Corticofugal influences of S1 cortex on ventrobasal thalamic neurons in the awake rat.

Corticofugal influences on the responses of 39 ventrobasal (VB) thalamic neurons to repetitive stimuli were studied in awake rats by focally suppressing the evoked and spontaneous electrocortical activity of the primary (S1) somatosensory cortex with magnesium or lidocaine plus magnesium. Suppression of the S1 cortex reduced the number of spikes discharged by 19 (66%) of 29 VB units in response to each of 25 electrical stimuli delivered to the medial lemniscus; 9 units were unaffected and 1 showed an increased response to 1-10 Hz stimuli. The responses of 6 (38%) of 16 VB units to electrical somatic stimuli were also reduced following S1 cortical suppression; 9 units were unaffected and 1 showed an increased response to 20-40 Hz stimuli. A comparison of the reduced responsiveness of 5 units studied during medial lemniscal and somatic stimulation did not reveal any additional response attenuation attributable to subthalamic corticofugal influences. We conclude that, in the awake rat, somatosensory transmission to VB thalamic neurons is primarily facilitated by S1 corticothalamic neurons.