The number of motor units in the medial gastrocnemius muscle of male and female rats.

The number of motor units in the hindlimb locomotor muscle--the medial gastrocnemius--was estimated in male and female Wistar rats by comparison of the whole muscle tetanic force to the mean tetanic force of its motor units. Functional isolation of motor units was achieved by electrical stimulation of single axons from ventral roots of L4 - L5 spinal nerves, while the whole muscle force was measured during stimulation of the sciatic nerve. The medial gastrocnemius muscle is approximately 1.5 times bigger by mass, and is innervated by 10% more motoneurones in males compared to females. Estimation of the force that can be generated by the three types of motor unit in the studied muscle revealed that in both sexes fast fatigable units are responsible for over 60% of the total force output whereas the percentage of the force output contributed by slow units in females is approximately twice that in males. Concluding, the motor innervation of hindlimb skeletal muscles is sexually dimorphic and in the rat male medial gastrocnemius muscle contains about 10% more motor units than in females.