Vestibular nuclei

Target:

• Cristae ampullaris of semicircular canals

• Maculae utriculi and sacculi of the utricle and saccule

“Vestibular nuclei are situated partly in the medulla and partly in the pons, immediately beneath the lateral part of the floor of the fourth ventricle called vestibular area. On the basis of cytoarchitecture and afferent and efferent connections, four distinct vestibular nuclei are recognised, viz. (a) inferior or spinal vestibular nucleus, (b) lateral vestibular nucleus (also called Dieter's nucleus), (c) superior vestibular nucleus, and (d) medial vestibular nucleus (Fig. 8.15).

• Inferior vestibular nucleus lies in the medulla, just medial to the inferior cerebellar peduncle. It is continuous rostrally with the lateral vestibular nucleus and related medially to the medial vestibular nucleus. It extends from the cranial end of nucleus gracilis to the pontomedullary junction. “

• Lateral vestibular nucleus lies immediately cranial to inferior vestibular nucleus and extends upwards in the pons almost to the level of nucleus of abducent nerve. It is composed of large multipolar cells resembling typical motor neurons. The cells of this nucleus give origin to the fibres of lateral vestibulospinal tract (Fig. 8.15).

• Superior vestibular nucleus is smaller in size and located entirely within the pons above the medial and lateral vestibular nuclei.

• Medial vestibular nucleus extends from medulla at the level of olive to the lower part of the pons. It is bounded laterally and rostrally by the other three vestibular nuclei. Its medial border is near the midline of the brainstem. The caudal end of this nucleus is near the caudal limit of the fourth ventricle.

“Connections of the vestibular nuclei

Afferents”

“• Fibres of the vestibular nerve (main afferents): Most of the vestibular nerve terminate in the four vestibular nuclei, however, few pass directly to the cerebellum by way of inferior cerebellar peduncle to the flocculonodular lobe.

• Cerebellovestibular fibres: The fibres from cerebellar cortex (flocculonodular lobe) relay in the nucleus fastigius which give rise to the fastigiobulbar tract. The fibres of this tract mostly pass through inferior cerebellar peduncle, some fibres, however, as they pass from cerebellum, loop around the superior cerebellar peduncle to form the uncinate fasciculus (tract of Russell) before joining the main tract which terminate in the vestibular nuclei.”

“Efferents

• To the cerebellum (vestibulocerebellar fibres): Most of these fibres arise from vestibular nuclei, however, few are the direct fibres of vestibular nerve from cells of vestibular ganglion as noted above. These fibres pass through the medial portion of the inferior cerebellar peduncle (juxta-restiform body) of the same side to relay into the cortex of archicerebellum.

• To the spinal cord: The principal connections between the vestibular nuclei and spinal cord are mediated through the vestibulospinal tract and the descending portion of the medial longitudinal fasciculus.”

“– Vestibulospinal tract (also called lateral vestibulospinal tract) (Fig. 8.15): Fibres of this tract arise exclusively from lateral vestibular nucleus. They descend downwards in the medulla dorsal to the inferior olivary nucleus and continue so in the anterior funiculus of the spinal cord, where they terminate on the anterior horn cells at all level of spinal cord especially in regions of cervical and lumbosacral enlargements. The vestibulospinal tract is uncrossed and regulates the muscle tone throughout the body in such a manner that the balance is maintained.

– Descending portion of medial longitudinal fasciculus: Fibres from vestibular nuclei, mainly from medial, project towards the midline and then turn caudally in the medial longitudinal fasciculi of both the sides which continue downward into the sulcomarginal fasciculus of anterior funiculus of spinal cord and terminate on anterior horn cells throughout the cervical part of the spinal cord. These connections provide for changes in the tone of neck muscles as required to support the head in various positions and during various movements.”

“ To the cranial nerve nuclei: Fibres from vestibular nuclei first project towards the midline than ascend up in ascending portions of the medial longitudinal fasciculi of both the sides and synapse with the cells of IIIrd, IVth and VIth cranial nerve nuclei; and downwards in the descending portions of the MLFs and synapse with spinal nucleus ”

“of XIth cranial nerve. These connections provide for the synchronized conjugate movements of the eyes, coordinated with the movements of the head (such coordination relies heavily on the information required by vestibular nuclei from semicircular canals or kinetic labyrinth).

Having considered the cranial nerve nuclei and there connections, it is now possible to workout the functional components of the individual cranial nerves