The response to stimulation of hindlimb nerves in fetal sheep, in utero, during the different phases of electrocortical activity.

The relationship between the amplitude of electrically evoked responses from nerves of a hindlimb and electrocortical activity has been determined in fetal sheep from 105-141 days gestation. Stimulation of the common sciatic, tibial or peroneal nerves and recording from the two nerves not stimulated revealed an early (4.5 ms) and late (18.6 ms) evoked neural response, probably reflex. These responses were reduced or absent in low voltage compared with high voltage electrocortical activity after 125 days gestation. Spontaneous gastrocnemius activity was more frequent during high voltage activity. The electrocorticogram was undifferentiated before 115 days, and the evoked responses in peripheral nerves were largest when breathing and eye movements were present. After electrocortical differentiation the responses were enhanced when brief episodes of augmented eye and breathing movements occurred during the transition between high and low voltage activity. The possibility that this indicates arousal or wakefulness in utero is discussed.