The differences in the structure of the motor nucleus of the medial gastrocnemius muscle in male and female rats

There are many reports describing sexual dissimilarities in the CNS, particularly in the brain and cortical regions. However, knowledge regarding sexual dissimilarities in the spinal cord and in particular in the hindlimb muscle-motoneuron connectivity controlling locomotion is limited. In order to recognise sex differences in the architecture of the medial gastrocnemius (MG) motor nucleus in rats of the same age, retrograde-labelled motoneurons were identified following a bath of the proximal stump of the transected MG nerve in horseradish peroxidase. The rostrocaudal distribution of motoneurons along the spinal cord and on transverse sections as well as the size and density of motoneurons in the motor nucleus were determined from serial microscopic images. It was shown that the MG pool extended throughout the L4-L6 segments, with a length that was 32% greater in males. The position of the moto pool within the ventral horn of the spinal cord transversal sections was also different in both sexes: the pool was located more dorsally and laterally in males. Three size categories of motoneurons with different soma diameters were distinguished: alpha(1) (27.5-40.0 mu m), alpha(2) (>40.0 mu m) and gamma (<27.5 mu m). The density of alpha (alpha(1) and alpha(2)) motoneurons as well as gamma motoneurons was higher in females, by about 13% and 23%, respectively. The number of alpha(1) motoneurons was 8% higher in females, whereas there were 46% more alpha(2) motoneurons in males. The most significant differences in the distribution concerned alpha(2) motoneurons, which revealed a lower density in the rostral parts of the MG motor nucleus in females. It was therefore concluded that the length of the MG motor nucleus was shorter, whereas the mean density of alpha and gamma motoneurons was higher in females. (C) 2018 Elsevier GmbH. All rights reserved.