DEVELOPMENT OF SPONTANEOUS ACTIVITY AND MECHANOSENSITIVITY IN AXOTOMIZED AFFERENT NERVE-FIBERS DURING THE FIRST HOURS AFTER NERVE TRANSECTION IN RATS

1. Spontaneous activity and ectopic mechanical excitability of axotomi zed unmyelinated and myelinated fibers in the sural nerve were examine d in anesthetized rats. The analysis was performed within 30 h after t he nerve lesion using single-fiber recordings that were performed prox imal to the severed nerve end. 2. Among all unmyelinated fibers tested (n = 865), 4-8% exhibited persistent spontaneous activity of low and irregular frequency. The percentage of spontaneously active C fibers d id not change significantly during the first 30 h. Only 6 of 796 A fib ers had spontaneous activity. 3. Mechanical stimulation of the cut ner ve end excited 5-8% of all C fibers under investigation. No developmen t with time could be detected in the frequency of mechanically excitab le C fibers. In contrast, beginning 6 h after nerve transection, the n umber of mechanically excitable A fibers rose with time, reaching 27% after 22-30 h. 4. Among the A fibers (C fibers) that exhibited mechani cal excitability or spontaneous activity, only 4% (25%) had both prope rties, whereas 96% (75%) were either mechanosensitive or spontaneously active. 5. With time after the nerve lesion, the mean discharge rate of all spontaneously discharging C fibers decreased significantly from 49 imp/min (0.5-9 h after nerve lesion) to 11 imp/min after 22-30 h. The mean discharge rate of C fibers exhibiting solely spontaneous acti vity and those C fibers that were additionally mechanosensitive did no t differ significantly. 6. In conclusion, the mechanically induced imp ulse activity in an increasing number of myelinated afferents and also the spontaneous activity and mechanically induced activity observed i n a small proportion of unmyelinated afferents hours after axotomy may contribute to the generation of paresthesias, painful sensations and associated changes following peripheral nerve lesion.