Physiology of Hearin - Copy

7/31/2019 Physiology of Hearin - Copy

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Sound results when the particles of amedium are set into vibration. Thedisplacement is around a mean position andthere is no net flow of air in the direction of

motion. A sound wave has 2 basic properties:-

- Intensity - It is the power transmittedby the wave through a unit area. It is a

subjective correlate of loudness.- Frequency It is the number of cycles

per second of back and forth motion of airparticles and is subjective correlate of pitch.


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Pure tone :single frequency sound

Complex sound: sound with more than one

frequency

Overtones: complex sound has a fundamentalfrequency ie. the lowest frequency at which

a source vibrates.All frequencies above that

tone are called overtones.

Impedance: it is a measure of opposition of asystem to movement and is a function of the

medium in which sound is traveling and its

surroundings.


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Fouriers analysis: Analysis of a complex sound intoits constituent sinusoids. It is one of the essentialtools for those attempting to understand theworkings of auditory system.


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Periodic sound: These are complex soundswith all the frequencies being multiples ofthe lowest or fundamental frequency

Such components at exact multiples of a

frequency are called harmonics.

Non-periodic sound


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Root Mean Square value is used to measure

the intensity of sound in air. It is useful

because the same relation between intensity,

pressure and velocity holds over all shapes ofwaveform.

It is common to use a logarithmic scale to

grade sound pressures because:-

- Ears sensitivity to pressures varies bymore than a million times.

- Human ear can discriminate fractional

changes in pressure.


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Formula for calculating decibel

Number of dB = 10 Log actual sound intensity

10 reference intensity

The decibel (1/10th of a bel) is a logarithmicmeasure of relative energy where 10 db (1

bel) represents an increase over a given

reference energy level of first order of

magnitude. Reference sound intensity 0.0002dynes/cm2

Corresponds to threshold of hearing in

normal subjects at 1000hz


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Human ear can hear frequencies from 20 Hz

to 20,000 Hz.

Subjects threshold is by definition 0 db

sensation level.

14 - 20 db- just audible whisper

40 - 60 db- normal conversation

60 - 90 db- noisy room.

90 - 120db- loud music.

130db Pain threshold.


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Sound conducting mechanism: Transmissionof sound waves through External auditorycanal, tympanic membrane and theossicular chain.

-- acoustically from sound in the middle earthat reaches the window of the cochleadirectly

--bone conduction

Perceptive neural mechanism: Transductionin the cochlea , auditory division of the 8th

nerve and its central connections.


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1.Pinna

2.Ear canal also known as external auditory meatus

3.Eardrum also known as tympanic membrane.)


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The external ear has 2 main influences on theincoming sound

It increases pressure at tympanic membranein a frequency sensitive way thus

emphasizing certain frequencies in the input. It increases pressure in a way that depends

on the direction of the sound sourcetherefore it aids in sound localisation.

This acoustic function of theexternal ear is also known as external ear gainand is dependent on frequency and directionof sound.


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Total effect of reflections from head,pinna and

various external ear resonances is to add 15 to 20

db to the sound pressure over the frequency rangefrom 2 to 7 kHz.

As a sound source is moved around the head, starting

in front and moving round to the side, the main

change produced is an attenuation of upto 10 db inthe frequency range of 2 to 7 kHz, therefore changes

in this frequency range could indicate whether the

source was in front of the subject or behind.

In the horizontal plane, determining the direction ofsound depends on the difference in time between

the arrival of stimulus in the two ears .


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Impedance of the tympanic membrane in

man seems to be 3-4 times that of the air in

the EAC over a wide frequency range of 1

kHz. This leads to some 50% of energy being

reflected back into the meatus.

Resonance in EAC at a frequency of about 3

kHz adds 10-12 dB at the TM

Resonance at concha at 5 kHz adds 10 dB


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Couples sound energy to cochlea

Acoustic transformer- Reduces the

impedance mismatch --auditory meatus air

cochlear fluids

--(low impedance) (high impedance)

Provides physical protection to cochlea.

Couples sound preferentially to only onewindow of cochlea required for movement

of cochlear fluids.


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Bekesy suggested thatTM moved like a stiffplate up to 2k Hz andfound that inferior

edge was flaccid andhence the movementswere greatest at thatpart.

Khanna & Tonndorf

suggested twomaxima of vibration,one on either side ofthe manubrium .


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Axis of rotation of the ossicles and the axis of

suspension by their ligaments nearly coincide

with their center of rotational inertia. Hence

the bones are able to vibrate with very little

loss through the suspending ligaments. At

low frequencies where mass effects are

small, ligaments play important role in

maintaining position of ossicles. Also the

coincidence of the center of inertia of the

ossicles with their center of rotation will

help reduce the perception of bone

conducted sound.


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When a sound wave meets a medium of

higher impedance from a lower impedance

medium, much of the sound energy is reflected.

The middle ear by acting as an acoustic

impedance transformer reduces this attenuation

substantially. An effective impedance transformer

will change the low pressure high displacement

vibrations of air into high pressure low

displacement vibrations suitable for drivingcochlear fluids.

According to Aibara the impedance of

cochlear fluid is 5.8*10 4 N.sec/m3.


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Area of tympanic membrane is larger than that of stapesfootplate. The forces collected over the TM are

concentrated on a smaller hence increased pressure at

oval window.

T f A i f Middl E


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Transformer Action of Middle Ear

Lever Action

Fulcrum Effect pressure gain: 2.1 times


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Impedence transformer ratio calculated as

follows.

Areal ratio= vibratory area of TM (60sqmm)

stapes footplate area(3.2sqmm)

= 18.75

Lever (ossicles) ratio- The arm of incus isshorter than that of malleus and this

produces a lever action that increases the

force and decreases the velocity at stapes.

