PHYSIOLOGY OF HEARING: SOUND TRANSMISSION AND PROCESSING

PHYSIOLOGY OF HEARING
-------Dr. SAYAN BANERJEE-------

AIR BORNE SOUND (Alternate phases
of condensation & rarefaction)
AURICLE COLLECTS SOUND WAVES
(of greater amplitude & lesser force)
EAC TYMPANIC MEMBRANE
CATENARY LEVER OSSICULAR LEVER HYDRAULIC LEVER
VIBRATION with greater
force & lesser amplitude
FOOT PLATE OF STAPES/
OVAL WINDOW
FLUIDS IN INNER EAR
(PERILYMPH) VIBRATE
DISPLACEMENT OF
BASILAR MEMBRANE
SHEARING MOVEMENTS BETWEEN
HAIR CELLS & TECTORIAL
MEMBRANE OF ORGAN OF CORTI
VIBRATIONS ARE THEN
TRANSMITTED TO ROUND WINDOW
NERVE IMPULSES IN
FIBRES OF AUDITORY
NERVE (EIGHTH NERVE)
DORSAL & VENTRAL
COCHLEAR NUCLEUS
SUPERIOR OLIVARY
NUCLEUS
LATERAL LEMNISCUS
INFERIOR
COLLICULUS
MEDIAL GENICULATE
BODY
AUDITORY CORTEX SOUND

EXTERNAL EAR
Increases pressure at the
TM in a frequency-
sensitive way
Increases the pressure in a way
that depends on the direction of
the sound source

Pinna, because of their
location and shape, serve to
gather sound arriving from
an arc of 135° relative to the
direction of the head.
This pattern rejects
sound arriving from
the ear and serves to
determine the origin
of the sound.
The horn shaped concha
then acts like a megaphone
to concentrate the sound
at the entrance of the
auditory canal.
This action
increases sound
pressure as much
as 6 dB (2 times).
External Auditory Canal is
acting in concert with the
effect of the pinna
Increases sound pressure at
the tympanic membrane by 15
to 22 dB at 4000 Hz.
Some of the incident sound is reflected off the head and maximum
amplification takes place when the source is in the horizontal plane and 90
degrees to the side

The pinna of the mammal is regarded as a simple funnel
that collects and filters sound → aids in sound localization,
especially front-to-back and high-to-low distinctions
Along with the external ear canal, it increases
acoustic pressure at the tympanic membrane in the
1.5 to 5 kHz range, which is the frequency range
most important for speech perception.
The concha has a resonant frequency of around
5300 Hz, and the external auditory canal has a
resonant frequency of around 3000 Hz.

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.
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.

• The plots describe the gains for a
sound source that is positioned on
the same horizontal plane as the
interaural axis (elevation of 0
degrees) and which is 45 degrees off
of the midline towards the ear that is
measured (azimuth of 45 degrees).
• The gains of the different
components are all multiplied (added
in dB) together to achieve the total
gain.

Resonance in EAM changes the
sound pressure at the TM in a
frequency selective way
Reduces the loss of
incident energy
Frequencies b/w 2kHz &
5kHz are selectively
amplifying in the EAC
because of their
peculiar size, shape and
placement
Hearing b/w 2kHz &
5kHz is very important
for intelligibility of
human speech as most
consonants are centred
in this frequency range
Amplifies sound by 5-10
dB
The resonance is more
marked at 2700Hz where
the amplification is as
much as 15-20dB
Hence if pinna and
meatus is bypassed
or altered by surgical
process like
meatoplasty
Amplification at 2-
5kHz of around 15
dB is lost

OCCLUSION EFFECT
• Blocking the EAC by hearing aid
mould or by any other object
leads to unnatural hearing
inside
When the sound enters
EAC, there is vibration of
walls of external ear
Generates added
vibration of air in EAM
Air-bone vibration exit
through open meatus
But when occluded
vibration can not exit
and a part of it enters
the cochlea
Gets extra-amplified

SOUND
LOCALISATION:
Because of its shape, the pinna shield the sound
from rear end, change timbre, and helps to localize
sound in front from those behind and above from
below the head
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
Maximum amplification
takes place when the
source is in the
horizontal plane and
90 degrees to the side

