Auditory Pathway

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.