CORTICAL AUDITORY CENTERS
Cortical auditory centers
are in the temporal lobe of cerebral cortex.
Auditory areas are:
1. Primary auditory area,
which includes area 41, area 42 and Wernicke area
2. Secondary auditory area
or auditopsychic area, which includes area 22.
Areas 41 and 42 are the primary auditory areas situated in the anterior transverse gyrus and lateral surface of superior temporal gyrus. Wernicke area is in upper part of superior temporal gyrus, posterior to areas 41 and 42. Area 22 occupies the superior temporal gyrus.
FUNCTIONS OF CORTICAL AUDITORY CENTERS
Cortical
auditory centers are concerned with the perception of auditory impulses,
analysis of pitch and intensity of sound
and determination of source of sound. Areas 41
and 42 are concerned with the perception of
auditory impulses only. However, analysis and interpretation
of sound are carried out by Wernicke area,
with the help of area 22.
APPLIED PHYSIOLOGY – EFFECT OF LESION
1. Lesion of cochlear
nerve causes deafness of the ear
2. Unilateral lesion of
auditory pathway, above the level of cochlear nuclei causes diminished
hearing
3. Degeneration of hair cells
in the organ of Corti leads to presbycusis. Presbycusis
is the gradual loss of
hearing. It is common in
old age.
4. Lesion in superior
olivary nucleus results in poor localization of sound.
Sound waves travel through
external auditory meatus and produce vibrations in the tympanic membrane.
Vibrations from tympanic
membrane travel through malleus and incus and reach the stapes resulting in the
movement of stapes. Movements of stapes produce vibrations
in the fluids of cochlea. These vibrations stimulate
the hair cells in organ of Corti. This, in turn,
causes generation of action potential (auditory impulses) in the auditory nerve fibers. When auditory impulses reach the cerebral cortex, the perception of hearing
occurs. Thus, during the process of hearing, ear converts energy
of sound waves into action potentials in auditory nerve
fibers. This process is called sound transduction.
ROLE OF EXTERNAL EAR
External ear directs the
sound waves towards tympanic membrane. Sound waves produce pressure
changes over the surface
of tympanic membrane. Accumulation of wax prevents conduction of sound.
In many
animals, the auricle (pinna) can be turned to locate the source of
sound. Auricle can be folded to
avoid
unwanted sound. But in man, the extrinsic and intrinsic muscles of
auricle are rudimentary and the
movement is not possible. auditory nerve fibers. This process is called sound transduction.
ROLE OF EXTERNAL EAR
External ear directs the sound
waves towards tympanic membrane. Sound waves produce pressure
changes over the surface
of tympanic membrane. Accumulation of wax prevents conduction of sound.
In many
animals, the auricle (pinna) can be turned to locate the source of
sound. Auricle can be folded to
avoid
unwanted sound. But in man, the extrinsic and intrinsic muscles of
auricle are rudimentary and the
movement is not possible.
ROLE OF MIDDLE EAR
ROLE OF TYMPANIC
MEMBRANE
Due to the pressure
changes produced by sound waves, tympanic membrane
vibrates, i.e. it moves in
and out of
middle ear. Thus, tympanic membrane acts as a resonator that
reproduces the vibration of sound.
ROLE OF AUDITORY
OSSICLES
Vibrations set up in
tympanic membrane are transmitted through the malleus and incus and reach
the
stapes, causing to and fro movement of stapes against oval window and
against perilymph present in
scala vestibuli of
cochlea.
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