Physiology of HearingPhysiology of Hearing

Prof. Vajira Weerasinghe Prof. Vajira Weerasinghe

Dept of PhysiologyDept of Physiology

SoundSound Sound is a form of energySound is a form of energy

It is transmitted through a medium as a It is transmitted through a medium as a longitudinal pressure wavelongitudinal pressure wave

The wave consists of a series of The wave consists of a series of compressions and rarefactions of the compressions and rarefactions of the molecules in the medium molecules in the medium

The ear is capable of capturing this energy The ear is capable of capturing this energy and perceiving it as sound informationand perceiving it as sound information

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RarefactionRarefaction Rarefaction

the graph showing a sine wave refers only to variations in pressure or compression, not to the actual displacement of air

Sound wavesSound waves

Properties of soundProperties of sound

The wave motion of sound can be The wave motion of sound can be described in terms of described in terms of AmplitudeAmplitude, , FrequencyFrequency, , Velocity Velocity and and WavelengthWavelength  

Properties of soundProperties of sound

WavelengthRefers to the physical distance between successive compressions and is thus dependant on the speed of sound in the medium divided by its frequency

Amplitude (Intensity or loudness) Refers to the difference between maximum and minimum pressure

Frequency (pitch)Refers to the number of peak-to-peak fluctuations in pressure that pass a particular point in space in one second

VelocityRefers to the speed of travel of the sound wave. This varies between mediums and is also dependant on temperature (in air at 20°C it is 343 m/s)

Loudness (or amplitude)Loudness (or amplitude) The intensity of sound is perceived as loudnessThe intensity of sound is perceived as loudness

It is measured on a relational scale with the unit of It is measured on a relational scale with the unit of measurement being the decibel (after Alexander measurement being the decibel (after Alexander Graham Bell)Graham Bell)

Sound intensities require a standard sound level Sound intensities require a standard sound level against which they are comparedagainst which they are compared

The standard sound pressure level (SPL = 0dB) is The standard sound pressure level (SPL = 0dB) is 0.0002 dynes/cm20.0002 dynes/cm2

The decibel is a numeric value that represents sound The decibel is a numeric value that represents sound intensity with respect to the reference sound pressure intensity with respect to the reference sound pressure levellevel

Loudness (or amplitude)Loudness (or amplitude) sound pressure levels sound pressure levels

of common soundsof common sounds

Sound intensity is Sound intensity is measured on a measured on a logarithmic scalelogarithmic scale

An increase of 6 dB of An increase of 6 dB of sound pressure is sound pressure is perceived as double perceived as double the intensity of the the intensity of the soundsound

SOUND dB SPL

Rocket Launching pad 180

Jet plane 140

Gunshot blast 130

Car horn 120

Pneumatic drill 110

Power tools 100

Subway 90

Noisy restaurant 80

Busy traffic 75

Conversational speech 66

Average home 55

Library 40

Soft whisper 30

Frequency (or pitch)Frequency (or pitch) Frequency is perceived as the pitch of a Frequency is perceived as the pitch of a

soundsound The higher the frequency, the higher the The higher the frequency, the higher the

pitch and vice versapitch and vice versa

The range of human hearing is said to be The range of human hearing is said to be from 20 - 20,000 Hzfrom 20 - 20,000 Hz

The speech frequencies; those frequencies The speech frequencies; those frequencies most important for human hearing are from most important for human hearing are from approximately 250 - 4000 Hzapproximately 250 - 4000 Hz

Listen to sound clipsListen to sound clips

Sound spectrum analyserSound spectrum analyser

Neurolab Neurolab

Songs with different frequencies Songs with different frequencies

Transmission of sounds Transmission of sounds through the earthrough the ear External earExternal ear

– Mostly through air (External acoustic meatus)Mostly through air (External acoustic meatus)

Middle earMiddle ear– Through solid medium - bone (ossicles)Through solid medium - bone (ossicles)

