Physiology of hearing ppt

PHYSIOLOGY OF HEARING
BY: Dr.Humra shamim

IS HEARING IMPORTANT?
 Communication: hearing is essential to
language
 Localisation: determination of location of
unseen sound sources

WHAT IS REQUIRED FOR NORMAL HEARING?
 Adequate stimulus (sound)
 Conduction of stimulus to sensory organ of
hearing
 Sensory transduction of stimulus at organ of
hearing
 Neural transmission of the signal
 Central auditory processing of the signal at
brain

SOUND
 Sound is a form of energy that propagates in the form of
waves
 The speed of sound depends on the medium through which
the wave passes.
 Speed of sound in air is 343m/s in water is 1482m/sec
 The sound frequencies audible to humans range from about
20 to 20,000 cycles per second (cps, Hz).
 sound intensity is expressed by taking the logarithmic ratio
of two sound intensities (the numerator being the sound
intensity of interest, and the denominator being a reference
sound intensity) and multiplying by 10.
 dB = 10 log10 J/Jr, where J is the intensity of the sound of
interest, and Jr is the intensity of reference

TECHNICAL JARGON:
• Strength of the sound
• Loudness denotes the appreciation of sound intensity
• Expressed in decibel (dB)
Amplitude/loudness
• Number of cycles per second
• Pitch /Tone denotes the appreciation of frequency
• Expressed in Hertz(Hz)
Frequency/pitch /tone
• Resistance offered by a medium to sound wavesImpedance

INTENSITY
 Intensity is defined as the power transmitted
by sound wave a unit area.
 Intensity is dependent on pressure and
velocity average taken over whole cycle
 Intensity =peak pressure x peak velocity/2
 Displacement produced by sound waves
vary with frequency if the intensity is constant
 Low frequency vibrations produce greater
displacements

Simple harmonic motion. Simple harmonic motion is a periodic motion that undulates around a null point with equal
amplitudes. The amplitude is the maximum amount of displacement from the null point in one direction. The frequency of a
simple harmonic motion is the number of cycles per second, and is measured in Hertz (Hz). The period of a cycle is the
inverse of its frequency (1/f), and represents the duration of a single cycle

HUMAN AUDITORY FIELD
 The human ear is sensitive to sound over wide range of
amplitudes:0.0002—200 dyne/cm2
 It can detect the difference between two sounds occuring
10micro seconds apart in time.

EAR ACTS AS A TRANSDUCER
SOUNDENERGY
MECHANICAL
ENERGY
ELECTRICAL
ENERGY

NATURAL RESONANT FREQUENCY
EXTERNAL AUDITORY CANAL--------------- 3000Hz
TYMPANIC MEMBRANE----------------------- 800-1600Hz
MIDDLE EAR---------------------------------------- 800Hz
OSSICULAR CHAIN------------------------------ 500-
2000Hz

FUNCTIONS OF EXTERNAL EAR:
Sound
collection
Increasing
pressure on
tympanic
membrane in a
frequency
sensitive way
Sound
localisation

EXTERNAL EAR
 Act as a resonator
 It increases the
pressure at the ear
drum in a frequency
sensitive way
 Helps in localisation of
direction of sound

SOUND COLLECTION
 Pinna- concha system catches sound over large area and
concentrate it to smaller area of ext. auditory meatus.
 This increases the total energy available to the tympanic
membrane

FEATURES OF EXTERNAL CANAL
 Open on one end only
 The impedance of ear drum is about 3-
4times more than air
 30% of incident energy gets reflected from
external canal
 Efficient in conducting sound in frequency
range of 3-5kHz
 Cuts off unwanted frequency helping in better
speech discrimination

PRESSURE INCREASE BY EAC
 If a tube which is closed at one end and open at other
is placed in a sound field then pressure is low at open
end and high at closed end.
 This phenomenon is seen in EAC at 3kHz frequency ,
and at concha at 5kHz
 The two main resonance are complementary , and
increases sound pressure in range of 2-7kHz.

