Primary and Secondary Vestibular Connections in the Brain Stem and Cerebellum: An Axoplasmic Transport Study in the Monkey and Cat

The projections of the vestibular ganglion were explored in the monkey by anterograde transport of tritiated amino acids from the labyrinth. The central connections of the principal vestibular nuclei VN were studied by retrograde and anterograde axoplasmic transport technics in the monkey and cat. Primary vestibular fibers terminate in virtually all parts of the superior SVN and medial vestibular nuclei MVN. Projections to the inferior vestibular nucleus IVN terminate in all areas except cell group f, while peripheral inputs to the lateral vestibular nucleus LVN are restricted to its ventral part. Of the small cell groups associated with the VN, only the interstitial nucleus of the vestibular nerve and cell group y receive primary vestibular fibers. Primary vestibular fibers project beyond the VN to terminate in the ipsilateral accessory cuneate nucleus, subtrigeminal lateral reticular nucleus and reticular formation RF. Primary vestibulocerebellar projections terminate as mossy fibers in the ipsilateral nodulus, uvula, flocculus, lingula and deep folia of vermal lobules V and VI no primary fibers terminate in the fastigial nuclei FN. The most impressive central afferents to the VN are commissural projections from the contralateral MVN, SVN, IVN and cell group y. Other brain stem afferents are derived from the perihypoglossal nuclei PH, the interstitial nucleus of Cajal INC, the visceral nuclei of the oculomotor nuclear complex OMC and the RF. Spinovestibular projections arise from the contralateral central cervical nucleus CCN. Cerebellovestibular projections differ for LVN, IVN and MVN. LVN receives cortical inputs from the ipsilateral anterior lobe vermis, and bilateral projections from rostral FN. IVN receives fewer fibers from the anterior lobe, but has substantial inputs from the nodulus, uvula and middle portions of FN. MVN receives cortical projections primarily from the ipsilateral flocculus.