Ingestion in fetal sheep and its relation to sleep states and breathing movements

Swallowing was recorded electromyographically in unanaesthetized fetal sheep in utero during the last third of gestation. In control fetuses bouts of swallowing were, on average, 2.0 min in duration and occurred at intervals of 2.31 h. Swallowing bouts were associated with a low-voltage electrocorticogram, eye movements and, in 82% of instances, with phasic activity in the dorsal neck muscles. These data indicate that fetuses are not in quiet (non-rapid eye movement; n.r.e.m.) sleep when swallowing and also suggest that they are not in r.e.m. sleep as conventionally defined; we consider that fetuses may be in a state of heightened activity resembling wakefulness. Swallowing bouts were always accompanied by fetal breathing movements. When a swallow coincided with an inspiratory effort the diaphragm was briefly inhibited. Evidence was obtained that bouts of fetal ingestive activity were not dependent upon outpourings of fluid from the lungs and were not significantly influenced by volume information from the gastric compartments.