Ultrastructure of contralaterally projecting vestibular efferent neurons in the cat.

Numerous reports have demonstrated that primary vestibular afferent axons terminate in the vicinity of centrally located vestibular efferent (VE) neurons. The goal of this study was to morphologically demonstrate contact of primary vestibular afferents directly on the soma of contralaterally projecting VE neurons, as well as develop a quantitative description of these neurons. A right vestibular neurectomy was performed in three young adult cats followed up three days later by the placement of horseradish peroxidase into the left vestibule. Labeled contralateral (right) VE neurons were examined in a transmission electron microscope for degenerating synaptic profiles contacting the soma. Ultrastructural characteristics of VE neurons, including nuclear size, volume fraction of intracellular organelles, number of synaptic profiles, and cellular shape, were quantitatively analyzed. No degenerating synaptic profiles were observed on labeled VE soma, although degenerating axons were present nearby. Statistical analysis of volume fraction data of the VE neurons suggests a continuum of a single subpopulation of cells rather than multiple subpopulations.