Functional reorganization in the rat dorsal horn during an experimental myositis.

In anaesthetized rats, the influence of an acute inflammation (2-8 h duration) of the gastrocnemius-soleus (GS) muscle on the excitability of dorsal horn neurones was studied using a mapping procedure. One of the main effects of the myositis was that the neurone population responding to GS A-fibre input increased in size. The increase was most marked in the lateral segments L6-L3 which received little input from the GS muscle in control animals. Excitability testing showed a myositis-induced lowering in threshold, combined with an increase in latency, jitter and input convergence. This suggests that new oligo- or polysynaptic connections become functional under the influence of a myositis. Neuronal effects induced by C fibres in the GS nerves were not significantly altered by a myositis, but C fibre-induced activations from the peroneal and sural nerves increased in the lateral dorsal horn. The results show that an acute myositis leads to marked changes in the functional connectivity of the dorsal horn within a few hours. The main increase in excitability took place in the lateral dorsal horn, where many neurones acquired a new input from the GS muscle. This mechanism may be involved in the spread or referral of muscle pain.