Endurance exercise does not modify nerve fibre morphology in the rat soleus nerve

The effects of the myelination and growth of axons in the nerve to the soleus muscle (NSM) was investigated in young male Wistar rats. Experimental animals were run on a treadmill for 13 weeks (70 min/day, 6 days/week), while sedentary weight-matched animals of the same age and sex served as controls. The activity of the enzymes, phosphofructokinase (PFK) and succinate dehydrogenase (SDH) were measured in the soleus muscle to assess the effects of training on the anaerobic glycolytic and oxidative capacities. Axon and myelin sheath cross-sectional areas were measured from electron micrograph montages of the whole NSM with the aid of a digitizing tablet. The exercise programme produced large adaptive increases in the capacity of the soleus muscle for both oxidative and anaerobic glycolytic metabolism. The specific activity of PFK and SDH increased in the soleus muscle of exercised animals by 42.8% and 68.2%, respectively. In the NSM, however, there were no differences between control and exercised animals concerning the total number of myelinated nerve fibers, the size of axons and myelin sheaths of nerve fibres and the degree of myelination of axons (myelin area divided by axon area). Comparison with other studies suggests that the intensity of exercise may be the critical parameter responsible for discrepancies in the literature. Reports suggesting that exercise increases the size of nerve fibres should not be generalized to all exercise programmes.