2. IS HEARING IMPORTANT?
Communication: hearing is essential to
language
Localisation: determination of location of
unseen sound sources
3. 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
4. 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 log10J/Jr, where J is the intensity of the sound of
interest, and Jr is the intensity of reference
5. 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 waves
Impedance
6. 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
7. 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 singlecycle
8. HUMAN AUDITORYFIELD
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.
9. EAR ACTS AS A TRANSDUCER
SOUNDENERGY MECHANICAL
ENERGY
ELECTRICAL
ENERGY
11. FUNCTIONS OF EXTERNAL EAR:
Sound
collection
Increasing
pressure on
tympanic
membrane in a
frequency
sensitive way
Sound
localisation
12. 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
13. 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
14. 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
15. PRESSURE INCREASE BYEAC
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.
16. 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
17. 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.
18. 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
19. 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)
20. 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
21. 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
22. 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
23. 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
25. A) AREA OF THE TYMPANICMEMBRANE
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
26. B) LEVER ACTION OF EAROSSICLES
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
27. C) SHAPE OF THE TYMPANICMEMBRANE
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
28. HYDRAULIC ACTION OF TYMPANICMEMBRANE
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
29. 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
30. 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 20dB;this is
mostly due to the fact that the tympanic
membrane does not move as a rigid diaphragm
Total transformer ratio=14x1.3=18.2:1
31. 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
32. ROLE OF MIDDLE EARMUSCLES:
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 THEIROWN
SPEECH
33. 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
34. 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
35. DAMAGED MIDDLE EARSCENARIOS
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
36. 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
37. BONE CONDUCTION INNER EARFACTORS:
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
38. BONE CONDUCTION MIDDLE EARFACTORS
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
39. BONE CONDUCTION EXTERNAL EARFACTORS:
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.
40. 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.
41. 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
42. 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
43. 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.
44. 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
45. 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.
46. 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 andcontaining
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
47. 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 +80mVrelative to perilymph. The maintenance of
such a large electrochemical gradient is performed by the stria
vascularis
48. 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
49. 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.
50. 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
51. 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
52. 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.
53. 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
54. 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
55. 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
56. 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
57. 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
58. 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
59. 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.
60. 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
61. 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.
62. 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.
63. 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
64. 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
65. 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
66. FUNCTIONS OF AUDITORY CORTEX
Perception of sound
Judging the intensity of the sound
Analysis of different properties ofsound
67. 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
68. DETERMINATIONOF 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.
69. DETERMINATIONOFSOUNDFREQUENCY—
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
70. 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
71. FREQUENCY CODING AT AUDITORY NERVE
Phase locking
Temporal properties (timing of action
potential)
Frequency selectivity(place coding)
72. THEORIES OF HEARING
Place theory of Helmholtz
Temporal theory of Rutherford
Volley theory of Wever
Place theory of Lawrence
Travelling wave theory of Bekesy
73. 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
74. 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.
75. 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
76. PLACE VOLLEY THEORY
Proposed by lawrence
Combines both volley and place theory
This theory thus attemps to explain sound
transmission and perception
77. 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
78. 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
79. 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
80. 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
