Quantitative and qualitative morphological properties of the soleus motor nerve and the L5 ventral root in young and old rats. Relation to the number of soleus muscle fibers.

The motor nerve to the soleus muscle and the L5 ventral root from young adult (3-6 months) and old (20-25 months) male Wistar rats were studied with regard to total number of myelinated nerve fibres, calibre spectra of myelinated fibres and morphological properties. The soleus muscle was examined with respect to total number of muscle fibres. A significant decrease in the number of myelinated nerve fibres was found in the old soleus nerves, and was mainly confined to the large-diameter fibers. Similarly, there was a significant decrease in the number of large myelinated fibres in the L5 ventral roots of old animals. Morphological changes in old nerves and spinal roots consisted of axonal degeneration with areas devoid of myelinated nerve fibres, and also myelin sheath irregularities, including myelin splitting with myelin balloon formation, infolded myelin loops and myelin reduplication. The axonal degeneration was more pronounced in the peripheral nerves than in spinal roots. In conformity with other studies, the soleus muscle showed an age-related decrease in the number of muscle fibres. These findings indicate that the decrease in fibre number in old soleus muscle is associated with degenerative neuronal changes and loss of alpha-motoneurons.