Changes with age in the number and size of myelinated axons in the rat L4 dorsal spinal root.

The number and size of myelinated axons in the L4 dorsal spinal root were determined in young rats and compared to those in the adult (older than 1 year) and in the old rats (older than 2 years). While the total number of myelinated axons was about the same at all ages, the average overall axon diameter showed a growth-related increase in adult animals, followed by a decrease reflecting senile atrophy in old rats. It is suggested that in aging laboratory rats the extent of axon shrinkage could account for the occurrence of secondary radicular demyelination.