Inflammation and peripheral nerve sensitisation

It is a well recognised phenomenon that acute inflammation often interferes with the success of local analgesia. It has been suggested recently that this apparent reduction in the efficiency of the local anaesthetic is due to an increase in peripheral nerve activity. This investigation was undertaken to identify possible alternations in the impulses evoked in sensory nerves in the presence of inflammation. The saphenous nerve was exposed in the thigh of an anaesthetised rat. Stimulating and recording electrodes were applied under paraffin oil and an area of skin supplied by the nerve was identified. An electronically driven mechanical probe was used to apply a repeatable tap of short duration to the skin, and the force was adjusted so that a submaximal evoked potential was produced. A number of these potentials were averaged electronically. Inflammatory changes were induced in he skin by electrical antidromic stimulation and the mechanical stimulation schedule was repeated. The evoked action potential was shown to have increased substantially. These results are considered in conjunction with findings of other investigators who have shown a similar sensitisation using multifibre preparations.