Spatial and cortical influences exerted on cuneothalamic and thalamocortical neurons of the cat.

This work aimed to study the responses of cuneothalamic and thalamocortical cells to electrical stimulation of the body surface in alpha-chloralose-anaesthetized cats. It was found that both classes of cells had a central excitatory receptive field, an edge overlapping the field centre whose stimulation elicited inhibitory-excitatory (cuneothalamic cells) and excitatory-inhibitory (thalamocortical cells) sequences, and a surrounding or peripheral area usually being inhibitory. Manipulating the descending corticofugal activity by removing the fronto-parietal cortex, electrical stimulation, or by placing picrotoxin or muscimol over the sensorimotor cortex demonstrated that the cortical feedback potentiated effects driven from the field centre and the surround. In particular this potentiated centre-driven excitation and surround-driven inhibition, but some of the data points to more complex patterns. The inhibition elicited in cuneothalamic cells from the edge and the surround of the field was faster than the excitation induced from the field centre. Effects at the edge of the field centre included late excitatory responses relayed via the cerebral cortex. There were also direct corticofugal excitatory inputs to the field centre. Excitatory surrounds were occasionally observed, the assumption being that in most cases these were suppressed by the enhanced inhibition driven from the cortex. The data indicate that the cortico-subcortical feedback contributes not only to enhance the surround antagonism of a centre response but also to increase the time resolution of thalamic and cuneate relay somesthetic neurons.