The relationship between nerve conduction velocity and fiber morphology during peripheral nerve regeneration

We analyzed the relationship between motor nerve conduction velocity (MCV) and morphological changes in regenerating nerve fibers at different times after sciatic nerve transection to identify reliable indices of functional recovery. Thirty rats were divided into five equal groups, one control group and four groups subjected to sciatic nerve transection and immediate suturing, followed by regeneration for 50, 100, 150, and 200 days, respectively. MCV was measured in each group, followed by morphometric analyses of fibers of the common peroneal nerve. MCV increased progressively with time after nerve transection, although it remained lower than the control velocity. Mean fiber diameter (axon plus myelin sheath) also increased with time after nerve transection. Recovery of mean fiber diameter was well correlated with MCV, even though regenerating nerves likely contained many small nonconducting fibers. In contrast, the change in the mean diameter of regenerating axons and relative myelin thickness (g-ratio) did not provide an accurate measure of recovery as they were not increasing in a time-dependent manner. Furthermore, internodal length changed only slightly with increasing fiber diameter in regenerating nerves; therefore, the regression relation between fiber diameter and internodal length was not a sensitive index of recovery. MCV and mean fiber diameter were the most sensitive indices of functional recovery during sciatic nerve regeneration.