Postlesional vestibular reorganization in frogs: Evidence for a basic reaction pattern after nerve injury

Nerve injury induces a reorganization of subcortical and cortical sensory o r motor maps in mammals. A similar process, vestibular plasticity 2 mo afte r unilateral section of the ramus anterior of N. VIII was examined in this study in adult frogs. The brain was isolated with the branches of both N. v m attached. Monosynaptic afferent responses were recorded in the vestibular nuclei on the operated side following ipsilateral electric stimulation eit her of the sectioned ramus anterior of N. VIII or of the intact posterior v ertical canal nerve. Excitatory and inhibitory commissural responses were e voked by separate stimulation of each of the contralateral canal nerves in second-order vestibular neurons. The afferent and commissural responses of posterior vertical canal neurons recorded on the operated side were not alt ered. However, posterior canal-related afferent inputs had expanded onto pa rt of the deprived ramus anterior neurons. Inhibitory commissural responses evoked from canal nerves on the intact side were detected in significantly fewer deprived ramus anterior neurons than in controls, but excitatory com missural inputs from the three contralateral canal nerves had expanded. Thi s reactivation might facilitate the survival of deprived neurons and reduce the asymmetry in bilateral resting activities but implies a deterioration of the original spatial response tuning. Extensive similarities at the syna ptic and network level were noted between this vestibular reorganization an d the postlesional cortical and subcortical reorganization of sensory repre sentations in mammals. We therefore suggest that nerve injury activates a f undamental neural reaction pattern that is common between sensory modalitie s and vertebrate species.