Aging effects on conduction velocities of myelinated and unmyelinated fibers of peripheral nerves.

Conduction velocities of myelinated and unmyelinated fibers of cutaneous sural (SU), muscular gastrocnemius soleus (GS) and parasympathetic vagus nerves were measured in Wistar rats, ranging in age from 28 to 920 days. The maximum conduction velocity for myelinated fibers of all nerves tested gradually increased with age up to 300 days and then remained unchanged until 700 days. Although the maximum conduction velocity of SU myelinated fibers was stable up to 900 days, that of GS and vagus nerves decreased by about 10-15% between 700 and 900 days. In contrast to myelinated fibers, the conduction velocity of unmyelinated fibers showed only a small increase between 28 and 100-200 days and remained quite constant thereafter up to the maximum age tested, 900 days. The reason for the stability of unmyelinated fiber conduction velocity may depend on the fact that the majority of age-related changes in nerve are in the myelin sheaths.