RESPONSES OF MEDULLARY RETICULOSPINAL NEURONS TO STIMULATION OF CUTANEOUS LIMB NERVES DURING LOCOMOTION IN INTACT CATS

The present study was designed to determine whether the transmission o f cutaneous afferent information from the limbs to the medullary retic ular formation is phasically modulated during locomotion. Experiments were carried out in three chronically prepared, intact cats in which n erve cuff electrodes were placed, bilaterally, on the superficial radi al and the superficial peroneal nerves. Thirty-seven reticulospinal ne urones (RSNs) were identified by stimulation of their axons in the lum bar spinal cord (L2): 29 of 37 of these were recorded with the car at rest. 28 of 37 during locomotion and 20 of 37 both at rest and during locomotion. Low-threshold stimulation of the cutaneous nerves evoked e xcitatory responses in the majority of RSNs both at rest and during lo comotion. In the 28 of 37 RSNs recorded during locomotion. it was poss ible to record the evoked response to stimulation of all four limb ner ves, giving a total of 184 tested cases [RSNs testedxnumber of nerves stimulatedxphase of stimulation (swing or stance)]. The responses of m ost RSNs to cutaneous stimulation were modulated in a phase-dependent manner during locomotion. The maximal responses in most, but not all, cases were obtained during the swing phase of the limb that was stimul ated and were largely independent of the discharge pattern of the cell . We interpret this result as indicating that the efficacy of transmis sion of the afferent information is determined more by the excitabilit y of the spinal relay neurones than by the level of excitability of th e RSNs in the brainstem. It is suggested that the base discharge patte rn of RSNs might be largely determined by their central afferent input , while peripheral afferent inputs would primarily serve to modify the RSN discharge pattern in response to perturbations.