Inner earInner ear– Through fluid medium – endolymph (cochlea)Through fluid medium – endolymph (cochlea)

Parts of the earParts of the ear

Air and bone conduction Air and bone conduction

There are two methods by which hair There are two methods by which hair cells can be stimulatedcells can be stimulated– Air conduction Air conduction

Sound stimulus travelling through the external and Sound stimulus travelling through the external and middle ear and activating the hair cellsmiddle ear and activating the hair cells

– Bone conduction Bone conduction Sound stimulus travelling though the bones of the Sound stimulus travelling though the bones of the

skull activating the hair cellsskull activating the hair cells

Whatever method it takes, the sound Whatever method it takes, the sound stimulus finally activate hair cells in the stimulus finally activate hair cells in the cochleacochlea

External earExternal ear

Consists of Consists of – PinnaPinna– External auditory meatus External auditory meatus

Middle earMiddle ear composed of composed of

– the tympanic membranethe tympanic membrane

– the tympanic cavitythe tympanic cavity

– the ossicles the ossicles MalleusMalleus IncusIncus Stapes (connected to the oval window of the cochlea)Stapes (connected to the oval window of the cochlea)

– two musclestwo muscles the tensor tympani attached to the malleus the tensor tympani attached to the malleus the stapedius muscle attached to the stapes the stapedius muscle attached to the stapes

– the Eustachian tubethe Eustachian tube

Inner earInner ear Consists of two main partsConsists of two main parts

– the cochlea (end organ for hearing) the cochlea (end organ for hearing) – the vestibule and semicircular canals (end organ for balance)the vestibule and semicircular canals (end organ for balance)

The inner ear can be thought of as a series of tunnels or canals The inner ear can be thought of as a series of tunnels or canals within the temporal bonewithin the temporal bone

Within these canals are a series of membranous sacs (termed Within these canals are a series of membranous sacs (termed labyrinths) which house the sensory epitheliumlabyrinths) which house the sensory epithelium

The membranous labyrinth is filled with a fluid termed endolymphThe membranous labyrinth is filled with a fluid termed endolymph

It is surrounded within the bony labyrinth by a second fluid termed It is surrounded within the bony labyrinth by a second fluid termed perilymph perilymph

The cochlea can be thought of as a canal that spirals around itself The cochlea can be thought of as a canal that spirals around itself similar to a snail. It makes roughly 2 1/2 to 2 3/4 turnssimilar to a snail. It makes roughly 2 1/2 to 2 3/4 turns

Cross section through Cross section through cochleacochlea

CochleaCochlea The bony canal of the cochlea is The bony canal of the cochlea is

divided into an upper chamber, divided into an upper chamber, the scala vestibuli and a lower the scala vestibuli and a lower chamber, the scala tympani by chamber, the scala tympani by the membranous labyrinth also the membranous labyrinth also known as the cochlear ductknown as the cochlear duct

The floor of the scala media is The floor of the scala media is formed by the basilar membrane, formed by the basilar membrane, the roof by Reissner's membranethe roof by Reissner's membrane

The scala vestibuli and scala The scala vestibuli and scala tympani contain perilymphtympani contain perilymph

The scala media contains The scala media contains endolymphendolymph

Endolymph and perilymphEndolymph and perilymph

Endolymph is similar in ionic content to Endolymph is similar in ionic content to intracellular fluid (high K, low Na) intracellular fluid (high K, low Na)

Perilymph resembles extracellular fluid Perilymph resembles extracellular fluid (low K, high Na)(low K, high Na)

The cochlear duct contains several types The cochlear duct contains several types of specialized cells responsible for of specialized cells responsible for auditory perceptionauditory perception

CohleaCohlea

The sensory cells responsible for hearing are located on the basilar The sensory cells responsible for hearing are located on the basilar membrane within a structure known as the organ of Cortimembrane within a structure known as the organ of Corti

