Vestibular system nerve supply

NERVE SUPPLY TO VESTIBULAR APPARATUS

Impulses from the hair cells of crista ampullaris and maculae are transmitted to medulla oblongata and other parts of central nervous system (CNS) through the fibers of vestibular division of vestibulocochlear (VIII cranial) nerve.

FIRST ORDER NEURON

First order neurons of the sensory pathway are bipolar in nature. The soma of bipolar cells is present in

vestibular or Scarpa ganglion, which is situated in the internal auditory meatus. Dendrites of bipolar cells reach the receptor organs, i.e. crista ampullaris and maculae in vestibular apparatus. Branches of the dendrites have close contact with basal part of hair cells. Dendrites terminating on type I hair cells are comparatively larger than those ending on type II hair cells. Axons of the first order neurons (bipolar cells) form vestibular division of vestibulocochlear nerve. These fibers reach the medulla oblongata and terminate in vestibular nuclei. These nerve fibers are called primary vestibular fibers.

Vestibular Nuclei

There are four vestibular nuclei in the medulla oblongata, viz. superior, inferior, lateral and medial

nuclei. Most of the primary vestibular fibers reaching superior and medial nuclei come from crista ampullaris of semicircular canals. Lateral vestibular nucleus receives fibers mainly from maculae of otolith organ and inferior vestibular nucleus receives fibers from both crista ampullaris and maculae.

Efferent nerve fibers to hair cells

Some neurons in vestibular nuclei send efferent fibers, which run back to the hair cells along with primary

vestibular fibers. It is believed that these efferent fibers to hair cells provide tonic inhibition of hair cells.

Fibers to Cerebellum

Fibers from some bipolar cells reach cerebellum directly and terminate in flocculonodular lobe or the fastigial nucleus in cerebellum.

SECOND ORDER NEURON

Second order neurons of this pathway are located in the four vestibular nuclei. Axons from vestibular nuclei form the secondary vestibular fibers. Secondary vestibular fibers form four tracts:

1. Vestibulo-ocular tract

2. Vestibulospinal tract

3. Vestibuloreticular tract

4. Vestibulocerebellar tract.

1. Vestibulo-ocular Tract

Fibers from superior, medial and inferior vestibular nuclei descend downwards for short distance along

with vestibulospinal tract. Afterwards, these fibers ascend through the medial longitudinal fasciculus

and terminate in the nuclei of III, IV and VI cranial nerves, thus forming vestibulo-ocular tract. This tract

is concerned with movements of eyeballs in relation to the position of the head.

2. Vestibulospinal Tract

Fibers from lateral nucleus descend downwards and form the vestibulospinal tract. Some fibers from this

nucleus ascend upward and join medial longitudinal fasciculus. Fibers of vestibulospinal tract are involved

in reflex movements of head and body during postural changes.

3. Vestibuloreticular Tract

Some fibers from vestibular nuclei reach the reticular formation of brainstem forming reticulospinal tract.

These fibers are concerned with the facilitation of muscle tone.

4. Vestibulocerebellar Tract

Some fibers arising from all four vestibular nuclei form vestibulocerebellar tract and terminate in flocculonodular lobe and fastigial nuclei of cerebellum. This tract is involved in coordination of movements according to body position.