Morphologic correlates of vestibular compensation in the cat

Vestibulo-ocular (VO) neurons in the superior vestibular nucleus (SVN) were labeled with horseradish peroxidase (HRP) and studied quantitatively in the electron microscope to determine the morphologic correlates of vestibular compensation. Eighteen cells from four normal cats were compared to 26 cells from four 8-week vestibular neurectomy animals and 29 cells from four 1-year neurectomy animals. There was a greater than 70% decrease in synaptic profiles (SP) on the VO soma in both experimental groups. Increased excitatory activity in the residual SP are indicated by (1) smaller synaptic vesicles (SV) with increasing survival after neurectomy, (2) the association of the SP with a higher ratio of asymmetric to symmetric synapses, and (3) a higher volume fraction of mitochondria in the SP. The experimental VO soma showed a 17% reduction in size (cytoplasm), and a decrease in rough endoplasmic reticulum and polyribosomes. The target cell also showed an increased number of somal spines associated with the remaining SP in the 1-year neurectomy group.