The efferent vestibular system in the cat: a horseradish peroxidase and fluorescent retrograde tracers study.

Neurons of the efferent vestibular system were investigated in the cat using retrograde axonal transport of horseradish peroxidase and fluorescent retrograde double labelling techniques. The number of efferent neurons was clearly higher than previously reported. A three dimensional reconstruction of the location of these neurons showed that they constitute a single group and did not give evidence of an eventual specialization based on neuron subpopulations. However, a study of cross-sectional areas of the horseradish peroxidase-labelled efferent neurons detected that the ipsilateral population contained a larger number of small neurons than the contralateral one. Double labelling by means of either 4',6-diamidino-2- phenylindol 2HCl in combination with horseradish peroxidase or Fast Blue in combination with Nuclear Yellow showed that 20% of efferent neurons project to both labyrinths. Such a high percentage raises the question of the role of these double-projecting cells and the specificity of their branching on vestibular receptors. This study expands previous work in the cat demonstrating that a much greater number of efferent neurons exists than had hitherto been assumed, among them 20% have both crossed and uncrossed projections.