54272776 the Sense of Hearing

8/6/2019 54272776 the Sense of Hearing

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The Sense Of Hearing


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Main Components of the

Hearing Mechanism:Divided into 4 parts (by function):

Outer Ear

Middle Ear

Inner Ear

Central Auditory Nervous System


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External Ear

The external ear includes the auricle

(pinna) and external auditory canal.

The auricle is composed of elasticfibrocartilage covered by perichondrium

and skin.

The ear acts to gather acoustic energy

and transform it into neural stimuli which is

transmitted tothe brain for processing.


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Middle Ear

The middle ear is composed ofthe

tympanic membrane, the tympanic cavity,

t

heo

ssicles andt

he eust

achiant

ube. The tympanic membrane forms the lateral

wall ofthe middle ear.

The middle ear acts as an impendance-

matching device.


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Sound waves travel much easierthrough air

(low impedance) than water (high impedance).

If sound waves were directed atthe oval window

(water) almost all ofthe acoustic energy would

be reflected back tothe middle ear (air) and only

1% would enterthe cochlea .

the

middle ear ac

ts

totransf

or

mthe ac

ous

ticenergy tomechanical energy which then

stimulates the cochlear fluid.


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The middle ear also acts to increase theacoustic energy reaching the cochlea byessentially twomechanical phenomenon .

First, the area ofthe tympanic membrane ismuch greaterthan thatofthe stapes footplate(oval window) causing the force applied atthefootplate per square area to be greaterthan thetympanic membrane and second, the ossicles

actas a le

ver increasing

once again

the f

orceapplied atthe stapes footplate

Overall, the increase in sound energy reachingthe cochlea is approximately 22 times.


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The tympanic cavity is a cleftor spacewithin the temporal bone located betweenthe tympanic membrane laterally and the

inner earmedially . The middle ear contains three bones or

ossicles which transmit sound vibrations tothe inner ear.

They are from lateral tomedial, themalleus, the incus and the stapes.


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The malleus is firmly attached tothe tympanicmembrane and the stapes sits within the oval window ofthe cochlea. Between them lies the incus.

The ossicles are held in place,by their joints with each

other, by ligaments and twomuscles; the tensortympanitothe malleus and the stapedius muscle tothe stapes.

The articulation ofthe incus with the stapes causes thestapes to push forward on the oval window and on thecochlear uid on the other side of window every time thetympanic membrane moves inward.


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Internal Ear

The inner ear consists oftwomain parts, thecochlea (end organ for hearing) and thevestibule and semicircular canals (end organ for

balance). The cochlea consists of a fluid filled bony canal

within which lies the cochlear duct containing thesensory epithelium.

Energy enters the cochlea via the stapes boneatthe oval window and is dissipated through asecond opening (which is covered by amembrane) the round window.


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Cochlea

The cochlea is a systemof coiled tubes.

It consists ofthree tubes coiled side by

side:(1)the scala vestibuli, (2) the scalamedia, and (3) the scala tympani.

The scala vestibuli and scala media are

separated from each other by Reissners

membrane (also called the vestibular

membrane)


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the scala tympani and scala media are

separated from each other by the basilar

membrane.

On the surface ofthe basilarmembrane

lies the organ ofCorti, which contains a

series of electromechanically sensitive

cells,the hair cells.


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Transmission of Sound Waves in

the

Cochlea When the footofthe stapes moves inward

againstthe oval window,the round window must

bulge outward because the cochlea is bounded

on all sides by bony walls. The initial effectof a sound wave entering atthe

oval window is to cause the basilarmembrane at

the base ofthe cochlea to bend in the directionofthe round window.


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the elastic tension

that is built up in the

basilar bers as they

bend toward theround window initiates

a uid wave that

travelsalong the

basilarmembranetoward the

helicotrema.


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Function ofthe Organ ofCorti

The organ ofCorti,is the receptororgan

that generates nerve impulses in response

tovibration ofthe basilarmembrane.

The actual sensory receptors in the organ

ofCorti are two specialized types of nerve

cells called hair cellsa single row of

internal (or inner) hair cells,and three orfour rows of external (or outer) hair cells.


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The nerve bers stimulated by the hair

cells lead tothe spiral ganglion ofCorti,

which lies in the centerofthe cochlea.The

spiral ganglion neuronal cells send axon

intothe cochlear nerve and then intothe

central nervous system atthe level ofthe

uppermedulla.


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Excitation ofthe HairCells

Hair cells contain minute stereocilia, projectupward fromthe hair cells and eithertouch orare embedded in the surface gel coating ofthetec

torial

me

mbrane, which lies ab

ove

thestereocilia in the scala media.

Bending ofthe hairs in one direction depolarizesthe hair cells,and bending in the oppositedirection hyperpolarizes them.

This in turn excites the auditory nerve berssynapsing with their bases.


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The outer ends ofthe hair cells are xed

tightly in a rigid structure,called the

reticular lamina, supported by triangular

rods ofCorti,which are attached tightly to

the basilar bers.The basilar bers,the

rods ofCorti,and the reticular lamina move

as a rigid unit.


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The inward and outward motion causes

the hairs on the hair cells to shear back

and forth againstthe tectorial membrane.

Thus,the hair cells are excited whenever

the basilarmembrane vibrates.


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54272776 the Sense of Hearing