This is partitioned by two rows of peculiar shaped cells known as pillar This is partitioned by two rows of peculiar shaped cells known as pillar cellscells

The pillar cells enclose the tunnel of CortiThe pillar cells enclose the tunnel of Corti

Situated on the basilar membrane is a single row of inner hair cells Situated on the basilar membrane is a single row of inner hair cells medially and three rows of outer hair cells laterallymedially and three rows of outer hair cells laterally

The hair cells and other supporting cells are connected to one another The hair cells and other supporting cells are connected to one another at their apices by tight junctions forming a surface known as reticular at their apices by tight junctions forming a surface known as reticular laminalamina

The cells have specialized stereocilia on their apical surfacesThe cells have specialized stereocilia on their apical surfaces

Organ of CortiOrgan of Corti

Attached to the medial aspect of the Attached to the medial aspect of the scala media is a fibrous structure called scala media is a fibrous structure called the tectorial membranethe tectorial membrane

It lies above the inner and outer hair cells It lies above the inner and outer hair cells coming in contact with their stereociliacoming in contact with their stereocilia

The fluid in the space between the tectorial The fluid in the space between the tectorial membrane and reticular lamina is endolymph membrane and reticular lamina is endolymph

Thus the endolymp bathes the stercocillia Thus the endolymp bathes the stercocillia

But the body of the hair cells which lies below But the body of the hair cells which lies below the reticular lamina is bathed by perilymph the reticular lamina is bathed by perilymph

Hair cellsHair cells

Synapsing with the base of the hair cells Synapsing with the base of the hair cells are dendrites from the auditory nerveare dendrites from the auditory nerve

The auditory nerve leaves the cochlear The auditory nerve leaves the cochlear and temporal bone via the internal and temporal bone via the internal auditory canal and travels to the auditory canal and travels to the brainstembrainstem

Transmission of sound Transmission of sound waveswaves The outer ear and external auditory canal act The outer ear and external auditory canal act

passively to capture the acoustic energy and passively to capture the acoustic energy and direct it to the tympanic membranedirect it to the tympanic membrane

There, the sound waves strike the tympanic There, the sound waves strike the tympanic membrane causing it to vibratemembrane causing it to vibrate

These mechanical vibrations are then These mechanical vibrations are then transmitted via the ossicles to the perilymph of transmitted via the ossicles to the perilymph of the inner earthe inner ear

The perilymph is stimulated by the mechanical The perilymph is stimulated by the mechanical (vibrations) energy vibrations to form a fluid (vibrations) energy vibrations to form a fluid wave within the cochleawave within the cochlea

Middle earMiddle ear The middle ear acts as an impendance-matching deviceThe middle ear acts as an impendance-matching device

Sound waves travel much easier through air (low Sound waves travel much easier through air (low impedance) than water (high impedance)impedance) than water (high impedance)

If sound waves were directed at the oval window If sound waves were directed at the oval window (water) almost all of the acoustic energy would be (water) almost all of the acoustic energy would be reflected back to the middle ear (air) and only 1% reflected back to the middle ear (air) and only 1% would enter the cochlea. This would be a very would enter the cochlea. This would be a very inefficient method. inefficient method.

To increase the efficiency of the system, the middle ear To increase the efficiency of the system, the middle ear acts to transform the acoustic energy to mechanical acts to transform the acoustic energy to mechanical energy which then stimulates the cochlear fluidenergy which then stimulates the cochlear fluid

Middle ear Middle ear The middle ear also acts to increase the The middle ear also acts to increase the

acoustic energy reaching the cochlea by acoustic energy reaching the cochlea by essentially two mechanical phenomenonessentially two mechanical phenomenon

1.1. The area of the tympanic membrane is much The area of the tympanic membrane is much greater than that of the stapes footplate (oval greater than that of the stapes footplate (oval window) causing the force applied at the window) causing the force applied at the footplate per square area to be greater than footplate per square area to be greater than the tympanic membrane the tympanic membrane

