EFFECTS OF AN ACUTE MUSCLE NERVE-SECTION ON THE EXCITABILITY OF DORSAL HORN NEURONS IN THE RAT

A previous study of this laboratory showed that an acute myositis (which is associated with increased activity in slowly conducting muscle afferent fibres) is followed by marked excitability changes in the dorsal horn within a few hours. The present work addresses the question as to how the responsiveness of dorsal horn neurones changes when the same muscle nerve is transected. In anaesthetized rats, an axotomy of the gastrocnemius-soleus (GS) muscle nerves was performed and the electrical and mechanical excitability of single dorsal horn neurones in the lumbar spinal cord determined 2-8 h after the lesion. Axotomy led to a decrease in the proportion of neurones responding to A-fibre input from the cut nerve and to an increase in the efficacy of sural and peroneal nerve stimulation. The change in GS input became significant only 5-8 h after the lesion and could reflect the beginning of neuroplastic changes. The change in sural and peroneal input was most marked 2-5 h after axotomy and is probably due to fast neuronal processes. The efficacy of C-fibre input from the sural and peroneal nerves increased significantly in the lateral dorsal horn only. In comparison with the effects of an acute myositis, the axotomy had opposite effects with regard to the GS input, but similar effects with regard to the C-fibre drive from the other nerves. It is concluded that transection of a muscle nerve is similarly effective in inducing acute changes in dorsal horn excitability as is an increase in muscle nerve activity.