SOUND LOCALISATION:
 Because of its shape, the pinna shield the
sound from rear end,change timbre,and helps to
localize sound from in front or back
 Cues for sound localization from right/left
 Sound wave reaches the ear closer to sound
source before it arise in farthest ear
 Sound is less intense as it reaches the
farthest ear because head act as barrier
 Auditory cortex integrates these cues to
determine location

TOTAL GAIN
 The total effect of reflection of sound from head,pinna and
external canal resonances is to add 15-20dB to sound
pressure, over frequency range of 2-7kHz.

FUNCTIONS OF MIDDLE EAR:
 Couples sound energy to the cochlea
 Impedance matching
 Attenuation reflex
 Physically protects the cochlea
 Phase differential effect :Couples sound
preferentially to only one window ,thus
producing a differential pressure between the
windows required for the movement of
cochlear fluid

IMPEDANCE TRANSFORMER
 Impedance is defined as the resistance
offered by a medium for transmission of
sound
 middle ear acts as impedance transformer
 Cochlear fluids have an impedance equall
that of sea water (1.5X10 N.sec/m3)

IMPEDANCE TRANSFORMER
 Impedance is defined as the resistance offered by a
medium for transmission of sound
 Middle ear ossicles are suspended by ligaments
 Axis of rotation of ossicles and axis of suspension by
ligaments virtually coincides with their centre of
inertia
 At low frequencies the ligaments play an important
role in maintaining ossicular positions(elastic effect)
 Middle ear converts the low pressure high
displcement vibrations of ear drum into high pressure
low displacement vibrations this is suitable to drive
cochlear fluids

IMPEDANCE MISMATCH
IF THERE WAS NO MIDDLE EAR
SYSTEM ,99% OF SOUND WAVES
WOULD HAVE REFLECTED BACK
FROM OVAL WINDOW
MIDDLE EAR BY ITS IMPEDENCE
MATCHING PROPERTY ALLOWS
60% OF SOUND ENERGY TO
DISSIPATE IN INNER EAR

IMPEDANCE EFFICIENCY
 Only 60%of sound energy from TM gets
transmitted &absorbed in the cochlea
 Without the middle ear only 1%of sound
energy will be absorbed by the cochlea

LOW FREQUENCY SOUND DAMPENERS
 Middle ear efficiency is the best at 1kHz
 There is transmission loss of low frequency
sounds due to elastic stiffness of middle ear
ligaments(annular ligament is the most
important)
 Air inside middle ear cavity also dampens
low frequency sound transmission
 Grommet insertion improves transmission of
low frequency sounds

“IMPEDANCE MATCHING” BY THE MIDDLE EAR
SYSTEM
c) The shape of tympanic membrane
b) The lever action of middle ear ossicles
a) Area of tympanic membrane relative to oval window

A) AREA OF THE TYMPANIC MEMBRANE
RELATIVE TO OVAL WINDOW
 Total effective area of
tympanic membrane
69mm2
 Area of stapes footplate is
3.2mm2
 Effective areal ratio is 14:1
 Thus by focusing sound
pressure from large area of
tympanic membrane to
small area of oval window
the effectiveness of energy
transfer between air to fluid
of cochlea is increased

B) LEVER ACTION OF EAR OSSICLES
Handle of malleus is 1.3
times longer than long
process of incus
Overall this produces
a lever action that
converts low pressure
with a long lever action
at malleus handle to
high pressure with a
short lever action at tip
of long process of
incus

C) SHAPE OF THE TYMPANIC MEMBRANE
 TM buckles as it
moves to and fro
 This reduces malleolar
movement
 TM thus acts as a
mechanical lever
 This causes high
pressure low
displacement system

