Effects of long-term physical exercise on age-related changes of spinal motoneurons and peripheral nerves in rats

We examined the number and size of motoneurons and myelinated nerve fibers innervating the medial gastrocnemius (MG) muscle in the hindlimb of rats that were subjected to swimming for 10 months (30 min, 3 days/week) from 17 to 27 months of age. The number of MG motoneurons in exercised aged rats was slightly, but not significantly, greater than that in sedentary aged rats, and the number for both aged groups was significantly less compared to that in middle-aged (17-month-old) rats. The size of motoneuronal somata in the exercised aged rats was significantly larger than that in sedentary aged rats. Morphological parameters (fascicular area, fiber density, and axonal diameter) for the MG nerve in the exercised rats showed intermediate values between those for middle-aged and sedentary aged rats, although all of the differences in the means between groups were not significant. Thus, the overall changes seen in the exercised rats seem less compared to the changes in the sedentary rats, suggesting that long-term moderate exercise retards the progressive changes in motoneurons and peripheral nerves that develop in old age.