MODULE 12
The VESTIBULAR SYSTEM
- - - Vestibular fibers of the vestibulocochlear nerve enter at the level of slide 10 and pass dorsomedially through the small gap between the inferior cerebellar peduncle and the spinal tract of the trigeminal nerve (Cu Slide 10). Most of the fibers will terminate within the vestibular nuclei, but a few will pass directly through the juxtarestiform body to the posterior vermis and flocculonodular lobe of the cerebellum. At the level of slide 10 two vestibular nuclei, the lateral and superior nuclei, are present (Figure 12-5) and vestibular fibers may terminate in either of them. Almost all vestibular fibers bifurcate in this region and give off a descending branch which passes caudally through the inferior vestibular nucleus (giving it a "speckled" appearance on the slides) before ending in the inferior and medial vestibular nuclei at more caudal levels.
- - - The lateral and superior vestibular nuclei, like the dorsal motor nucleus of the vagus and the nucleus ambiguus, cannot be seen clearly on our slides. They have, however, been shown with reasonable accuracy in the views that follow. The superior nucleus is the only one to extend rostral to slide 10 - perhaps to slide 11. The lateral nucleus is also limited in extent, but is present in slides 9 and 8. The inferior and medial nuclei appear slightly caudal to the point of entrance of the vestibular nerve and are present as far caudally as slide 6. Use Figure 12-5 to call up the slides, or click on them below. The slides are from the ATLAS 1 series and many structures, in addition to the vestibular nuclei, are labeled.
- - - Blumenfeld describes the vestibular pathways starting on his Page 489 and in his Figure 12.18. You should have no trouble finding the medial longitudinal fasciculus, which connects the superior and medial nuclei with the oculomotor and trochlear nuclei. The medial vestibular nucleus projects to the cervical cord by way of the medial vestibulospinal tract. The lateral vestibular nucleus projects to the entire cord by way of the lateral vestibulospinal tract. These tracts were described when we considered the descending motor pathways, and are depicted in Figure 5-6. It is enough for you to know that they descend into the cord along with the other components of the medial pathway.
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