Physiology of hearing

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PHYSIOLOGYOF HEARING

Dr Seema S

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Outline

Introduction to sound Ear

Function as a transducer Parts and how it functions

External ear Middle ear Internal ear

Electrical potential Auditory path way

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SOUND

A form of energy propagates in the form of waves The speed of sound depend on the medium

through which the wave pass air - 343m/s water - 1482m/sec

Audible frequencies t for humans 20 to 20,000 cycles per second (cps, Hz).

It can detect the difference between two sounds occurring 10micro seconds apart in time

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EAR AS A TRANSDUCER

SOUNDENERGY

MECHANICAL ENERGY

ELECTRICAL ENERGY

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Technical jargons

•Strength of the sound•Loudness denotes the appreciation of sound intensity•Expressed in decibel (dB)

Amplitude/loudness

•Number of cycles per second•Pitch /Tone denotes the appreciation of frequency•Expressed in Hertz(Hz)

Frequency/Pitch/Tone

•Resistance offered by a medium to sound waves

Impedence

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RESONANCE•Resonance is the tendency of a system to oscillate with larger amplitude at some frequencies than at others

ATTENUATION•Attenuation is a general term that refers to any reduction in the strength of a signal

Values of hearing:

15-25dB —Whisper 35dB —Background noise 40-60dB —Background noise

( home ) 65—70dB –- normal speaking voice 130dB —painful noise 140-180dB —jet air craft engine noise

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Natural resonant frequencyEXTERNAL AUDITORY CANAL---------------

3000Hz

TYMPANIC MEMBRANE----------------------- 800-1600Hz

MIDDLE EAR--------------------------------------- 800Hz

OSSICULAR CHAIN--------------------------- 500-2000Hz

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External ear

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Functions of EXTERNAL EAR

Sound collection

Increasing pressure on the tympanic membrane in a frequency sensitive way

Sound localization

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Sound collection

Pinna- concha system catches sound over large area and concentrate it to smaller area of ext. auditory meatus.

This increases the total energy available to the tympanic membrane

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Pressure increase by EAC

If a tube which is closed at one end and open at other is placed in a sound field then pressure is low at open end and high at closed end.

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Sound localization

Because of its shape, the pinna shield the sound from rear end,change timbre,and helps to localize sound from infront or back

Cues for sound localization from right/left Sound wave reaches the ear closer to sound source before

it arise in farthest ear Sound is less intense as it reaches the farthest ear

because head act as barrier

Auditory cortex integrates these cues to determine location.

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Middle ear

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FUNCTIONS OF MIDDLE EAR

IMPEDENCE MATCHING

ATTENUATION

PHASE DIFFERENCIAL EFFECT

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Impedence mismatch

IF THERE WAS NO MIDDLE EAR SYSTEM ,99% OF SOUND WAVES WOULD HAVE REFLECTED BACK FROM OVAL WINDOW

MIDDLE EAR BY ITS IMPEDENCE MATCHING PROPERTY ALLOWS 60% OF SOUND ENERGY TO DISSIPATE IN INNER EAR

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“Impedance Matching” by the middle ear System

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(a) HYDRAULIC ACTION OFTYMPANIC MEMBRANE

Total effective area of tympanic membrane 45mm2

Area of stapes footplate is 3.2mm2

Effective areal ratio is 14:1

Thus by focusing sound pressure from large area of tympanic membrane to small area of oval window the effectiveness of energy transfer between air to fluid of cochlea is increased

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(b) Lever action of ossicles Handle of malleus is 1.3

times longer than long process of incus

Overall this produces a lever action that converts low pressure with along lever action at malleus handle to high pressure with a short lever action at tip of long process of incus

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(c) Action of tympanic membrane

Eustachian tube equilibrates the air pressure in middle ear with that of atmospheric pressure, thus permitting tympanic membrane to stay in its most neutral position.

A buckling motion of tympanic membrane result in an increased force and decreased velocity to produce a fourfold increase in effectiveness of energy transfer

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Total gain

Total transformer ratio=14x1.3x4=73:1

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Attenuation reflex

When loud sounds are transmitted through the ossicular system and from there into the central nervous system, a reflex occurs after a latent period of only 40 to 80 ms to cause contraction of the stapedius muscle and the tensor tympani muscle

The tensor tympani muscle pulls the handle of the malleus inward while the stapedius muscle pulls the stapes outward. These two forces oppose each other and thereby cause the entire ossicular system to develop increased rigidity, thus greatly reducing the ossicular conduction of low frequency sound

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Function of attenuation reflex To protect the cochlea from

damaging vibrations caused by excessively loud sound.

To mask low-frequency sounds in loud environments. This usually removes a major share of the background noise

To decrease a person’s hearing sensitivity to his or her own speech

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PHASE DIFFERENTIAL EFFECT

Sound waves striking the tympanic membrane do not reach the oval and round window simultaneously.

There is preferential pathway to oval window due to ossicular chain.

This acoustic separation of windows is achieved by intact tympanic membrane and a cushion of air around round window

This contributes 4dB when tympanic membrane is intact

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INNER EAR

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COCHLEA ---TWO FUNCTIONS…. A TRANSDUCER that translates

sound energy into a form suitable for stimulating the dendrites of auditory nerve.

AN ENCODER that programs the features of an acoustic stimulus so that the brain can process the information contained instimulating sound.

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Electrical potential of cochlea and CN VIII

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Endocochlear potential

An electrical potential of about +80 millivolts exists all the time between endolymph and perilymph, with positivity inside the scala media and negativity outside.

This is called the endocochlear potential, and it is generated by continual secretion of positive potassium ions into the scala media by the stria vascularis

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Cochlear microphonic

When basilar membrane move in response to sound stimulus electrical resistance at the tip of hair cells change allowing flow of K+ through hair cells and produce voltage fluctuations called cochlear micro phonic.

This is AC potential

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Summating potential

Produced by hair cells DC potential superimposed on VIII

nerve action potential

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Compound action potential

All or none response of auditory nerve fibres

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Central auditory pathway

• nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nuclei

• second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus

• the superior olivary nucleus,the auditory pathway passes upward through the lateral lemniscus.

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Some of the fibers terminate in thenucleus of the lateral lemniscus, but many bypass this nucleus and travel on to the inferior colliculus, where all or almost all the auditory fibers synapse From there, the pathway passes to the medial geniculate nucleus, where all the fibers do synapse

Finally, the pathway proceeds by way of the auditory radiation to the auditory cortex, located mainly in the superior gyrus of the temporal lobe.

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Pecularities of auditory pathway

First,signals from both ears are transmitted through the pathways of both sides of the brain, with a preponderance of transmission in the contralateral pathway

Second, many collateral fibers from the auditory tracts pass directly into the reticular activating system of the brain stem

Third, a high degree of spatial orientation is maintained in the fiber tracts from the cochlea all the way to the cortex

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Function of auditory cortex

Perception of sound

Judging the intensity of the sound

Analysis of different property of sound

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