HYDRAULIC ACTION OF TYMPANIC MEMBRANE
 The most important factor in the middle ear's impedance
matching capability comes from the “area ratio” between
the tympanic membrane and the stapes footplate
 Total area of tympanic membrane 90mm2
 Functional area of tympanic membrane is two third
(69mm2).Area of stapes footplate is 3.4mm2.So ,
 Effective areal ratio is 14:1
 Thus by focusing sound pressure from large area of
tympanic membrane to small area of oval window the
effectiveness of energy transfer between air to fluid of
cochlea is increased

ACTION OF TYMPANIC MEMBRANE
 Eustachian tube equilibriates the
air pressure in middle ear with
that of atmospheric pressure,thus
permitting tympanic membrane to
stay in its most neutral position.
 A buckling motion of tympanic
membrane result in an increased
force and decreased velocity to
produce a fourfold increase in
effectiveness of energy transfer

 Total transformer ratio=14x1.3=18.2:1
The combined effects of the area ratio and
the lever ratio give the middle ear output a 28-
dB gain theoretically. In reality, the middle ear
sound pressure gain is only about 20 dB; this is
mostly due to the fact that the tympanic
membrane does not move as a rigid diaphragm

PHASE DIFFERENTIAL EFFECT
 Sound waves striking the tympanic membrane
do not reach the oval and round window
simultaneously.
 There is preferential pathway to oval window
due to ossicular chain.
 This acoustic separation of windows is achieved
by intact tympanic membrane and a cushion of
air around round window
 This contributes 4dB when tympanic membrane
is intact

ROLE OF MIDDLE EAR MUSCLES:
 TENSOR TYMPANI MUSCLE
ATTACHES TO THE HANDLE OF
MALLEUS.IT PULLS THE DRUM
MEDIALLY.
 STAPEDIUS MUSCLE ATTACHES
TO THE POSTERIOR ASPECT OF
STAPES
 CONTRACTION OR THESE
MUSCLE INCREASES THE
STIFFNESS OF OSSICULAR
CHAIN THUS BLUNTING LOW
FREQUENCIES
 DECREASES A PERSON’S
SENSITIVITY TO THEIR OWN
SPEECH

PROTECTIVE FUNCTIONS OF MIDDLE EAR
MUSCLES
 Stapedius contraction can reduce
transmission by upto 30dB for frequencies
less than 1-2 kHz. for higher frequencies this
is limited to 10dB.
 Only the stapedius muscle contracts in
response to loud noise in humans
 The whole stapedial reflex arc has 3-4
synapses
 Stapedial reflex latency is 6-7ms

ATTENATION REFLEX
 When loud sounds are transmitted through the
ossicular system and from there into the central
nervous system, a reflex occurs after a latent period
of only 40 to 80 ms to cause contraction of the
stapedius muscle and the tensor tympani muscle
The tensor tympani muscle pulls the handle of the
malleus inward while the stapedius muscle pulls the
stapes outward. These two forces oppose each other
and thereby cause the entire ossicular system to
develop increased rigidity, thus greatly reducing the
ossicular conduction of low frequency sound

DAMAGED MIDDLE EAR SCENARIOS
 Damaged middle ear can cause loss of
transformer mechanism
 Differntial pressure levels between the two
windows could not be maintained
 Scala vestibuli is more yielding than scala
tympani .differential movements of fluid is still
possible .
 Small compliance of annular ligament in
comparison to much larger compiant round
window could again cause differential pressure

BONE CONDUCTION
 Normal route for hearing some component of
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(s.vestibuli is larger than s.tympani
)could cause movement of cochlear fluid
 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 FACTORS
 Vibration of the skull faithfully transmitted to
the ossicles of middle ear cavity
 Inertia of middle ear ossicles doesn’t
coincide with their point of attachment
 Middle ear acts as a band pass filter with
peak transmission around 1kHz
 This accounts for carharts notch though at a
slightly higher frequency

BONE CONDUCTION EXTERNAL EAR FACTORS:
 Bone 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
conduct ion
 External radiation of sound is best for low
frequencies, hence change with occlusion Is
greatest for these frequencies.

