Ectopic neural activity from myelinated afferent fibres in the lingual nerve of the ferret following three types of injury.

Sensory disturbances following nerve injury may result from abnormal neural activity initiated at the injury site. We have studied the activity generated in the lingual nerve after three types of injury which may have variable potentials for the initiation of sensory disturbances. We have also compared the results with those found after damage to the inferior alveolar nerve, another branch of the trigeminal nerve, to determine whether differences in nerve fibre type or location affect the level of abnormal activity. In anaesthetised adult male ferrets the left lingual nerve was either ligated and cut distally, chronically constricted, or sectioned and allowed to regenerate. Following recovery periods of 3 days-6 months, single unit electrophysiological recordings were made from central to the injury site. After all three types of injury, some of the damaged axons at the injury site developed spontaneous activity (up to 36% of units) and mechanical sensitivity (up to 35% of units). There were significantly fewer spontaneously active units after ligation than after the other two types of injury but the level of mechanical sensitivity was not significantly different between the three types of injury. There was a significant increase in the spontaneous activity between 3 weeks and later recovery periods following both ligation and section injuries, and this late increase was not seen in our previous studies on the inferior alveolar nerve. Differences in the time-course of ectopic activity in adjacent branches of the trigeminal nerve suggest that the fibre types or anatomical relationships affect the outcome of injury.