Gender differences in the morphometric properties of muscle fibres and the innervation ratio of motor units in rat medial gastrocnemius muscle

With 2 figures and 2 tables SUMMARY: Previous studies of motor unit contractile properties in the rat medial gastrocnemius revealed that these units generate higher forces in males than in females. Therefore, in the present study the number and morphometric parameters of muscle fibres and the innervation ratio of motor units in the medial gastrocnemius muscle were studied in male and female Wistar rats. The study additionally aimed at determining reasons of gender differences in motor unit force parameters, i.e. the number and diameter of muscle fibres, and mean values of the motor unit innervation ratios. Following staining of reticuline fibres by silver impregnation, the number, diameter and cross-section area of muscle fibres were determined on microscopic images of transverse muscle sections. In males, the muscles were approximately 1.5 times larger by mass and contained about 11 800 muscle fibres, whereas in females the muscles contained around 8000 fibres. In addition, the mean diameter and mean cross-section area of muscle fibres were 14 and 29% larger in males, respectively. Based on previously determined numbers of motoneurons innervating the medial gastrocnemius muscle in male and female rats, the mean innervation ratio, i.e. the number of muscle fibres innervated by one motoneuron, was estimated. This ratio was approximately 26% greater in males compared to females, with values of 207 and 153 fibres per motoneuron, respectively. Therefore, the differences in muscle fibre morphometric parameters and in the innervation ratio are responsible for higher forces of motor units in male muscles.