Abstract
Specificity of otolith and canal input was investigated in second-order vestibular neurons (2°VN). Especially the convergence of monosynaptic input from the three semicircular canals, the utricle, the lagena and the saccule was studied. Each of the six nerve branches on one side was stimulated separately in in vitro frog brain preparations. Antidromic spikes evoked from the spinal cord or the oculomotor nuclei were used to further identify the projection of these 2°VN. About 42% of the 2°VN received a monosynaptic input from only one ipsilateral labyrinthine organ. A small number of 2°VN showed a monosynaptic convergence of canal (8%) or of utricular and lagenar inputs (11%) or of utricular and saccule (4%) or lagenar and saccule (5%). Monosynaptic convergence of canal and utricular (24%) or of canal and lagenar inputs (31%) was more frequent. Monosynaptic utricular inputs were observed in 2°horizontal and 2°vertical canal neurons with a ratio of 3:1. Monosynaptic lagenar inputs were detected in 2°vertical but not in 2°horizontal canal neurons. In a separate group of 2°VN the convergence pattern of monosynaptic saccular, canal, lagenar and utricular inputs was studied. A monosynaptic convergence of signals from the saccule and any other labyrinthine organ was observed only in 3% of 2°VN. Descending and/or ascending projections were found for 2°canal and 2°utricular neurons, 2°lagenar neurons projected only to the spinal cord and 2°saccular neurons projected neither to the spinal cord nor to the oculomotor nuclei. The results show that afferent vestibular inputs converge at the level of 2°VN in a canal plane-specific way. Furthermore, these results are consistent with a role of lagenar input for spinal but not oculomotor reflexes and a non-vestibular function of saccular input in the frog.
