The age dependent effect of partial denervation of rat fast muscles on their activity

In 3 or 18 day old Wistar rats the hindlimb muscles were partially denervat ed by cutting the L4 spinal nerve. Three months later, the effects of parti al denervation of the fast extensor digitorum longus (EDL) muscle on the ac tivity of its remaining motor units were studied using electromyographic (E MG) recordings in freely moving animals. In spite of a reduced number of mo tor units the amount of aggregate EMG activity was greater in the partially denervated EDL muscle in all experimental conditions. This increase was mo re obvious at rest than during exploratory behaviour, and was significantly greater in muscles that were partially denervated at 3 days than at 18 day s of age. On the other hand, the effect of partial denervation on the EMG a ctivity pattern during locomotion was similar in animals partially denervat ed at 3 or 18 days of age. Unlike in intact EDL, in the partially denervate d EDL muscle the duration of the bursts was influenced by the step cycle du ration. Thus, we conclude that although partial denervation of EDL muscle i nfluences the amount and pattern of activity of the remaining undamaged mot or units in all animals, some of the alterations of EMG activity were more pronounced in animals denervated at younger age.