Evolution and resolution of stimulation-induced axonal injury in peripheral nerve.

We describe the evolution of axonal injury following the induction of neural damage by electrical stimulation. The sciatic nerves of cats were stimulated continuously for 8 h with charge-balanced waveforms at high intensities, 50 Hz and 2100-4500 microA, using circumneural helical electrodes. Computer-assisted morphometric and ultrastructural studies indicate that many of the damaged fibers had not regenerated by 125 days after stimulation. Functional deficits were not observed in any of the animals, and most of the fibers appeared to be histologically normal at 125 days after stimulation. These findings indicate that there is relatively little late-onset injury associated with the stimulation. However, the slow, and possibly incomplete, recovery of the damaged axons emphasizes the importance of using stimulus protocols with adequate margins of safety.