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

J Comp Neurol. 1988 Mar 15;269(3):425-30. doi: 10.1002/cne.902690309.


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 microliter) 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 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 rats aged 26 months and older, with most of the decrease occurring among the larger alpha-motoneurons.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Aging / physiology*
  • Animals
  • Cell Count
  • Horseradish Peroxidase
  • Male
  • Motor Neurons / classification
  • Motor Neurons / cytology
  • Motor Neurons / physiology*
  • Muscle Development*
  • Muscles / innervation
  • Nerve Fibers, Myelinated / physiology
  • Rats
  • Rats, Inbred F344
  • Spinal Cord / cytology
  • Spinal Cord / growth & development*
  • Spinal Cord / physiology


  • Horseradish Peroxidase