Auditory Pathway
Fibers of auditory pathway pass through cochlear trdivision of vestibulocochlear nerve (VIII cranial nerve). It is also known as auditory nerve. Major part of the auditory pathway lies in medulla oblongata, midbrain and thalamic region. Higher center for hearing is in temporal lobe ofcerebral cortex, where the fibers of auditory pathway initially terminate. Fibers are both crossed and uncrossed, so that each cochlea is represented in the cortex on both sides.
RECEPTORS
Hair cells
in organ of Corti are the receptors of the auditory sensation.
Hair cells are of two types, outer hair cell and inner hair cell. All the hair
cells are innervated by afferent and efferent nerve fibers.
Afferent nerve fibers from the hair cells form auditory nerve.
FIRST ORDER NEURONS
First order neurons of
auditory pathway are the bipolar cells of spiral ganglion, situated
in modiolus of cochlea. Peripheral short processes (dendrites) of the bipolar
cells are distributed around hair cells of organ of Corti as afferent nerve fibers. Their long processes (axons) leave the ear as cochlear nerve fibers and enter
medulla oblongata. In medulla oblongata, these fibers divide into two groups, which end on ventral cochlear
nucleus and dorsal cochlear nucleus of the same side
in medulla oblongata.
Efferent Nerve Fibers
to Hair Cells
Efferent
nerve fibers of hair cells arise from superior olivary nucleus. Fibers
from this nucleus reach the
hair cells by passing
through the ventral and dorsal cochlear nuclei and cochlear nerve of the same
side.
Efferent
nerve fiber to outer hair cell terminates directly on the cell body and
controls the motility of
this cell.
Efferent nerve fiber to inner hair cell terminates on the auditory
(afferent) nerve
fiber,
where it leaves the inner hair cell. It controls the impulse
output from this hair cell.
SECOND ORDER NEURONS
Neurons of dorsal and ventral
cochlear nuclei in the medulla oblongata form the second order neurons of
auditory pathway. Axons of
the second order neurons run in four different groups:
1. First
group of fibers cross the midline and run to the opposite side, to form trapezoid
body. Fibers from
trapezoid body go to the superior
olivary nucleus Second group of fibers terminate at
superior olivary
nucleus of same side via
trapezoid body of the same side
3. Third
group of fibers run in lateral lemniscus of the same side and terminate in nucleus of
lateral
lemniscus of same
side
4. Fourth
group of fibers run into reticular formation, cross the midline as intermediate trapezoid fibers
and
finally join the nucleus of lateral lemniscus of opposite side.
THIRD ORDER NEURONS
Third
order neurons are in the superior olivary nuclei and nucleus of
lateral lemniscus. Fibers of the third order
neurons end in medial geniculate body, which forms the
subcortical auditory center.
Fibers from medial
geniculate body go to the temporal cortex, via internal capsule as auditory radiation.
Some fibers from medial geniculate body go to inferior
colliculus of tectum in midbrain. The fibers
of
auditory radiation are involved in reflex movement of head, in
response to auditory stimuli.
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