Malleus is 2.1 times longer than incus, solever action multiplies force 2.1 times but

velocity decreases 2.1 times. Thus

impedance ratio is increased by 4.4 times.


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Final transformer ratio is calculated as ratio

of specific impedances, obtained by

multiplying these 2 factors together,

as 18.75*4.4 = 82.5

The result of the transformer action of

middle ear(combined with effect of external

ear) is that upto 50% of the incident energyis transmitted to cochlea as against 3%(a 15

db loss) expected in absence of middle ear

transformer.


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Entire TM does not move as a rigid body.

at low freq-entire tm moves with the samephase with varying magnitude

at freq above 1000Hz-more complicatedpattern of vibration

Energy used up in middle ear to stretch tmand ossicular ligaments

Middle ear air spaces load the motion of TM

Slippage in the ossicular system at freqabove 1000 to 2000hz


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Effective stimulus to the inner ear is adifference in the sound pressure between theround and oval windows

Middle ear maximizes pressure diff between

the two windows by:a) A tympano-ossicular system whichpreferentially increases pressure at ovalwindow.

b)Round window shielding effect by a intact

tympanic membrane which reduces soundpressure in middle ear by 10-20 db comparedto EAC.

c) Presence of air in middle ear around theround window allows its movement.


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Influence of middle ear muscles

Tensor tympani-pulls malleusmedially

(T.T. attached to

manubrium ofmalleus)

Stapedius- pullsstapes posteriorly

(stapedius insertsto post aspect ofstapes)


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Contraction of both muscles- increases

stiffness of ossicular chain- influences

transmission

It can change the direction of vibration of

ossicles n thus lead to less effective coupling

wit the cochlea.

Middle ear muscle reflex has various

functions:

1. Protection from loud noise( esp long

lasting)

2. Selective attenuation of low frequencystimulus components improves

intelligibility of speech.

3. Reduces influences of some of the

unwanted resonances in middle ear


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INNER HAIR CELLS( single row)

make synaptic contact with approx 95% of afferentfibres of auditory nerve

IHC detect basilar membrane movement , transfer

it to auditory nerve

Response can be divided into a osscilating ACresponse f/b DC depolarisation

OUTER HAIR CELLS( three rows)

More sensitive, more susceptible to damage.

amplifies basilar membrane vibration, generates

cochlear microphonics

produce only AC response


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Inner ear: hair cells

Outer hair cells 30,000 in number

3-5 rows

Cylindrical in shape

50-150 stereo cilia arranged in2-3 rows that assume V or Wshape

Tallest tips are embedded inoverlying membrane

Those in apex are longer 8mmthan in base 2mm

Innervated by type 2 auditorynerve fibers

Efferent nerve fibersterminate directly onto cells

Inner hair cells 10,000 in number

Single row

Flask shaped cells

Stereo cilia in 3-4 ascendingrows assuming a flattened Ushape or st line

No such difference in height

Same dimension throughoutentire length of cochlea

Innervated by type 1 auditorynerve fibers

Efferent nerve fibersterminate on dendrites ofnerve fibers


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The mechanical travelling wave incochlea forms the basis of frequencyselectivity of whole organ and in addition isthe basis of our extreme sensitivity to sound.

Cochlea travelling wave was originallydescribed by Bekesy.

According to him response is large for only avery narrow range of sound frequencies and if

the sound freq is changed the response dropssharply. As a wave moves up the cochleatowards its peak it encounters a region inwhich the membrane is mechanically active.


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Sterocilia - present on apical surface of hair cell. Theyare mechanically rigid, cross linked to each other.

Stimulated by shear or relative motion betweentectorial membrane and reticular lamina

Moves away from and towards modiolus If they are deflected in the direction of tallest sterocilia

there is associated opening of ionic channel

K+ ions from endolymph move inside the cell

Energy for this whole process comes from striavascularis, which by ion pumping , stores energy inbattery ofendolymph

This is the battery or resistance modulation theory ofDavis


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Travelling wave

Von Bekesy noted that the motion of the basilar

membrane was in the form of a travelling wave, like the

one that occurs when you flick a rope.

The wave oscillates at the frequency of stimulation, but it

is not a sinusoidal wave.


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Travelling wave characteristics

Always starts at the base ofthe cochlea andmoves toward the apex

Its amplitude changes as ittraverses the length of thecochlea

The position along thebasilar membrane at whichits amplitude is highestdepends onthe frequency of the

stimulus

All of these characteristicsdepend on the change instiffness along the length ofthe basilar membrane.


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PHASE LOCKING

FREQUENCY SELECTIVITY (PLACE CODING)

TEMPORAL CODING OF STIMULI

PHASE LOCKING

neurotransmitter- released in synapses at the base of IHC as a result of

depolarisation of IHC-gives rise to AP in auditory nerve fibres

synchrony between sound stimulus,transmitter release and AP generation in

individual cycles of stimulus is known as phase locking.

FREQUENCY SELECTIVITY (PLACE CODING)

each nerve fibre has a frequency of stimulus for which it is most sensitive

it is possible to determine the frequency of stimulus depending upon which fibres

were activated

TEMPORAL CODING OF STIMULI

for stimulus above 5kHz the timing of AP in nerve is able to signal details oftem oral ro erties of sound wave form


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Helmholtz theory: (Place theory) The

selectivity of cochlea and pitchdiscrimination is based on the place of

displacement of BM.


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Volley theory

As Wever pointed out, if different nerve fibers

respond on different cycles and the brain has a way

of adding up the responses of all the neurons,

then it would have a perfect representation of the

frequency of the tone. This is called the volley

theory. It is a combination of the above two

theories.


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Physiology of Hearin - Copy