ULTRASTRUCTURAL-CHANGES IN SUPERIOR VESTIBULAR COMMISSURAL NEURONS FOLLOWING VESTIBULAR NEURECTOMY IN THE CAT

The ultrastructural changes of the feline superior vestibular commissu ral neurons (CNs) were quantitatively assessed 8 weeks following ipsil ateral vestibular neurectomy. Results indicated a slight degeneration of synaptic profiles (SPs; 25%) representing the primary vestibular af ferent input onto CN soma. The synaptic vesicles of the remaining SPs, which likely originate from the cerebellum and the contralateral CNs, were smaller and rounder, suggesting a transition from an inhibitory to an excitatory mode of response. The SP loss had little impact on th e CNs' capacity for protein synthesis and structural maintenance, sinc e there was no change in the volume fraction of intracellular organell es. These data suggest that CNs do not degenerate and are likely funct ional after vestibular compensation. These findings support the role o f the commissural pathway in vestibular compensation as proposed by Ga liana et al, which is based on the assumption that the intervestibular commissural connections remain intact following vestibular neurectomy .