Contractile responses of rat plantaris muscles following partial denervation, and the influence of daily exercise

Abstract

The study was designed to determine the influence of increased daily neuromuscular activity on the sprouting response of motoneurones following partial denervation. Female Sprague-Dawley rats had one hindlimb partially denervated by transecting lumbar radicular nerve L4, and were subsequently subjected to a daily programme of increased activity, including grid climbing and voluntary wheel exercise, for 9 days. Functional sprouting was estimated on day 10 by comparing the L5-evoked plantaris muscle forces, measured in situ, with those of the contralateral L5. Comparisons were made between responses from exercised and non-exercised rats. Tetanic (200 Hz) contribution of L5 axons to plantaris muscle force doubled during the period following partial denervation, but did not attain the equivalent of whole muscle tetanic tension of normal controls. Twitch: tetanic ratios were elevated, and tetanic contractions "fatigued" to a greater extent in partially denervated muscles, signifying limitations on the part of each sprouting motoneurone to tetanically activate its enlarged complement of muscle fibres. No influence of daily exercise following the lesion on any of these functional indices of motoneurone sprouting was evident. Increased daily neuromuscular activity, performed within the restrictions imposed by the neuromuscular deficit, does not influence motoneurone sprouting responses. This is in contrast to the enhancement of sprouting previously reported for motoneurones of rats subjected to intense, prolonged, daily exercise preceding the partial denervation, and when neurones remaining following partial denervation are electrically stimulated for relatively short periods of time (1 h), the day of the lesion.