2.2. The ossicles act as a lever increasing once The ossicles act as a lever increasing once again the force applied at the stapes footplateagain the force applied at the stapes footplate

Overall, the increase in sound energy reaching Overall, the increase in sound energy reaching the cochlea is approximately 22 timesthe cochlea is approximately 22 times

CochleaCochlea The cochlea consists of a fluid filled bony canal within The cochlea consists of a fluid filled bony canal within

which lies the cochlear duct containing the sensory which lies the cochlear duct containing the sensory epitheliumepithelium

Energy enters the cochlea via the stapes bone at the Energy enters the cochlea via the stapes bone at the oval window and is dissipated through a second opening oval window and is dissipated through a second opening (which is covered by a membrane) the round window(which is covered by a membrane) the round window

Vibrations of the stapes footplate cause the perilymph to Vibrations of the stapes footplate cause the perilymph to form a waveform a wave

This wave travels the length of the cochleaThis wave travels the length of the cochlea

It takes approximately 5 msec to travel the length of the It takes approximately 5 msec to travel the length of the cochleacochlea

CochleaCochlea As it passes the basilar membrane of the As it passes the basilar membrane of the

cochlear duct, the fluid wave causes the basilar cochlear duct, the fluid wave causes the basilar membrane to move in a wave-like fashion (i.e. membrane to move in a wave-like fashion (i.e. up and down)up and down)

The wave form travels the length of the cochlea The wave form travels the length of the cochlea and is dissipated at the round windowand is dissipated at the round window

Due to changes in the mechanical properties of Due to changes in the mechanical properties of the basilar membrane, the amplitude of the basilar membrane, the amplitude of vibration changes as one travels along the vibration changes as one travels along the basilar membranebasilar membrane

The place principleThe place principle

Low frequency stimuli cause the greatest Low frequency stimuli cause the greatest vibration of basilar membrane at its apex, vibration of basilar membrane at its apex, high frequency stimuli at its basehigh frequency stimuli at its base

Neurolab

As the basilar membrane is displaced superiorly by the perilymph As the basilar membrane is displaced superiorly by the perilymph wave, the stereocilia at the apex of each inner and outer hair cell, wave, the stereocilia at the apex of each inner and outer hair cell, which are imbedded in the tectorial membrane undergo a shearing which are imbedded in the tectorial membrane undergo a shearing force (i.e. they are bent)force (i.e. they are bent)

This shearing force causes a change in the resting membrane This shearing force causes a change in the resting membrane potential of the hair cell which is transmitted to its basal endpotential of the hair cell which is transmitted to its basal end

There a synapse is formed with a dendrite from the auditory nerveThere a synapse is formed with a dendrite from the auditory nerve

The hair cell membrane potential change is transmitted across this The hair cell membrane potential change is transmitted across this synapse (? via acetylcholine) causing depolarization of the nerve synapse (? via acetylcholine) causing depolarization of the nerve fiberfiber

This neural impulse is then propagated to the auditory centres of This neural impulse is then propagated to the auditory centres of the brainthe brain

From the ear to the auditory From the ear to the auditory cortexcortex

Processing of auditory Processing of auditory signalsignal Auditory nerveAuditory nerve

– The place principleThe place principle– Intensity of the stimulus is coded as an Intensity of the stimulus is coded as an

increase in the frequency of action potentialsincrease in the frequency of action potentials– There is also recruitment of additional nerve There is also recruitment of additional nerve

fibres as the intensity increasesfibres as the intensity increases Cochlear nucleiCochlear nuclei

– There is tonotopic organisation (neurons are There is tonotopic organisation (neurons are arranged according to the sensitivity to each arranged according to the sensitivity to each frequency)frequency)

– Further processing happensFurther processing happens

Processing of auditory Processing of auditory signalsignal Superior olivary complexSuperior olivary complex

