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 denervated by cutting the L4 spinal nerve. Three months later, the effects of partial denervation of the fast extensor digitorum longus (EDL) muscle on the activity of its remaining motor units were studied using electromyographic (EMG) recordings in freely moving animals. In spite of a reduced number of motor units the amount of aggregate EMG activity was greater in the partially denervated EDL muscle in all experimental conditions. This increase was more obvious at rest than during exploratory behaviour, and was significantly greater in muscles that were partially denervated at 3 days than at 18 days of age. On the other hand, the effect of partial denervation on the EMG activity pattern during locomotion was similar in animals partially denervated at 3 or 18 days of age. Unlike in intact EDL, in the partially denervated EDL muscle the duration of the bursts was influenced by the step cycle duration. Thus, we conclude that although partial denervation of EDL muscle influences the amount and pattern of activity of the remaining undamaged motor units in all animals, some of the alterations of EMG activity were more pronounced in animals denervated at younger age.