BY

MD.ROOHIA

PHYSIOLOGY OF HEARING

It is a form of energy produced by a vibrating object.

A sound wave consists of compression and rarefaction of molecules of the medium in which it travels.

Sound wave shows variations in pressure of the air, and the velocity and displacement of molecules.

When pressure of wave is at a maximum, the forward velocity of air molecules is also at a maximum.

Sound Graphic representations of a sound wave. (A) Air at equilibrium, in the absence of a sound wave; (B) compressions and rarefactions that constitute a sound wave; (C) transverse representation of the wave, showing amplitude (A) and wavelength (λ).

A Sound wave has two properties

Frequency Intensity

Frequency is the number of cycles per second.

The wavelength of sound is the distance between analogous points of two successive waves.

Unit of frequency is hertz (Hz).If the frequency of a wave is f cycles/s (Hz),

then f waves must pass any point in one sec

Frequency and Wave Length

Speech FrequenciesFrequencies of 500, 1000

and 2000 Hz are called speech frequencies as most of human voice fall within this range.

PTA (Pure Tone Average) is the average threshold of hearing in these three speech frequencies.

Normal hearing frequency range is 20 to 20,000Hz

Routine in audiometric testing only 125 to 8000Hz evaluated

It is the strength of the sound which determines its loudness.

It is usually measured by decibels.

Intensity

It is 1/10th of a bel.It is named after Alexander Graham BellSound can be measured as power

(watts/cm2) or as pressure (dynes/cm2) or in physical units (N/m2 or pascals).

Decibel notation was introduced in audiology to avoid dealing with large figures of sound pressure level.

In audiology sound is measured as sound pressure level (SPL).

Decibel (dB)

Sound Pressure LevelThe SPL of a sound in

decibels is 20 times the logarithm to the base 10, of the pressure of a sound to the reference pressure.

The reference pressure is taken as 0.0002 dynes/cm2or 20µPa for a frequency of 1000 Hz and represents the threshold of hearing in normally hearing young adults.

Pure ToneA single frequency

sound is called a pure tone.

Complex SoundSound with more

than one frequency is called a complex sound.

It is a subjective sensation produced by frequency of sound.

Higher the frequency, greater is the pitch.

Pitch

A complex sound has a fundamental frequency i.e., the lowest frequency at which a source vibrates.

All the frequencies above that tone are called overtones.

Overtones determine the quality or timbre of sound.

Overtones

It is the subjective sensation produced by intensity.

More the intensity of the sound, greater the loudness.

Loudness

Noise It is defined as an aperiodic

complex sound. There are 3 types of noise:

White noise – contains all frequencies in audible spectrum. It is a broad band noise and used for masking.

Narrow band noise – white noise with certain frequencies, above and below the given noise filtered out. Frequency range is smaller than the broad band white noise. It is used to mask test frequency in pure tone audiometry.

Speech noise – noise having frequencies in the speech range (300-3000 Hz). All other frequencies are filtered out.

Audiometric zero is the mean value of minimum audible intensity in a group of normally hearing healthy young adults.

Audiometric Zero

It is the sound pressure level produced by an audiometer at a specific frequency.

It is measured in decibels with reference to audiometric zero.

Hearing Level

It is the level of sound above the threshold of hearing for an individual.

Sensation level refers to the sound which will produce the same sensation, as in a normally hearing person.

Sensation Level

Intensity (Loudness) level of sound that is most comfortable for the person.

Most Comfortable Level

Level of sound which produces discomfort in the ear.

It is usually 90 – 105 dB SL.It is important to find the loudness discomfort

level of a person when prescribing a hearing aid.

Loudness Discomfort Level

PROPAGATION OF SOUNDATTENUATION BY

DISTANCE.Propagation of sound

is like a ripples on pond.

Dicreases in amplitude as they move away from the source.

For sound if distance doubles amp drops by half.

Transmission between different mediaAir is light and compressible, only small sound

pressures will be needed to give a certain velocity of vibration, and hence displacement of air molecules.

In a medium with higher impedance the pressure will be inadequate to give similar velocities of vibration.

So when sound in air meets a medium of higher impedance it can not produce same amount of vibration in that medium, so the result is much of the sound is reflected with only small proportion being transmitted.

Fourier AnalysisThe analysis of the

complex sound into its constituent sinusoids is known as FOURIER ANALYSIS.

Any realistic waveforms can be made out of sums of sinusoids.

Sinusoidal sound behave in a simple way in many complex environments.

Cochlea itself performs Fourier Analysis.

FOURIER TRANSFORMSound just a

combination of sine functions.

We can assign each sine-function,&therefore the original sound, to a distinct energy or power spectrum which gives us the energy/amplitude/freq.

This process called FT.

EXTERNAL EAR Features Of External CanalOpen on one end only.The impedance of ear drum

is around 3 to 4 times more than air.

