Medial gastrocnemius motor nucleus in the XXXX : age-related changes in the number and size of motoneurons.

@entity35 @entity10036
The age-related alterations in the number and size of alpha- and gamma-motoneurons were studied in the @entity10036 ( @entity10036 ) motor nuclei in @entity35 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 microliter) of 40% horseradish peroxidase (HRP) solution were injected into the cut @entity10036 nerve bilaterally by using glass micropipettes and a pressure injection system. The number, position, and soma size (average soma diameter) of @entity10036 motoneurons were determined by using photographic maps of each TMB-stained section. The total number of myelinated axons was counted in seven @entity10036 nerves from the same animals. The average soma diameters in each @entity10036 nucleus were distributed bimodally; cells with average diameter greater than 21.0-24.0 micron 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 @entity35 aged 26 months and older, with most of the decrease occurring among the larger alpha-motoneurons.
