Somatosensory-evoked potential induced by stimulation of the caudal tibial nerve in awake and barbiturate-anesthetized sheep.

The somatosensory-evoked potential (SEP) from caudal tibial nerve stimulation was recorded in sheep, using surgically implanted electrodes. Peak latencies of the SEP remained stable throughout the 3- to 5-month recording period. Wave-form and peak latencies from awake sheep and concomitant changes during barbiturate anesthesia were compared with those reported for other animal species, including man. Wave form components representing activities from the primary somesthetic cortex, association cortex, and non-specific diffuse projections were differentially susceptible to barbiturate anesthesia. The neural source of the initial positive peak (mean latency = 19 ms) was resistant to anesthesia and represented primary somesthetic activity of the cortex. Sheep with surgically implanted electrodes yielded stable latencies with SEP wave-form features essentially homologous to those reported for other animal species.