Spontaneous and mechanically evoked afferent activity originating from myelinated fibres in ferret inferior alveolar nerve neuromas

The inferior alveolar nerve is a predominantly sensory branch of the trigeminal nerve which runs within a bony canal, and is frequently damaged in patients. A small proportion of these patients develop neuropathic pain, and this may result from neural activity generated at the injury site. To investigate this abnormality we have used electrophysiological techniques in an animal model to determine the level of spontaneous activity and mechanical sensitivity of myelinated fibres ending in a neuroma of the inferior alveolar nerve. In 20 anaesthetised adult male ferrets the left inferior alveolar nerve was ligated in the region of the third premolar tooth, cut distally, and recovery permitted for periods of 3-113 days prior to making single unit recordings from the nerve central to the injury. The proportion of units which were spontaneously active ranged from 0% to 26%, with discharge rates of 0.3-12.9 Hz. Discharge in response to mechanical stimulation of the neuroma was found in 0-36% of the units. Both spontaneous activity and mechanical sensitivity were significantly higher after shorter recovery periods and the majority of the spontaneously active units was also mechanically sensitive. These data reveal that the inferior alveolar nerve responds to injury in a similar way to some other peripheral nerves, and the neural activity generated at the injury site may play a role in the development of dysaesthesia.