INNER EAR PHYSIOLOGY
 The two important functions of the inner ear are HEARING
and BALANCE.
 The portion of the inner ear that deals with hearing is the
cochlea, and that deals with balance is collectively known as
the vestibular organs (semicircular canals, utricle, and
saccule).
 COCHLEA acts as a TRANSDUCER that translates sound
energy into a form suitable for stimulating the dendrites of
auditory nerve.

STRUCTURE OF COCHLEA:
The cochlea is a fluid-filled space with three
compartments: scala tympani, scala media, and scala
vestibuli
The scala tympani and the scala media are separated
by the basilar membrane, and the scala media and the
scala vestibuli are separated by Reissner's membrane.
The scala media contains the organ of Corti which
contains inner and outer hair cells

 The inner hair cells are flask-shaped cells,3000 approx in
number and arranged in a single row
 the outer hair cells are cylindrical-shaped,12000 approx in
number arranged in 3-4 rows
 The hair cells derive their names from having hairlike
projections on their apical surface. These hair like
projections are stereocilia, which play an important role in
the signal transduction properties of the hair cells

ENDOLYMPH
 Formed by stria vascularis
 Endolymphatic sac maintains homeostsis of
endolymph
 It has a high sodium and low potssium
content
 Endolymph has positive potential gradient
+50-120mv(endocochlear potential)
 Na k ATPase is responsible for this gradient.

PERILYMPH:
 Site of production is controversial ?CSF
 Occupies perilymphatic space. continuous
between vestibular &cochlear divisions
 Ionic concentration resembles extracellular fluid
 Perilymph from s.vestibuli originates from
plasma ,while perilymph from s.tympani
originates from plasma and CSF
 Electric potential of s.tymapani is 7mv and s.
vestibuli is +5mv

BASILIAR MEMBRANE
 Separates s.media from s .tympani
 Length’s of basilar membrane increases
progressively from oval window to the apex
(0.04mm near oval window and 0.5mm at
helicotrema )12 fold increase
 Diameters of basilar fibres decrease from oval
window to helicotrema
 The stiff short fibres near the oval window
vibrate best at very high frequency,while long
limber fibres near the tip of cochlea vibrate best
at a low frequency.

Schematic cross-sectional view of the human cochlea. The scala media
(cochlear duct) is filled with endolymph, and the scala vestibuli and tympani
are filled with perilymph. The endolymph of the scala media bathes the organ
of Corti, located between the basilar and tectorial membranes and containing
the inner and outer hair cells.
Hair cells contain stereocilia along the apical surface and are connected by
tip links. In response to mechanical vibration of the basilar membrane,
deflection of stereocilia, displacement of tip links, and opening of gated
potassium channels. Epithelial supporting cells (connexin channels, red) allow
for the flow of potassium ions

 The scala vestibuli and the scala tympani are filled with
perilymph, which has a low potassium concentration.
 The scala media is filled with endolymph, which has a high
potassium concentration.
 The unique electrolyte composition of the scala media sets up a
large electrochemical gradient, called the endocochlear potential,
which is about +80 mV relative to perilymph. The maintenance of
such a large electrochemical gradient is performed by the stria
vascularis

ENDOCOCHLEAR POTENTIAL
 The importance of is that the tops
of hair cells project through the
reticular lamina and are bathed by
the endolymh of the scala media
,whereas perilymph bathes the
lower bodies of the hair cells.
further more the hair cells have a
negative intracellular potential of -
70mv wrt the perilymphbut -
150mv wrt endolymph at their
upper surfaces where the hair cells
project through the reticular lamina
and into the endolymph

COCHLEAR MECHANICS:
 Mechanical travelling wave
in the cochlea is the basis
of frequency selectivity
 The travelling wave
reaches a peak and dies
away rapidly
 As the wave moves up the
cochlea towards its peak ,it
reaches a region in which
the membrane is
mechanically active. In this
region the membrane stars
putting energy into the
wave .the amplitude raises
rapidly only to fall rapidly.

