Vestibulocochlear (VIII) Nerve

VESTIBULAR NERVE

At the fundus of the internal acoustic meatus, the vestibular part of the vestibulocochlear nerve expands to form the vestibular ganglion before dividing into superior and inferior divisions. Both divisions contain peripheral processes of the vestibular bipolar cells, which penetrate tiny foramina in the superior and inferior vestibular areas of the fundus of the internal meatus. The peripheral processes spread to contact hair cell receptors embedded in the neuroepithelium lining the ampullae of the semicircular ducts (canals) and the maculae of the saccule and utricle. The longer central processes of the bipolar cells transmit impulses from these vestibular hair cells to the brainstem. Passing backward in the pontomedullary junctional area, the central processes divide into ascending and descending branches, which end predominantly in the superior (cranial), inferior (caudal), medial, and lateral vestibular nuclei located in the medulla oblongata and lower pons. Other branches proceed directly through the homolateral inferior cerebellar peduncle to the flocculonodular cerebellar lobe. Fibers from the superior nucleus enter the ipsilateral medial longitudinal fasciculus and ascend to end on cells of cranial nerves III, IV, and VI. Fibers from the inferior, medial, and lateral nuclei all terminate on the contralateral medial longitudinal fasciculus, in addition to connections with the autonomic nuclei, reticular formation, and the intermediolateral column of the cord. These connections play a crucial role in regulating posture and coordinating head, body, and eye movements. Separate vestibular-cerebellum pathways, mainly through the fastigial nuclei, also influence posture and movement coordination. Connections with the autonomic centers and the intermediolateral column likely account for the nausea and vomiting seen, at times, with overstimulation of the vestibular system.

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