Discharge pattern of afferent fibers from a neuroma

Discharge properties of afferent units from experimentally produced stump neuromata in the superficial peroneal nerve of the cat hind limb were investigated electrophysiologically. The superficial peroneal nerves were cut and ligated 6-245 days before the experiments. Myelinated and chiefly unmyelinated axons were analyzed. The following results were obtained: (1) 3.9 +/- 3% (mean +/- S.D.) of axons from early neuromata (days 6-27) and 13.4 +/- 10.7% of axons from old neuromata (more than 50 days after nerve severance) showed ongoing activity. The rate of ongoing activity was usually below 1 imp/sec (73%) and rarely above 4 imp/sec and its pattern, in most cases, was irregular. Some myelinated afferents had regular or irregular bursting patterns. (2) Mechanical stimulation of the neuroma excited 19.4 +/- 9.6% of the axons from young neuromata and 32.8 +/- 14.9% of the axons from old neuromata. Part of these mechanosensitive units exhibited pronounced after-discharges. Some 20% of the units which could be excited, probably ephaptically, by stimulation of other afferent fibers in the common peroneal nerve were excited by pressure applied to the neuroma. (3) About 20-40% of the units with ongoing activity (3-5% of all axons) responded weakly to intravenous injections of adrenaline and noradrenaline and to repetitive stimulation of the lumbar sympathetic trunk. (4) Recording from distally cut fiber bundles showed that part of the axons could be activated by electrical stimulation of the nerve distal to the recording site and by mechanical stimulation of the neuroma. Most of these axons were unmyelinated. This result indicates that afferent axons either branch or interact ephaptically a long distance proximal to the neuroma in the neuroma nerve. (5) The results are discussed with respect to similar results obtained on afferent fibers from experimentally produced neuromata of the sciatic nerve of mice and rats.