural muscle tone. 3. The percentage of time the fetuses spent in each state was: NREM sleep 53%, REM sleep 41.4%, and AW 5.6%. 4. Spontaneous intrauterine breathing activity occurred only during REM sleep, but 35% of REM sleep was not associated with FB. 5. Gasps (isolated deep inspirations) appeared occasionally throughout the recording and were not related to any specific sleep state or wakefulness. 6. In the fetus, the monosynaptic reflex (MSR) induced by direct electrical stimulation of the fibular nerve was enhanced by about 75% during REM sleep compared to NREM and AW. In the newborn lamb the adult pattern of suppression of MSR during REM sleep was not seen until several days after birth.