Activity-induced morphologic changes in rat soleus nerve.

The effects of 8 to 12 weeks of intense running or swimming on the morphology of the soleus nerve of the adult male albino rat were investigated. Nerve fiber diameter and number, axon diameter, and myelin area were determined. Mean fiber diameter (5.50 +/- 0.47) of the trained rats was smaller (P less than 0.05) than for sedentary control rats (6.19 +/- 0.27). The decrease in fiber size in the trained rats appeared to be reflected by a combination of smaller (P greater than 0.05) mean axon diameter and myelin area. Total fiber number was not different in the control (means = 113 +/- 6) and trained (means = 108 +/- 4) rats. These results provide further evidence that a chronic, intense but physiologic exercise can impose a functional demand on peripheral nerves that can alter morphologic features that are closely related to conduction velocity and commonly are associated with neuronal cell size.