Role of Inner Ear

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.

Role of Inner Ear


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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