Recovery of the jaw-opening reflex after lesions of the lingual nerve in the rat.

In three groups of rats, lesions were produced in the right lingual nerves near the base of the tongue; the three types of injury inflicted (cryogenic, crush, and stretch) are reputed to spare the epineurium but produce different degrees of intraneural damage. In regular assessments of recovery, an electrical stimulus (sufficient to elicit the jaw-opening reflex) was applied to either side of the tongue in turn; the amplitude of the reflex was measured as the isometric force of jaw opening. The size of the reflex response to stimulation of the injured side was followed up to 4 months post-lesion, with the response elicited from the control side used as the reference level. The reflex was absent when the experimental side was stimulated immediately after creation of a lesion; the first sign of reflex recovery was found at about 15 days post-operative. Subsequently, in 84% of the animals, the reflex activity elicited from the experimental side increased until it exceeded that elicited from the reference side; this relative hyperreflexia started 1-4 months post-lesion and had a highly variable duration. There was no difference in the incidence, latency, or duration of the hyperreflexia following any of the three types of lesion. The hyperreflexia found in this study is not readily explained by existing hypotheses of the mechanisms underlying post-lesion hyperesthesia or central neuronal hyperexcitability.