PHYSIOLOGY OF HEARING

PHYSIOLOGY OF HEARING MODERATOR -Dr.Narayana Swami PRESENTER - Dr.Razal 1

SOUNDA form of energy Propagates in the form of wavesThe speed of sound depend on the medium through which the wave passAir - 343m/sWater - 1482m/sAudible frequencies for humans 20 to 20,000 cycles per second (cps, Hz). 2

Technical TermsAmplitude/loudness-Strength of the soundAppreciation of sound intensityExpressed in decibel (dB)Frequency/Pitch/ToneNumber of cycles per secondPitch /Tone denotes appreciation of frequencyExpressed in Hertz(Hz)ImpedenceResistance offered by a medium to sound waves

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

ATTENUATIONAttenuation is a general term that refers to any reduction in the strength of a signal

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Values Of Hearing 14-20 dB -Whisper40-60 dB -Conversational Speech60-90dB -Noisy room130 dB -Painful noise140-180 dB -Jet aircraft engine Noise, Gun Fire

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Mechanism of HearingSound collected by Pinna-passes through EAC-Strikes TMVibrations of TM Transmitted to Stapes foot plate through Ossicles Movements of footplate causes pressure change in Labyrinthine fluids-Moves the Basilar membrane This Stimulates the hair cell of organ of corti-acts as a transducer and convert the mechanical energy to electrical impulse-travel along the Auditory nerve to the Auditory cortex.

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EXTERNAL EAR7

Sound Collection

Pinna- concha system catches sound over large area and concentrate it to smaller area of EACThis increases the total energy available to the tympanic membrane

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Sound LocalizationBecause of its shape, the pinna shield the sound from rear end, change timbre, and helps to localize sound from in front or back

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EACit increases the pressure at the ear drum level in a frequency sensitive way by acting as a resonator. 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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It also plays a very important role in sound localization in difficult situations, especially front to back and high to low distinctions,

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MIDDLE EAR

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Functions Of Middle Ear

IMPEDENCE MATCHINGATTENUATIONPHASE DIFFERENTIAL EFFECT

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Impedance Matching

1.Area of tympanic membrane relative to oval window

2.The lever action of middle ear ossicles.

3.The shape of tympanic membrane13

Impedence Mismatch

If there was no middle ear system ,99% of sound waves would have reflected back from oval windowMiddle ear by its impedence matching property allows 60% of sound energy to dissipate in inner ear14

Hydraulic Action of Tympanic MembraneTotal effective area of tympanic membrane 45mm2Area of stapes footplate is 3.2mm2Effective arial ratio is 14:1Thus 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 15

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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Action of TMEustachian tube maintains 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 gainTotal transformer ratio=14x1.3x4=73:118

Attenuation ReflexWhen 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 sound19

Function of Attenuation ReflexTo 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 noiseTo decrease a persons hearing sensitivity to his or her own speech20

Phase Differential EffectSound 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 windowThis contributes 4dB when tympanic membrane is intact 21

INNER EAR

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COCHLEA-FunctionsA 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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Endocochlear PotentialAn 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 MicrophonicWhen 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 potential25

Summating PotentialProduced by hair cellsDC potential superimposed on VIII nerve action potential26

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Play from 2.38 min27

Theories of HearingTravelling Theory(Von Bekesy)This theory proposes frequency coding to take place at the level of cochleaHigh frequencies are represented towards the base while lower frequencies are closer to apex28

Place TheoryEach pitch has its own separate place on the basilar membrane.Telephone TheoryPitch depends upon the rate firing of individual nerve fiberVolley Theory Combination of bothHigher frequency -Place TheoryLower Frequency Telephone theory

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Hearing By Bone ConductionTransmitted by vibrations of Skull bones and then to the basilar membrane.

Translatory MechanismLow frequency sound produces Ossicular and Stapes movement with respect to oval windowCompressional MechanismHigh Frequency sound produce intermittent compression and expansion of skull bone and bony labyrinth 30

Auditory Neural PathwaysHair cells are innervated by dendrites of bipolar cells of spiral ganglion which is situated in Rosenthal's canal (canal running along the osseous spiral lamina) Axons of these bipolar cells form the cochlear division of CN VIII and end in the cochlear nuclei, the dorsal and ventral, on each side of the medulla. Further course of auditory pathways is complex. From cochlear nuclei

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(a) Superior olivary complex (b) Nucleus of lateral lemniscus (c) Inferior colliculus(el) Medial geniculate body Fig. 2.1 Structure of organ of corti. (e) Auditory cortex.

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The auditory fibres travel via the ipsilateral and contralateral routes and have multiple decussation points each ear is represented in both cerebral hemispheres. The area of cortex, concerned with hearing is situated in the superior temporal gyrus (Brodmann's area 41)

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Auditory Neural Pathways

Function of auditory cortexPerception of sound

Judging the intensity of the sound

Analysis of different property of sound33

THANK YOU34