Transcript Physiology of Hearing

Physiology of Hearing
Sound
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Sound is a form of energy
It is transmitted through a medium as a
longitudinal pressure wave
The wave consists of a series of compressions
and rarefactions of the molecules in the
medium
The ear is capable of capturing this energy and
perceiving it as sound information
Rarefaction
Compression
Compression
Compression
Sound waves
Rarefaction
Rarefaction
the graph showing a sine wave refers only to variations
in pressure or compression, not to the actual
displacement of air
Properties of sound
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The wave motion of sound can be described in
terms of Amplitude, Frequency, Velocity and
Wavelength
Properties of sound
Wavelength
Refers 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
Velocity
Refers 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)
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The intensity of sound is perceived as loudness
It is measured on a relational scale with the unit of
measurement being the decibel (after Alexander Graham Bell)
Sound intensities require a standard sound level against which
they are compared
The standard sound pressure level (SPL = 0dB) is 0.0002
dynes/cm2
The decibel is a numeric value that represents sound intensity
with respect to the reference sound pressure level
Loudness (or amplitude)
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sound pressure levels of
common sounds
Sound intensity is
measured on a
logarithmic scale
An increase of 6 dB of
sound pressure is
perceived as double the
intensity of the sound
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)
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Frequency is perceived as the pitch of a sound
The higher the frequency, the higher the pitch
and vice versa
The range of human hearing is said to be from
20 - 20,000 Hz
The speech frequencies; those frequencies
most important for human hearing are from
approximately 250 - 4000 Hz
Transmission of sounds through
the ear
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External ear
– Mostly through air (External acoustic meatus)
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Middle ear
– Through solid medium - bone (ossicles)
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Inner ear
– Through fluid medium – endolymph (cochlea)
Parts of the ear
Air and bone conduction
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There are two methods by which hair cells can
be stimulated
– Air conduction
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Sound stimulus travelling through the external and middle
ear and activating the hair cells
– Bone conduction
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Sound stimulus travelling though the bones of the skull
activating the hair cells
Whatever method it takes, the sound stimulus
finally activate hair cells in the cochlea
External ear
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Consists of
– Pinna
– External auditory meatus
Middle ear
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composed of
– the tympanic membrane
– the tympanic cavity
– the ossicles
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Malleus
Incus
Stapes (connected to the oval window of the cochlea)
– two muscles
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the tensor tympani attached to the malleus
the stapedius muscle attached to the stapes
– the Eustachian tube
Inner ear
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Consists of two main parts
– the cochlea (end organ for hearing)
– the vestibule and semicircular canals (end organ for balance)
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The inner ear can be thought of as a series of tunnels or canals within the
temporal bone
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Within these canals are a series of membranous sacs (termed labyrinths)
which house the sensory epithelium
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The membranous labyrinth is filled with a fluid termed endolymph
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It is surrounded within the bony labyrinth by a second fluid termed
perilymph
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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 turns
Cross section through cochlea
Cochlea
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The bony canal of the cochlea is
divided into an upper chamber, the
scala vestibuli and a lower chamber,
the scala tympani by the membranous
labyrinth also known as the cochlear
duct
The floor of the scala media is formed
by the basilar membrane, the roof by
Reissner's membrane
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The scala vestibuli and scala tympani
contain perilymph
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The scala media contains endolymph
Endolymph and perilymph
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Endolymph is similar in ionic content to
intracellular fluid (high K, low Na)
Perilymph resembles extracellular fluid (low K,
high Na)
The cochlear duct contains several types of
specialized cells responsible for auditory
perception
Cohlea
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The sensory cells responsible for hearing are located on the basilar
membrane within a structure known as the organ of Corti
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This is partitioned by two rows of peculiar shaped cells known as pillar cells
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The pillar cells enclose the tunnel of Corti
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Situated on the basilar membrane is a single row of inner hair cells medially
