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 ongoing discharges that originate in the dorsal root ganglion (DRG). We investigated in vivo which functional types of afferent neurons contributed to this ectopic activity. Six to twelve days after the gastrocnemius soleus (GS) nerve supplying skeletal muscle and the sural (SU) nerve supplying skin had been transected (experimental group E1), 20.4% of afferent neurons with myelinated axons projecting into the GS nerve produced ongoing discharges of irregular or bursting pattern. In contrast, all SU neurons were silent. Additional transection of peroneal and tibial nerves (group E2) induced ongoing 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 while 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 neurons with ongoing activity had an axonal conduction velocity of 5-30 m/sec indicating that some of these neurons may have had nociceptive function. These findings provide the first evidence that after peripheral nerve injury both 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.