– Impulses from both ears are comparedImpulses from both ears are compared– This is necessary for the localisation of sound This is necessary for the localisation of sound

Lateral leminscus, inferior colliculus, medial Lateral leminscus, inferior colliculus, medial geniculate bodygeniculate body– Further processing happensFurther processing happens

Temporal lobeTemporal lobe– Unique feature of cortical neuronal response to Unique feature of cortical neuronal response to

auditory stimulus is the brief duration of the responseauditory stimulus is the brief duration of the response– Localisation of sound and sound discrimination based Localisation of sound and sound discrimination based

on the sequence of sounds in the stimulus occurs in on the sequence of sounds in the stimulus occurs in the cortex the cortex

Perception of different Perception of different characteristics of soundcharacteristics of sound FrequencyFrequency

– Starts at the basilar membrane and frequency sharpening Starts at the basilar membrane and frequency sharpening occurs throughout the auditory pathway occurs throughout the auditory pathway

IntensityIntensity– Starts at the hair cells (OHC are stimulated by weaker stimulus)Starts at the hair cells (OHC are stimulated by weaker stimulus)– Frequency of impulsesFrequency of impulses

DirectionDirection– Inter-aural time difference Inter-aural time difference

Pattern recognition Pattern recognition – Cortical function Cortical function

Interpretation of speechInterpretation of speech– Complex cortical phenomenonComplex cortical phenomenon

Hearing lossHearing loss Hearing can be defined as the ability to receive Hearing can be defined as the ability to receive

and process acoustic stimuli (i.e. sound)and process acoustic stimuli (i.e. sound)

Hearing is an important function for Hearing is an important function for communication and provides people with communication and provides people with pleasurable experiences such as listening to pleasurable experiences such as listening to musicmusic

The loss of ability to hear has important The loss of ability to hear has important consequences in ones day to day life and ability to consequences in ones day to day life and ability to function within the hearing culture (vs the deaf function within the hearing culture (vs the deaf culture)culture)

Hearing loss can be broadly defined as the Hearing loss can be broadly defined as the decreased ability to receive or process acoustic decreased ability to receive or process acoustic stimulistimuli

Hearing lossHearing loss It has several causes: conduction, sensorineural, It has several causes: conduction, sensorineural,

mixed, central or functionalmixed, central or functional

Hearing loss is very common in our societyHearing loss is very common in our society

Its incidence is approximately 0.2% in those Its incidence is approximately 0.2% in those under 5 years of age, 5% in those 35-54 years of under 5 years of age, 5% in those 35-54 years of age, 15% of those 55-64 years of age and 40% (or age, 15% of those 55-64 years of age and 40% (or more) in those over 75 years of age (in the west)more) in those over 75 years of age (in the west)

As one ages, the likelihood of hearing loss As one ages, the likelihood of hearing loss increasesincreases

Conduction deafness (or conductive Conduction deafness (or conductive deafness)deafness) A conductive hearing loss exists when sound A conductive hearing loss exists when sound

waves for any reason are not able to stimulate waves for any reason are not able to stimulate the sensory cells of the inner ear (i.e. cause a the sensory cells of the inner ear (i.e. cause a fluid wave within the cochlea)fluid wave within the cochlea)

Examples of conditions causing a conductive Examples of conditions causing a conductive hearing loss include hearing loss include – impacted waximpacted wax– external auditory canal atreciaexternal auditory canal atrecia– perforation of the tympanic membraneperforation of the tympanic membrane– ossicular discontinuityossicular discontinuity– OtosclerosisOtosclerosis– Middle ear diseaseMiddle ear disease

Conduction deafness (or conductive Conduction deafness (or conductive deafness)deafness)

In a conductive hearing loss, the sound In a conductive hearing loss, the sound waves cannot be transformed into a fluid waves cannot be transformed into a fluid wave within the cochlea, thus the sensory wave within the cochlea, thus the sensory cells receive decreased or no stimulationcells receive decreased or no stimulation

