hearing
TRANSCRIPT
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)