Primary vestibulocerebellar fibers in the monkey: Distribution of fibers arising from distinctive cell groups of the vestibular ganglia

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Abstract
In a series of monkeys attempts were made to determine the distribution of primary vestibulocerebellar fibers arising from cell groups of the vestibular ganglia that innervate distinctive parts of the labyrinth. Lesions in the vestibular ganglia were produced by extradural surgical approaches in cell groups which innervate: (1) the cristae of the anterior and lateral canals, (2) the macula of the utricle, (3) the macula of the saccule, and (4) all receptor components of the labyrinth. In a control series selective lesions were produced in the cochlea and cochlear nerve. Vestibular and cochlear lesions were studied in serial sections of the petrous bones stained with Sudan black B. Degeneration in the brain stem and cerebellum was studied in sagittal and transverse sections stained by the Nauta‐Gygax and Wiitanen technics.Vestibular fibers projecting to parts of the cerebellum represent ascending branches of root fibers which largely traverse the superior vestibular nucleus and enter the cerebellum via the juxtarestiform body. In the cerebellum fibers divide into medial and lateral bundles. Medial branches projecting fibers to the ipsilateral nodulus, uvula (lamina d) and lingula appear to arise from cells in all parts of the vestibular ganglia. Cells in the superior vestibular ganglion (SVG) innervating the cristae of the anterior and lateral canals project fibers to floccular folia 5 to 10; the utricular cell groups of SVG projects fibers mainly to floccular folia 5 and 6 while the saccular cell group of the inferior vestibular ganglion (IVG) projects fibers to floccular folia 3, 4, 5 and 6. Collaterals of fibers passing laterally in the peduncle of the flocculus end in ventromedial parts of the dentate nucleus.Some vestibular fibers projecting to parts of the flocculus penetrate the entire thickness of the granular layer and break up into fragments near Purkinje cells and in basal parts of the molecular layer. Such fibers appear to represent climbing fibers.Cochlear lesions produce degeneration in localized regions of the ipsilateral dentate nucleus.Cell groups of the vestibular ganglia innervating different receptor components of the labyrinth have unique areas of predominant projections in different folia of the flocculus and common regions of termination in portions of the nodulus and uvula.