Modulation of the somatosensory evoked potentials by the input information originating from the gastrocnemius and sural nerves in the dog.

Input-output characteristics of the somatosensory system were studied in the dog. Somatosensory evoked potentials were recorded from the scalp at stimulations of the sural and gastrocnemius nerves. There was a sigmoid relationship between the stimulus strength and the amplitude of the sural nerve action potential, which was best described by a logistic equation. The early waves (P1, N1) increased especially at low threshold stimulations of the sural nerve; then they remained either unchanged or decreased at high threshold stimulations within the Group II and III ranges. The late waves (P2, N2) especially increased at high threshold Group II stimulations of the sural nerve, and increased further at strong stimuli. Similar changes were observed at stimulations of the gastrocnemius nerves. The attenuation of the early waves was explained by "gating" mechanisms acting from the sensorimotor cortex onto the sensory relay nuclei. The increase of the late waves at especially high threshold Group II and III stimulations was accounted for by the multisynaptic diffuse projection from the reticular formation and/or the nonspecific thalamic nuclei to the somatosensory cortex. In a three weeks old pup, stimulation of the gastrocnemius nerves evoked somatosensory responses with similar morphology observed in adult dogs, but the latencies of the evoked potential components were relatively long, presumably as a result of the uncompleted myelination in the somatosensory afferent system.