30% of incident sound energy is reflected from external canal.

It is efficient in conducting sound in frequency range of 3 to 5 kHz.

It cuts off unwanted frequencies helping in better speech discrimination.

It acts as a resonator.It increases the pressure at the ear drum in a

frequency sensitive way.Helps in localization of sound.Its length is 28mm.

Role of EAC

If a tube of one quarter wave length long and one end is open and the other end is blocked with hard termination, the pressure will be low at the open end and high at the closed end when the tube is placed in a sound field.

This phenomenon is seen in human external meatus at frequency of 3 kHz, resonance adds 10 to 12 dB at the tympanic membrane.

Resonator

Sound LocalisationBoth sound pressure

levels and phase of acoustic waves are important factors in sound localization.

Maximum time difference (phase difference) between two ears is 750 milliseconds.

Role Of Middle EarIt couples sound energy

to cochlea.It serves as an acoustic

transformer to match the impedance of air to cochlear fluids.

It couples sound preferentially to only one window, thus producing a differential pressure between the windows required for movement of cochlear fluids.

Middle EarTransformer Mechanism BY 3 WAYS

1) CATENARY LEVER(ear drum)

Buckling mechanism of TM

Force is transmitted from centre of TM.

TM memb doesn’t move as a plate.

This causes high pressure with low displacement.

2.OSSICULAR LEVER(lever ratio):Length of the handle

of malleus 1.3 times longer than long process of incus.

3.HYDRAULIC LEVER(areal ratio):Average area of TM

is larger(60mmsq0 than foot plate area(3.2mmsq)(OW).

Effective vibratory area of TM 65% that is 45mmsq.

Impedance EfficiencyOnly 65% of sound

energy from TM gets absorbed and transmitted to the cochlea.

Without middle ear only 1% of the sound energy will be absorbed by the cochlea.

Role Of Middle Ear MusclesTensor tympani attaches to

the handle of malleus. It pulls the drum medially.

Stapedius muscle attaches to the posterior aspect of stapes.

Contraction of these muscles increases the stiffness of ossicular chain thus blunting low frequencies.

These muscles decreases a person’s sensitivity to their own speech.

Protective Function Of Middle Ear MusclesStapedius contraction can

reduce transmission up to 30dB for frequencies less than 1 to 2 kHz. For higher frequencies it is limited to 10dB.

Only stapedius muscle contracts in response to loud noise in humans.

The whole stapedial reflex arc has 3 to 4 synapses.

Stapedial reflex latency is 6 to 7 ms.

Damaged middle ear can cause loss of transformer mechanism.

Differential pressure levels between the two windows could not be maintained.

Scala vestibuli is more yielding than scala tympani. Differential movements of fluid with in the cochlea is still possible.

Small compliance of annular ligament in comparison to much larger compliant round window could again cause differential pressure.

Transmission Through Damaged Middle Ear

Bone ConductionNormal route for

hearing one’s own voice.

Useful in cases of severe conductive losses.

Can be used as a diagnostic tool.

Bone Conduction Inner Ear Factors Intrinsic detection of

distortional vibrations of cochlear bone.

Differential distortion of bony structures of cochlea (scala vestibuli is larger than scala tympani) could cause movement of cochlear fluids.

Direct vibration of osseous spiral lamina.

Direct transmission of vibrations from the skull via CSF to the cochlear fluids.

Leaving one window open improved sound conduction.

Bone Conduction Middle Ear FactorsVibrations of the skull gets

faithfully transmitted to the ossicles of middle ear cavity.

Inertia of the middle ear ossicles doesn’t coincide with their points of attachments.

Middle ear acts as a band pass filter with peak transmission around 1kHz.

This accounts of carhart’s notch though at a slightly higher frequency.

Bone Conduction External Ear FactorsBone vibrations are

conducted through the external canal and the air within it.

Vibrations can escape externally if the canal is open.

Occlusion of external ear increases bone conduction.

External radiation of sound is best for low frequencies, hence change with occlusion is greatest for these frequencies.

Cochlea Scala vestibuli and scala

tympani contains perilymph. Scala media contains

endolymph. Perilymph space opens into CSF

via cochlear aqueduct. Endolymphatic space joins the

endolymphatic sac by endolymphatic duct.

Scala vestibuli is separated from scala media by reissner’s membrane. It is very thin and does not obstruct the passage of sound from s. vestibuli to s. media. They may even be considered to be a single chamber.

EndolymphFormed by stria vascularis.Endolymphatic sac

maintains homeostasis of endolymph.

It has high potassium and low sodium content.

Endolymph has positive potential gradient +50 to 120mV (endocochlear potential).

Na K ATPase is responsible for this gradient.

Stria VascularisSecretes Endolymph.Superficial dark

staining marginal cells.

Lightly staining basal cells.

Marginal cells are secretory in nature.

