Several techniques of nerve block have been used to relieve spasticity which is the disabled problem of many patients who had upper motor neuron lesion. It has been reported that the stretch reflex can be reduced selectively with little, if any, loss of voluntary motor power after phenol block.

Although spasticity is reduced by injections of dilute phenol into peripheral nerve, the mechanism for such an effect remain in doubt. Some authors have suggested the selective block of small diameter nerve fiber, but other have contradiction such findings.

The purpose of this study is to disclose the mechanism of action and pattern of regeneration after phenol injection through experiments involving the electron microscope. Sciatic nerves in rats were examined at various intervals after intraneural injection with 5% phenol solution.

The results were as follows:

1) The overall consequence of phenol injection into the nerves was Wallerian degeneration. The greatest degree of destruction was present at 2 weeks following injection. After 8 weeks, almost nerve fibers appeared to be regenerated. At 16 weeks, the phenol injected nerves looked almost completely normal except increase in endoneurial collagen and fibroblast.

2) The action of phenol on peripheral nerves was not selective; fibers of all sizes were affected, but it took larger for longer axones to degenerate and regenerate than small axons.