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.
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