Auditory pathways: anatomy and physiology Notes

Auditory Pathway Introduction

• The fibers of the auditory pathway pass through the cochlear division of the vestibulocochlear nerve (8 cranial nerve) is also known as the auditory nerve. The major part of my auditory pathway lies in the medulla oblongata, midbrain, and thalamic region.
• The higher center for hearing is in the temporal lobe of the cerebral cortex, where the fibers of the auditory pathway finally terminate. The fibers are both crossed and uncrossed so that, each cochlea is represented in the cortex on both sides.

Read And Learn More: Medical Physiology Notes

Receptors

The hair cells in the organ of Corti are the receptors of auditory sensation. The hair cells are of two types, outer hair cells, and inner hair cells. The hair cells are innervated by afferent and efferent nerve fibers. The afferent nerve fibers form the auditory nerve (see below).

First Order Neurons

• The first-order neurons of the auditory pathway are the bipolar cells. The bipolar cells form the spiral ganglion, situated in the modiolus of the cochlea.
• The peripheral short processes (dendrites) of the bipolar cells are distributed around the hair cells of the organ of Corti as afferent nerve fibers. Their long processes (axons) leave the internal auditory meatus, as the cochlear never fibers, and enter the medulla oblongata. Immediately after entering the medulla oblongata, the fibers divide into two groups which end on the ventral cochlear nucleus and co’sei cochlear nucleus of the same side in the medulla oblongata.
  • Efferent Nerve Fibers to Hair Cells:
    • The efferent nerve fibers of hair cells arise from the superior olivary nucleus. The fibers from here reach the hair cells bypassing’ through ventral and dorsal cochlear nuclei and cochlear nerve of the same side.
    • The efferent nerve fiber to the outer hair cell terminates directly on the cell body and controls the motility of this cell. The efferent nerve fiber to the 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

The neurons of dorsal and ventral cochlear nuclei in the medulla oblongata form the second-order neurons of the auditory pathway. The axons of the second-order neurons pass through a complex pathway, which runs in four different directions:

1. The first group of fibers crosses the midline and runs to the opposite side to form a trapezoid body. Fibers from the trapezoid body go to the superior olivary nucleus.
2. The second group of fibers terminates at the superior olivary nucleus of the same side via the trapezoid body of the same side
3. The third group of fibers runs in the lateral lemniscus of the same side and terminates in the nucleus of the lateral lemniscus of the same side
4. The fourth group of fibers runs into reticular formation, crosses the midline as intermediate trapezoid fibers, and finally joins the nucleus of the lateral lemniscus of the opposite side.

Third Order Neurons

• Third-order neurons are in the superior olivary nuclei and nucleus of the lateral lemniscus. The fibers of the third-order neurons end in the medial geniculate body which forms the subcortical auditory center.
• Fibers from the medial geniculate body go to the temporal cortex, via the internal capsule, as auditory radiation. Some fibers from the medial geniculate body go to the inferior colliculus of the tectum in the midbrain. The fibers of auditory radiation are involved in the reflex movement of the head in response to auditory stimuli.

Cortical Auditory Centers

The cortical auditory centers are in the temporal lobe of the cerebral cortex. The auditory areas are Area 41, area 42, and Wernicke’s area. Areas 41 and 42 are the primary auditory areas situated in the anterior transverse gyrus and lateral surface of the superior temporal gyrus. Wernicke’s area is in the upper part of the superior temporal gyrus, posterior to areas 41 and 42.

1. Functions Of Cortical Auditory Centers:

• The cortical auditory centers are concerned with the perception of auditory impulses, analysis of the pitch and intensity of sound, and determination of the source of sound.
• Areas 41 and 42 are concerned with the perception of auditory impulses only. However, the analysis and interpretation of sound are carried out by Wernicke’s area with the help of auditopsychic area.
  • Auditopsychic Area: It is area 22 which occupies the superior temporal gyrus. It is concerned with the analysis and interpretation of auditory sensation along with Wernicke’s area.

Applied Physiology Effect Of Lesion

• Lesion of the cochlear nerve causes deafness of the ear.
• Unilateral lesion of the pathway above the level of cochlear nuclei causes diminished hearing.
• Degeneration of hair cells in the organ of Corti leads to presbycusis. Presbycusis is the gradual loss of hearing that occurs in old age.
• The lesion in the superior olivary nucleus results in poor localization of sound.