The effects of aging on the total number of muscle fibres and dominant motor neurons of rats

Abstract

The effects of aging on the total number of muscle fibres and dominant alpha motor neurons were studied among three experimental age groups. HRP (horseradish peroxidase) was injected into the tibialis anterior (fast twitch type) and soleus (slow twitch type) muscles of Wistar strain albino female rats aged 10, 65, 135 weeks, to be labelled retrogradely the motor neurons of the spinal cord. The results obtained were summarized as follows: 1. Fast twitch (FOG, FG) muscle fibres decreased at 65 weeks. 2. Slow twitch (SO) muscle fibres and alpha motor neurons (large size and low oxidative enzyme activity) decreased at 135 weeks. The present study suggests that the decrease of muscle fibres in the early stage until 65 weeks is a result of selective atrophy, and this quantitative alteration may be due to disturbances of neuromuscular junction or to degeneration of the fibres themselves. On the other hand, the decrease of muscle fibres from 65 to 135 weeks may be due to regressive change, such as decrease of dominant motor neurons in the spinal cord.