TRAVELLING WAVE THEORY
 The movements of the
footplate of the stapes set
up a series of traveling
waves in the perilymph of
the scala vestibuli
 High-pitched sounds
generate waves that reach
maximum height near the
base of the cochlea; low-
pitched sounds generate
waves that peak near the
apex
 The basilar membrane is
not under tension, and it
also is readily depressed
into the scala tympani by
the peaks of waves in the
scala vestibuli

Schematic showing sound propagation in the cochlea. As sound energy travels
through the external and middle ears, it causes the stapes footplate to vibrate.
The vibration of the stapes footplate results in a compressional wave on the
inner ear fluid. Because the pressure in the scala vestibuli is higher than the
pressure in the scala tympani, this sets up a pressure gradient that causes the
cochlear partition to vibrate as a traveling wave. Because the basilar membrane
varies in its stiffness and mass along its length, it is able to act as a series of
filters, responding to specific sound frequencies at specific locations

HAIR CELLS:
 The hairs ends of the OUTER
HAIR CELLS are fixed tightly in
a rigid structure composed of a
flat plate, called the reticular
lamina, supported by triangular
rods of Corti,which are attached
tightly to the basilar fibers.
 The hairs of the INNER HAIR
CELLS are not attached to the
tectorial membrane, but they are
apparently bent by fluid moving
between the tectorial membrane
and the underlying hair cells.

INNER HAIR CELLS
 Makes large no. of synaptic contact with afferent
fibres of auditory neve
 95% of afferent auditory nerves make contact with
inner hair cells
 Detects basilar membrane movement
 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
groove”
 They are driven by viscous drag of endolymph
 inner hair cells respond to the velocity rather than
displacement of basilar membrane

OUTER HAIR CELLS
 Very few outer hair cells synapse with
auditory nerves
 Inside of outer hair cells have -70 mV
 They serve to amplify basilar membrane
vibration
 They increase the sensitivity and selectivity
of cochlea
 Cochlear microphonics are derived from
these cells

RESTING POTENTIAL OF HAIR CELLS
 Each hair cell has an intracellular potential of (-70mV) with
respect to perilymph.
 At upper end of hair cell the potential difference between
intracellular fluid and endolymph is (-150mV)
 This high potential difference makes the cell very sensitive.
Tip links
 The tops of the shorter
stereocilia are attached by thin
filaments to the back sides of
their adjacent longer stereocilia

 The basilar fibers, the rods of
Corti, and the reticular lamina
move as a rigid unit
 Upward movement of the basilar
fiber rocks the reticular lamina
upward and inward toward the
modiolus.Then, when the basilar
membrane moves downward, the
reticular lamina rocks downward
and outward.
 The inward and outward motion
causes the hairs on the hair cells
to shear back and forth against the
tectorial membrane.Thus, the hair
cells are excited whenever the
basilar membrane vibrates

Schematic showing the role of tip links in hair cell signal transduction
 As the stereocilia is deflected toward the direction of the tallest row, it
causes the tip links to stretch. The stretch of the tip links causes the
opening of stretch-sensitive cationic channels located on the stereocilia
 The opening of these stretch-sensitive cationic channels on the
stereocilia causes a large influx of cationic current, which leads to hair
cell depolarization.
 As the stereocilia is deflected away from the tallest row, it causes a
relaxation of the tip links, which decreases the probability of ion channel
opening. This leads to hyperpolarization of the hair cell

DEPOLARIZATION/ACTIVATION
 When the cilia are bent in
the direction of the longer
ones, the tips of the
smaller stereocilia are
tugged outward.This
causes a mechanical
transduction that opens
200 to 300 cation-
conducting channels,
allowing rapid movement
of potassium ions from the
surrounding scala media
fluid into the stereocilia,
which causes
depolarization of the hair

 The influx of potassium inside
the cell causes activation of
calcium channels
 This calcium drags the
neurotransmitter filled vesicle to
fuse with cell membrane at
base of cell.
 Neurotransmitter
(glutamate)releases and excites
the dendrites of afferent nerve
fibres.