Perilymph Site of production is

controversial - ? CSF Occupies perilymphatic space.

Continuous between vestibular and cochlear divisions.

Ionic concentration resembles extracellular fluid.

Perilymph from s. vestibuli originates from plasma, while perilymph from s. tympani originates from both plasma and CSF.

Electrical potential from s. tympani is +7mV and from s. vestibuli is +5mV.

Basillar Membrane It separates s. media from s.

tympani. Length’s of basillar membrane

increases from oval window to the apex (0.04mm near oval window and 0.5mm at helicotrema) 12 folds increase.

Diameters of basilar fibers decrease from oval window to helicotrema.

The stiff short fibers near the oval window vibrate best at very high frequency, while long limber fibers near the tip of cochlea vibrate best at a low frequency.

It is known as tonotopic presentaion.

Cochlear MechanicsBy the movement of

ossicles sound wave reaches through oval window to cochlea.

Here the fluid in sv &st set in motion as well as BM.

BM moved by travelling wave.

Location of max amplitude of this wave depends on freq of incomming sound signal,here freq analysis take place.

BM movts leads to stimulation of nerve cells In OC, & send electrical impulses to brain& sound percieved.

BM movt is amplified by OHC called active amplification.

Low input signals evoke larger BM displacements than high sound levels.

Transduction By Hair CellsWhen the steriocilia

are deflected in the direction of the tallest steriocilia, the links are stretched opening up calcium channels.

Inner Hair Cells Makes large number of synaptic

contact with afferent fibers of auditory nerve.

95% of afferent auditory nerves make contact with inner hair cells.

Detects basillar membrane movements.

Tips of inner hair cells are not embedded in the tectorial membrane as outer hair cells.

They fit loosely into a groove called “Henson’s stripe”.

The cilia are driven by vicious drag of endolymph.

Inner hair cells respond to the velocity rather than displacement.

Outer Hair CellsVery few outer hair cells

synapse with auditory nerves.

Inside of outer hair cells have -70mV.

They serve to amplify basillar membrane vibration.

They increase the sensitivity and selectivity of cochlea.

Cochlear microphonics are derived from these cells.

Cochlear Electrical PotentialsCochlear

microphonics – A/CSummating potential

– D/CNegative neutral

potentials – N1 & N2

Inner hair cells excite auditory nerves.Single auditory stimulus is always excitatory.Sound stimulus, transmitter release and

action potential generation occur in synchrony (Phase L

ocking). Commonly seen in low frequency.Timing AP in the nerve is able to signal

details of the temporal properties of the sound wave form is called TEMPORAL CODING.

Coding based on frequency selectivity is called PLACE CODING.

Phase coding at Auditory Nerve Fibers

AUDITORY

PATHWAY

Central Auditory MechanismsSignals from both ears are

transmitted to both sides of the brain.

Preponderance of transmission in contralateral pathway.

Three cross over points are: In the trapezoid body. In the commisure between

the two nuclei of lateral lemnisci.

In the commisure connecting the two inferior colliculi.

Sound localization and lateralizationAuditory discriminationTemporal aspects of audition including

Temporal resolutionTemporal maskingTemporal integrationTemporal ordering

Auditory performance with competing acoustic signals

Auditory performance with degraded signals.

Central Functions

1st GROUP THEORIES Telephonic Theory Of Rutherford(1880)Volley Theory of Waver & Bray(1949)2nd GROUP THEORIESResonance Theory of Helmholtz(1883)Place TheoryTravelling Wave Theory VonBekesy(1960)

Theories Of Hearing

Rutherford proposed that the entire cochlea responds as a whole to all frequencies instead of being activated on a plate.

Here the sounds of all frequencies are transmitted as in a telephone cable and frequency analysis is performed at a higher level (brain).

Damage to certain portions of the cochlea can cause preferential loss of hearing certain frequencies i.e., like damage to the basal turn of cochlea causing inability to hear high frequency sounds.

This can not be explained by telephonic theory.

Telephonic Theory

Proposed by Wever &Bray(1949)Volleys means groupsImpulses of frequency above 1000cyc/sec

were transmitted by diff group of nerve fibres

Volly Theory

RESONANCE THEORY OF HELMHOLTZBasilar memb acts as

series of tuned resonators as in piano string

Each pitch vibrate BM particular to its own place.

High freq at basal region, loe at apical region.

Individual resonators not found in cochlea so its modified to place theory.

According to Helmholtz basillar membrane has different segments that resonated to different frequencies.

Particular nerve fibre gives information frm org of corti to regarding region to brain.

Eg: boiler maker’s disease

Place Theory

Travelling Wave TheoryProposed by Bekesy.This theory proposes

frequency coding to take place at the level of cochlea.

High frequencies are represented towards the base while lower frequencies are closer to apex.

By MD.ROOHIA

MODERATOR

DR.JANARDHAN

Physiology of HearingTHANK YOU