The maximum conductive hearing loss is The maximum conductive hearing loss is approximately 60 dBapproximately 60 dB

Many conductive hearing loss can curedMany conductive hearing loss can cured

Sensorineural deafness or Sensorineural deafness or nerve deafnessnerve deafness Sensorineural hearing loss occurs when the Sensorineural hearing loss occurs when the

sensory cells of the cochlea (inner ear) or the sensory cells of the cochlea (inner ear) or the auditory nerve fibers are dysfunctionalauditory nerve fibers are dysfunctional

The acoustic energy (sound wave) is not The acoustic energy (sound wave) is not capable of being transformed inside the cochlea capable of being transformed inside the cochlea to nervous stimulito nervous stimuli

Reasons for this include Reasons for this include – noise damage to the cochleanoise damage to the cochlea– aging (presbycusis)aging (presbycusis)– ototoxic medicationsototoxic medications– tumours such as an acoustic neuromatumours such as an acoustic neuroma

Sensorineural deafness or Sensorineural deafness or nerve deafnessnerve deafness

Hearing loss can be in excess of 100 dBHearing loss can be in excess of 100 dB

Sensorineural hearing loss is, in general, Sensorineural hearing loss is, in general, cannot be curedcannot be cured

Cochlear implants are available as a Cochlear implants are available as a method of treatment method of treatment

Cochlear implantsCochlear implants

Mixed Hearing LossMixed Hearing Loss

Mixed hearing losses are simply the Mixed hearing losses are simply the combination of a conductive and combination of a conductive and sensorineural hearing losssensorineural hearing loss

For example, an elderly person with For example, an elderly person with presbycusis plus impacted wax (cerumen) presbycusis plus impacted wax (cerumen)

or a heavy metal musician with noise or a heavy metal musician with noise induced hearing loss who develops a induced hearing loss who develops a perforated tympanic membraneperforated tympanic membrane

Central Hearing LossCentral Hearing Loss Central hearing loss occurs in the auditory Central hearing loss occurs in the auditory

areas of the brainstem and higher levels areas of the brainstem and higher levels (temporal lobe)(temporal lobe)

Very little information is known about Very little information is known about lesions that cause this type of impairmentlesions that cause this type of impairment

Persons with central hearing loss have Persons with central hearing loss have normal hearing, but have difficulty with the normal hearing, but have difficulty with the processing of auditory information (word processing of auditory information (word deafness)deafness)

Functional Hearing LossFunctional Hearing Loss

Persons with functional hearing loss have Persons with functional hearing loss have no physiologic basis for a hearing deficitno physiologic basis for a hearing deficit

They are using their 'hearing loss' for They are using their 'hearing loss' for secondary gain and are called secondary gain and are called malingerersmalingerers

This is occasionally seen in adolescents or This is occasionally seen in adolescents or persons appying for pension benefits as a persons appying for pension benefits as a result of hearing lossresult of hearing loss

All of the different types of hearing loss All of the different types of hearing loss

– can be present at birth, i.e. congenital can be present at birth, i.e. congenital

– or acquired later on in lifeor acquired later on in life

Diagnosis of Hearing LossDiagnosis of Hearing Loss

The diagnosis of hearing loss can be The diagnosis of hearing loss can be relatively simple ("I can't hear from my relatively simple ("I can't hear from my right ear") to the more subtle (“Sunil seems right ear") to the more subtle (“Sunil seems to have difficulty saying some words")to have difficulty saying some words")

Auriscopic examination and identify the Auriscopic examination and identify the any structural defect in the ear canal any structural defect in the ear canal

Tests of hearing need to be done Tests of hearing need to be done

Tests of hearing Tests of hearing

Tuning fork tests Tuning fork tests – Rinne’s testRinne’s test– Weber’s testWeber’s test

Pure tone audiometry (PTA)Pure tone audiometry (PTA)