and three rows of outer hair cells laterally
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The hair cells and other supporting cells are connected to one another at
their apices by tight junctions forming a surface known as reticular lamina
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The cells have specialized stereocilia on their apical surfaces
Organ of Corti
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Attached to the medial aspect of the scala
media is a fibrous structure called the tectorial
membrane
It lies above the inner and outer hair cells
coming in contact with their stereocilia
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The fluid in the space between the tectorial
membrane and reticular lamina is endolymph
Thus the endolymp bathes the stercocillia
But the body of the hair cells which lies below
the reticular lamina is bathed by perilymph
Hair cells
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Synapsing with the base of the hair cells are
dendrites from the auditory nerve
The auditory nerve leaves the cochlear and
temporal bone via the internal auditory canal
and travels to the brainstem
Transmission of sound waves
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The outer ear and external auditory canal act
passively to capture the acoustic energy and direct it
to the tympanic membrane
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There, the sound waves strike the tympanic
membrane causing it to vibrate
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These mechanical vibrations are then transmitted via
the ossicles to the perilymph of the inner ear
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The perilymph is stimulated by the mechanical
(vibrations) energy vibrations to form a fluid wave
within the cochlea
Middle ear
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The middle ear acts as an impendance-matching device
Sound waves travel much easier through air (low impedance)
than water (high impedance)
If sound waves were directed at the oval window (water)
almost all of the acoustic energy would be reflected back to the
middle ear (air) and only 1% would enter the cochlea. This
would be a very inefficient method.
To increase the efficiency of the system, the middle ear acts to
transform the acoustic energy to mechanical energy which then
stimulates the cochlear fluid
Middle ear
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The middle ear also acts to increase the acoustic
energy reaching the cochlea by essentially two
mechanical phenomenon
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The area of the tympanic membrane is much greater
than that of the stapes footplate (oval window)
causing the force applied at the footplate per square
area to be greater than the tympanic membrane
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The ossicles act as a lever increasing once again the
force applied at the stapes footplate
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Overall, the increase in sound energy reaching the
cochlea is approximately 22 times
Cochlea
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The cochlea consists of a fluid filled bony canal within which
lies the cochlear duct containing the sensory epithelium
Energy enters the cochlea via the stapes bone at the oval
window and is dissipated through a second opening (which is
covered by a membrane) the round window
Vibrations of the stapes footplate cause the perilymph to form a
wave
This wave travels the length of the cochlea
It takes approximately 5 msec to travel the length of the
cochlea
Cochlea
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As it passes the basilar membrane of the cochlear
duct, the fluid wave causes the basilar membrane to
move in a wave-like fashion (i.e. up and down)
The wave form travels the length of the cochlea and is
dissipated at the round window
Due to changes in the mechanical properties of the
basilar membrane, the amplitude of vibration changes
as one travels along the basilar membrane
The place principle
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Low frequency stimuli cause the greatest
vibration of basilar membrane at its apex, high
frequency stimuli at its base
Neurolab
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As the basilar membrane is displaced superiorly by the perilymph wave, the
stereocilia at the apex of each inner and outer hair cell, which are imbedded
in the tectorial membrane undergo a shearing force (i.e. they are bent)
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This shearing force causes a change in the resting membrane potential of
the hair cell which is transmitted to its basal end
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There a synapse is formed with a dendrite from the auditory nerve
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The hair cell membrane potential change is transmitted across this synapse
(? via acetylcholine) causing depolarization of the nerve fiber
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This neural impulse is then propagated to the auditory centres of the brain
From the ear to the auditory
cortex
Processing of auditory signal
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Auditory nerve
– The place principle
– Intensity of the stimulus is coded as an increase in
the frequency of action potentials
– There is also recruitment of additional nerve fibres
as the intensity increases
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Cochlear nuclei
– There is tonotopic organisation (neurons are
arranged according to the sensitivity to each
frequency)
– Further processing happens
