Axotomized and intact muscle afferents but no skin afferents develop ongoing discharges of dorsal root ganglion origin after peripheral nerve lesion

After peripheral nerve lesions, some axotomized afferent neurons develop on going discharges that originate in the dorsal root ganglion (DRG). We inves tigated in vivo which functional types of afferent neurons contributed to t his ectopic activity. Six to twelve days after the gastrocnemius soleus (GS ) nerve supplying skeletal muscle and the sural (SU) nerve supplying skin h ad been transected (experimental group E1), 20.4% of afferent neurons with myelinated axons projecting into the GS nerve produced ongoing discharges o f irregular or bursting pattern. In contrast, all SU neurons were silent. A dditional transection of peroneal and tibial nerves (group E2) induced ongo ing activity in a similar percentage of GS neurons (22.1%), but their mean discharge frequency was higher (6.0 vs 2.7 Hz), and more of them exhibited bursting discharges (63 vs 17%). When the GS nerve had been left intact whi le tibial, peroneal, and SU nerve had been transected (group E3), 18.8% of unlesioned GS neurons developed ongoing discharges at a mean frequency of 6 .1 Hz; most of them exhibited a bursting pattern. Without a preceding nerve lesion, almost no GS neuron (1.1%) fired spontaneously. Most afferent neur ons with ongoing activity had an axonal conduction velocity of 5-30 m/sec i ndicating that some of these neurons may have had nociceptive function. The se findings provide the first evidence that after peripheral nerve injury b oth axotomized as well as intact afferent neurons supplying skeletal muscle but not skin afferents generate ongoing activity within the DRG, probably because of a yet unknown signal in the DRG triggered by axotomy.