Ongoing activity in peripheral nerves: The physiology and pharmacology of impulses originating from a neuroma

In rats, the sciatic nerve was cut, drawn into a polythene tube with one end sealed, and a neuroma allowed to develop in the chamber. Activity in the dorsal and ventral roots terminating in the neuroma was examined between 9 days and 4 months after the section. A fraction of the small myelinated afferent fibers originating in the neuroma were carrying a steady ongoing barrage of nerve impulses. Many of the fine terminals were excited by slight mechanical distortion. The fine sprouts in the neuroma were electrically excitable. The ongoing afferent barrage was highly dependent on blood flow. It was abolished for long periods of time after a brief antidromic tetanus had invaded the active fibers. This suggests that some of the pain relief obtained from peripheral nerve stimulation may have a peripheral rather than a central mechanism. No signs of excitatory on inhibitory interaction were detected between volleys in one group of nerve fibers and the activity in other groups of fibers in the neuroma. Alpha active sympathetic amines, noradrenaline, excited the ongoing activity while beta agents, isoprenaline, had no excitatory effect. This suggests that alpha blocking agents might be useful to test if the sympathetic system is involved in particular pains.