ROLE OF INNER EAR
TRAVELING WAVE
Movement of footplate of
stapes against oval window causes movement of perilymph
in scala vestibuli. This fluid does not move all the way from
oval window to round window through helicotrema. It immediately hits the vestibular membrane near oval window. This causes movement of fluid in scala media, since the vestibular membrane is flexible. Movement of fluid in scala media causes bulging
of basal portion of
basilar membrane towards scala tympani. Bulging
of basilar membrane increases the elastic tension in
basilar fibers in that portion of basilar membrane.
Elastic tension in basilar fibers initiates a wave, which travels along basilar membrane towards the
helicotrema like that of arterial pulse wave. It is called traveling
wave.
Resonance Point
Resonance
point is the part of basilar membrane, which is activated by traveling wave. In
the beginning,
each
traveling wave is weak. While traveling through basilar membrane from base
towards
apex
(helicotrema), the wave becomes stronger and stronger and at one point
(resonance point) of basilar membrane, it becomes very strong and activates
the basilar membrane. This resonance
point of basilar membrane immediately vibrates back and forth. The traveling
wave stops here and does not travel further. Distance
between stapes and resonance point is inversely
proportional to frequency of sound waves reaching the ear. Traveling wave
generated by highpitched sound disappears near the base of
the cochlea. Wave generated by medium-pitched
sound reaches half of the way and the wave generated by low-pitched sound
travels the entire distance of basilar membrane.
EXCITATION OF HAIR
CELLS
Stereocilia of hair cells in
organ of Corti are embedded in tectorial membrane. Hair
cells are tightly fixed by cuticular lamina reticularis and the pillar
cells or rods of Corti.
When
traveling wave causes vibration of basilar membrane at the resonance point, the
basilar fiber,
rods of
Corti, hair cells and lamina reticularis move as a single unit. It
causes movements of stereocilia
leading to excitement of
hair cells and generation of receptor potential.
ELECTRICAL EVENTS DURING PROCESS OF
HEARING
SOUND TRANSDUCTION
Sound transduction is a
type of sensory transduction in the hair cell (receptor
cells) in organ
of Corti by which the
sound energy is converted into action potentials in the auditory nerve fiber.
Electrical Events of
Sound Transduction
Three
types of electrical events that occur during sound transduction are:
1.
Receptor potential or the cochlear microphonic potential
2.
Endocochlear potential or endolymphatic potential
3. Action
potential in auditory nerve fiber.
Role of Efferent
Nerve Fibers of Hair Cells
Efferent
nerve fibers of hair cells also play important role during sound
transduction by
releasing acetylcholine. Efferent nerve fiber to inner hair cell terminates on the auditory (afferent) nerve fiber where it leaves the
inner hair cell. It controls the generation of action potential in auditory nerve fiber by inhibiting the release of glutamate
from inner hair cells. Efferent nerve fiber to outer hair
cell terminates
directly on the cell body.
It inhibits the electromotility of this cell.
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