Connections of Vestibular Nuclei
The vestibular nuclei receive the following afferents (Fig. 10.8).
1. The main afferents are central processes of bipolar neurons of the vestibular ganglion. These fibres constitute the vestibular part of the vestibulo-cochlear nerve. They convey impulses from end organs in the semicircular ducts, the utricle and the saccule: these are necessary for maintenance of equilibrium.After entering the medulla the fibres of the vestibular nerve divide into ascending and descending branches. The descending branches end in the medial, lateral and inferior vestibular nuclei. The ascending branches reach the superior vestibular nucleus.
2. The vestibular nuclei also receive fibres from some parts of the cerebellum.
The efferents from the vestibular nuclei are as follows.
a. Vestibulo-cerebellar fibres pass through the inferior cerebellar peduncle. The vestibulocerebellar fibres form a separate bundle, the juxtarestiform body, within the peduncle. Some fibres of the vestibular nerves by-pass the vestibular nuclei and go straight to the cerebellum.
b. Fibres arising in the vestibular nuclei establish connections with cranial nerve nuclei responsible for movements of the eyes (third, fourth, sixth) and of the neck (eleventh). These fibres form the medial longitudinal fasciculus (or bundle). .
c. Fibres from the lateral vestibular nucleus descend to the spinal cord as the vestibulospinal tract. Fibres from the medial (and other) nuclei descend to the spinal cord through the medial longitudinal fasciculus. These fibres are sometimes named the medial vestibulospinal tract. Some fibres reach the pontine reticular formation.
d. Some fibres from the vestibular nuclei enter the lateral lemniscus.
e. Some vestibular impulses reach the thalamus (ventro-posterior nucleus) and are relayed to the cerebral cortex. A vestibular centre is present in the parietal lobe just behind the postcentral gyrus.