CENTRAL AUDITORY PATHWAY
Inputs from auditory nerve drive
multiple cell types in different
subdivisions of the cochlear
nucleus, with each cell type
projecting centrally to different
targets in the superior olivary
complex, lateral lemniscus nuclei,
and inferior colliculus
Cochlear nucleus
is the critical first relay station for
all ascending auditory information
originating in the ear, and is
located in the pontomedullary
junction
its major subdivisions: the dorsal
cochlear nucleus, the anterior
ventral cochlear nucleus, and the
Cochlear
nuclei
Superior
olivary
complex
Nucleus
of lateral
lemniscus
Inferior
colliculus
Medial
geniculte
body
Auditory
cortex

 nerve fibers from the spiral
ganglion of Corti enter the dorsal
and ventral cochlear nuclei
 second-order neurons pass mainly
to the opposite side of the brain
stem to terminate in the superior
olivary nucleus
the superior olivary nucleus,the
auditory pathway passes
upward through the lateral
lemniscus.

 Some of the fibres
terminate in the nucleus of
lateral lemniscus ,but many
bypass this nucleus and
travel on to the inferior
colliculus,where all or
almost all the auditory fibres
synapse
 From there the pathway
passes to the medial
geniculate nucleus,where
all the fibres do synapse
 Finally the pathway
proceeds by way of the
auditory radiations to
auditory cortex.

The lateral lemniscus is formed by the three fiber tracts from the cochlear
nucleus
The inferior colliculus located in the midbrain just caudal to the superior
colliculus.
 receives projections directly from the cochlear nucleus and information
about interaural time and amplitude differences from the medial superior
olive and lateral superior olive
 processes the information it receives and sends fibers to the medial
geniculate body of the thalamus.
.
Functional magnetic resonance imaging showing the ascending pathways of auditory processing from the auditory brainstem to
the auditory cortex

THE MEDIAL GENICULATE
BODY
 is the thalamic auditory relay center
that receives auditory information
from the inferior colliculus.
 It has three divisions: ventral,
dorsal, and medial.
 Plays an important role in sound
localization and processing of
complex vocal communications,
such as human speech
AUDITORY CORTEX
 The main auditory portion of the
cerebral cortex resides in the
temporal lobe, close to the sylvian
fissure
 The primary auditory cortex is
located on the superior surface of
the temporal lobe (Heschl's gyrus).
This is also known as area A1, and

 The auditory association cortex is also
known as area A2, and corresponds to
Brodmann's areas 22 and 42.
 The primary auditory cortex is directly
excited by projections from medial
geniculate body,whereas the auditory
associaton area are excited by impulses
from primary auditory cortex as well as
some projections thalamic association
areas adjacent to MGB
 the primary auditory cortex is tonotopically
tuned, with high frequencies being
represented more medially, and low
frequencies being represented more
laterally
FUNCTIONS
 integrating and processing complex
auditory signals, including language
comprehension
 the auditory association cortex plays an
important role in speech perception
 auditory association cortex is located
lateral to the primary auditory cortex, and it
is part of a language reception area known

FUNCTIONS OF AUDITORY CORTEX
 Perception of sound
 Judging the intensity of the sound
 Analysis of different properties of sound

PECULARITIES OF AUDITORY PATHWAY
 First ,signals from both ears are transmitted
through the pathways of both sides of the brain
,with a preponderance of transmission in the
contralateral pathway
 Second ,many collateral fibres from the auditory
tracts pass directly into the reticular activating
system of the brain stem
 Third ,a high degree of spatial orientation is
maintained in the fibre tracts from the cochlea
all the way to the cortex

