Corticofugal action on somatosensory response properties of rat nucleus gracilis cells.

Single unit recordings were performed in the nucleus gracilis (Gr) of anesthetized rats to study the influences of the sensorimotor corticofugal projections on sensory responses of those cells. The effects of electrical stimulation of contralateral primary sensory cortex were studied in two conditions: when the receptive fields of the stimulated cortical area and the gracilis cells overlapped (matched) or when they were completely different (unmatched). Cortical stimulation at low intensities (<50 microA) evoked spike firing only in gracilis neurons with matched receptive fields. When the receptive fields were unmatched, the intensity of the stimulation had to be increased above 50 microA to elicit spike firing. To study the corticofugal actions on the responses of Gr neurons, the onset of peripheral stimulation was likened to a single cortical shock in the sensorimotor cortex. When receptive fields matched, cortical stimulation facilitated the cellular responses to the natural sensory stimulation of their RF in most of the Gr neurons (86%). In the unmatched receptive fields, cortical stimulation could either inhibit (66.7%), facilitate (20.8%) or did not modify (12.5%) the sensory response at all. Trains of cortical shocks during sensory stimulation demonstrated that the facilitatory and inhibitory effects on Gr neurons outlasted the period of stimulation by 30-60 s. Results indicate that the sensorimotor cortex exercises a very precise control of sensory transmission throughout the Gr nucleus and suggest that the corticofugal projection may play an important role in the plasticity of the sensorimotor system.