Diverging influences on Renshaw cell responses and monosynaptic reflexes from stimulation of capsula interna

In anaesthetized cats responses of lumbar Renshaw cells to anti- and orthodromic spinal root and muscle nerve shocks were recorded with and without conditioning stimulation of the contralateral capsula interna. Responses to antidromic test stimuli were decreased by supraspinal conditioning in 48 of 65 cases over their whole course and over a range up to 60 ms after the conditioning stimulation. The corresponding monosynaptic reflexes, however, were facilitated up to 250% under these conditions. Reduction of orthodromic responses was apparent in 12 of 44 cases, but might have been masked in others due to concomitant motoneurone facilitation. These results indicate divergent descending influences on motoneurones and related Renshaw cells, mediated via the capsula interna.