DETERMINATION OF LOUDNESS
Determined by the auditory system in at least three ways.
 First, as the sound becomes louder, the amplitude of
vibration of the basilar membrane and hair cells also
increases, so that the hair cells excite the nerve endings at
more rapid rate
 Second, as the amplitude of vibration increases, it causes
more and more of the hair cells on the fringes of the
resonating portion of the basilar membrane to become
stimulated, thus causing spatial summation of impulses.
 Third, the outer hair cells do not become stimulated
significantly until vibration of the basilar membrane reaches
high intensity, and stimulation of these cells presumably
apprises the nervous system that the sound is loud.

DETERMINATION OF SOUND FREQUENCY—
THE “PLACE” PRINCIPLE
There is spatial organization of the nerve fibers in the cochlear
pathway, all the way from the cochlea to the cerebral cortex
 Specific brain neurons are activated by specific sound
frequencies
 The major method used by the nervous system to detect different
sound frequencies is to determine the positions along the basilar
membrane that are most stimulated. This is called the place
principle

AUDITORY NERVE FIBRES:
 Inner hair cells excite auditory nerves
 Sound stimulus, transmittor release and
action potential generation occur in
synchrony (phase locking)
 Commonly seen at low frequencies

FREQUENCY CODING AT AUDITORY NERVE
 Phase locking
 Temporal properties (timing of action
potential)
 Frequency selectivity(place coding)

THEORIES OF HEARING
 Place theory of Helmholtz
 Temporal theory of Rutherford
 Volley theory of Wever
 Place theory of Lawrence
 Travelling wave theory of Bekesy

PLACE THEORY
 Acc to helmholtz basilar memebrane has
different segments that respond to different
frequencies
 Sharply tuned resonators dampen slowly this
could cause after ringing cessation of stimuli
 This theory fails to explain why a stream of
clicks of frequencies ranging from 1220,1300
and 1400 Hz is heard as 1000 Hz

TELEPHONIC THEORY
 Rutherford proposed that entire cochlea
responds as a whole to all freqquencies instead
of being activated on a plate by place basis.
 Here the sound of all frequencies are
transmitted as in a telephone cable and
frequency analysis is done at a higher
level(brain)
 Damage to certain portion of cochla 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 cannot be explained by telephonic theory.

VOLLEY THEORY
 Proposed by Wever
 Several neurons acting as a group can fire in
response to high frequency sound even
though none of them could do it individually

PLACE VOLLEY THEORY
 Proposed by lawrence
 Combines both volley and place theory
 This theory thus attemps to explain sound
transmission and perception

TRAVELLING WAVE THEORY
 Proposed 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 closes
to apex

TUNING BY OUTER HAIR CELLS
 Tuning of sound in basilar membrane requires local addition
of mechanical energy
 There are efferent fibres from crossed olivocochlear bundle
supplying the outer cells
 The inputs from these bundle causes contraction of outer
cells located close to maximum of travelling wave give rise
to extra distortion of basilar membrane
 This provides an extra gain of 40-50dB to the system

CENTRIFUGAL INNERVATION OF COCHLEA
 Cochlea recieves centrifugal or efferent
nerve supply,i.e. olivococchlear bundle
 It reduces the magnitude of travelling wave
,and possibly protects the ear against
moderate level of noise damage
 Reduces the masking effect of background
noise in complex tasks

COCHLEAR ECHOES/OTOACOUSTIC EMISSIONS
 Energy produced by outer hair cell motility serves as an
amplifier within the cochlea, contributing to better hearing
 OAEs are produced by the energy from outer hair cell
motility that makes its way outward from the cochlea
through the middle ear, vibrating the tympanic membrane,
and propagating into the external ear canal

Physiology of hearing ppt

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Physiology of hearing ppt