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

Authors

  • Ken Hashizume,

    1. Department of Kinesiology, Tokyo Metropolitan Institute of Gerontology, Tokyo 173, Japan
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  • Dr. Kenro Kanda,

    Corresponding author
    1. Department of Physiology, Tokyo Metropolitan Institute of Gerontology, Tokyo 173, Japan
    • Department of Physiology, Tokyo Metropolitan Institute of Gerontology, 35-2, Sakaecho Itabashi-ku, Tokyo 173, Japan
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  • Robert E. Burke

    1. Department of Physiology, Tokyo Metropolitan Institute of Gerontology, Tokyo 173, Japan
    Current affiliation:
    1. Laboratory of Neural Control, National Institute of Neurological and Communication Disorders and Storke, National Institutes of Health, Bethesda, MD 20892
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Abstract

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.

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