Medial gastrocnemius motor nucleus in the rat: Age-related changes in the number and size of motoneurons

The age‐related alterations in the number and size of alpha‐ and gamma‐motoneurons were studied in the medial gastrocnemius (MG) motor nuclei in rats at four ages: young (5 months), middle aged (10–13 months), old (26 months), and very old (31 months). Small volumes (0.1–0.5 μl) of 40% horseradish peroxidase (HRP) solution were injected into the cut MG nerve bilaterally by using glass micropipettes and a pressure injection system. The number, position, and soma size (average soma diameter) of MG motoneurons were determined by using photographic maps of each TMB‐stained section. The total number of myelinated axons was counted in seven MG nerves from the same animals. The average soma diameters in each MG nucleus were distributed bimodally; cells with average diameter greater than 21.0–24.0 μm were presumed to be alpha‐motoneurons and those with smaller diameters were presumed to be gamma. The mean number of presumed alpha‐motoneurons was significantly less in the old and very old groups as compared with the young and middle‐aged. In contrast, the number of presumed gamma‐motoneurons was the same across age groups. The mean average soma diameter of both alpha‐ and gamma‐motoneurons was smaller in the old animals. The apparent decrease in the total number of labeled motoneurons in old animals was also reflected in a decrease in myelinated axon counts. We conclude that there is a significant decrease in the absolute numbers of motoneurons in rats aged 26 months and older, with most of the decrease occurring among the larger alpha‐motoneurons.