Processing of auditory signal
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Superior olivary complex
– Impulses from both ears are compared
– This is necessary for the localisation of sound
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Lateral leminscus, inferior colliculus, medial geniculate
body
– Further processing happens
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Temporal lobe
– Unique feature of cortical neuronal response to auditory
stimulus is the brief duration of the response
– Localisation of sound and sound discrimination based on the
sequence of sounds in the stimulus occurs in the cortex
Perception of different
characteristics of sound
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Frequency
– Starts at the basilar membrane and frequency sharpening occurs
throughout the auditory pathway
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Intensity
– Starts at the hair cells (OHC are stimulated by weaker stimulus)
– Frequency of impulses
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Direction
– Inter-aural time difference
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Pattern recognition
– Cortical function
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Interpretation of speech
– Complex cortical phenomenon
Hearing loss
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Hearing can be defined as the ability to receive and
process acoustic stimuli (i.e. sound)
Hearing is an important function for communication
and provides people with pleasurable experiences
such as listening to music
The loss of ability to hear has important consequences
in ones day to day life and ability to function within
the hearing culture (vs the deaf culture)
Hearing loss can be broadly defined as the decreased
ability to receive or process acoustic stimuli
Hearing loss
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It has several causes: conduction, sensorineural,
mixed, central or functional
Hearing loss is very common in our society
Its incidence is approximately 0.2% in those under 5
years of age, 5% in those 35-54 years of age, 15% of
those 55-64 years of age and 40% (or more) in those
over 75 years of age (in the west)
As one ages, the likelihood of hearing loss increases
Conduction deafness (or conductive
deafness)
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A conductive hearing loss exists when sound waves
for any reason are not able to stimulate the sensory
cells of the inner ear (i.e. cause a fluid wave within
the cochlea)
Examples of conditions causing a conductive hearing
loss include
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impacted wax
external auditory canal atrecia
perforation of the tympanic membrane
ossicular discontinuity
Otosclerosis
Middle ear disease
Conduction deafness (or conductive
deafness)
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In a conductive hearing loss, the sound waves
cannot be transformed into a fluid wave within
the cochlea, thus the sensory cells receive
decreased or no stimulation
The maximum conductive hearing loss is
approximately 60 dB
Many conductive hearing loss can cured
Sensorineural deafness or nerve
deafness
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Sensorineural hearing loss occurs when the sensory
cells of the cochlea (inner ear) or the auditory nerve
fibers are dysfunctional
The acoustic energy (sound wave) is not capable of
being transformed inside the cochlea to nervous
stimuli
Reasons for this include
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noise damage to the cochlea
aging (presbycusis)
ototoxic medications
tumours such as an acoustic neuroma
Sensorineural deafness or nerve
deafness
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Hearing loss can be in excess of 100 dB
Sensorineural hearing loss is, in general, cannot
be cured
Cochlear implants are available as a method of
treatment
Cochlear implants
Mixed Hearing Loss
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Mixed hearing losses are simply the
combination of a conductive and sensorineural
hearing loss
For example, an elderly person with
presbycusis plus impacted wax (cerumen)
or a heavy metal musician with noise induced
hearing loss who develops a perforated
tympanic membrane
Central Hearing Loss
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Central hearing loss occurs in the auditory
areas of the brainstem and higher levels
(temporal lobe)
Very little information is known about lesions
that cause this type of impairment
Persons with central hearing loss have normal
hearing, but have difficulty with the processing
of auditory information (word deafness)
Functional Hearing Loss
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Persons with functional hearing loss have no
physiologic basis for a hearing deficit
They are using their 'hearing loss' for
secondary gain and are called malingerers
This is occasionally seen in adolescents or
persons appying for pension benefits as a result
of hearing loss
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All of the different types of hearing loss
– can be present at birth, i.e. congenital
– or acquired later on in life
Diagnosis of Hearing Loss
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The diagnosis of hearing loss can be relatively
simple ("I can't hear from my right ear") to the
more subtle (“Sunil seems to have difficulty
saying some words")
Auriscopic examination and identify the any
structural defect in the ear canal
Tests of hearing need to be done
Tests of hearing
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Tuning fork tests
– Rinne’s test
– Weber’s test
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Pure tone audiometry (PTA)