SOUND LOCALIZATION IS ACHIEVED BY 2 MAJOR MECHANISMS
INTERAURAL TIME DIFFERENCE
The differences in the time arrival
of the sound stimulus between the
two ears can be used as a cue for
sound localization.
INTERAURAL AMPLITUDE DIFFERENCE
The differences in amplitude
perceived by the two ears
can also be used as a cue
for sound localization.
This difference in amplitude is increased
further by the “HEAD SHADOW” effect
Sound coming from one side is
attenuated by the head as it
travels to the contralateral ear.
The head shadow effect in binaural
hearing helps to improve the signal to
noise ratio in adverse listening
environments.
One ear can be closer to the source of
sound or speech, whereas the contra
lateral ear is exposed to the background
noise.

In the frequency range of 2-7kHz, there is loss of up to 10dB, if the sound source is
moved around the head due to the interference between the wave transmitted directly,
and the wave that is scattered off the pinna.

When the sound
source is raised
above the
horizontal plane
The low frequency edge
of the dip moves to higher
frequencies as seen in the
frequency gain curve.
This phenomenon occurs
due to cancellation between
multiple out of phase
reflections off the back wall
of the pinna and concha.
At very high frequencies, the wave
length is short compared to the
dimensions of the pinna. The pinna
is highly directional and produces
high gain on a narrow axis.

MIDDLE EAR
ACOUSTIC
TRANSFORMER
To match impedance of air to
much greater impedance of
cochlear fluid. Transformer action
of middle ear changes low
pressure high displacement
vibration into high pressure low
displacement vibration suitable
for driving cochlear fluid.
COUPLING SOUND
ENERGY TO COCHLEA
By preferential conduction of
sound to oval window and
producing differential
pressure between the oval
and round windows, which is
required for movement of
cochlear fluid.
PHYSICAL
PROTECTION AGAINST
LOUD SOUND
PHASE
DIFFERENCE

MECHANICAL
CONDUCTION OF
SOUND
(ACOUSTIC
TRANSFORMER)
A sound wave, on arriving at the boundary of
its supporting medium, may be reflected or
absorbed by the material of which the
boundary is constructed. For example, if the
medium is air and the boundary is water, 99.9
percent of the sound energy is reflected.
The resistance to the
passage of sound through a
medium is its ACOUSTIC
RESISTANCE OR
IMPEDANCE. A similar
situation exists in the ear
when air conducted sound
has to travel to cochlear
fluids.
So to compensate this loss of sound
energy, nature has made middle ear
to convert sound of greater
amplitude, but lesser force, to that of
lesser amplitude and greater force.
This function of middle
ear is called
IMPEDANCE
MATCHING.
The major contributors to the human
acoustic transformer are the pinna,
external auditory canal, and the middle
ear sound conduction system.

• Principles of mechanical impedance. Frictional
resistance is represented by a “dashpot”—a perforated
piston operating inside a fluid-filled cylinder.
• The diagram could be converted to a representation of
acoustic impedance by interposing a cylinder, or
diaphragm, between the driving force (F) and the driven
mass (M) and expressing the displacing input as
pressure, that is, force per unit area.
ACOUSTIC IMPEDANCE represents
a special type of mechanical impedance
in which force is replaced by pressure,
that is, force per unit area, and the
system is driven by sound.
When air conducts sound, the stiffness
component of its acoustic impedance is
determined by the elastic coupling between
air molecules, the mass component is
determined by the mass of the air molecules,
and the frictional component is determined by
frictional resistance between the molecules.
Because fluid is much denser and less
compressible than air, it might seem at first that
mass and stiffness create the principal
difference between the acoustic impedance of
the cochlea and that of air.

ACOUSTIC
IMPEDANCE
STIFFNESS
Determined by the elastic
coupling between air molecules
If the membrane were stiffer than normal, the
volume velocity generated by the acoustic
stimulus would be decreased
In a simple acoustic resonator, stiffness varies
inversely with frequency and dominates the
acoustic impedance at low frequencies
RESISTANCE
(DAMPING)
Determined by frictional resistance
between the molecules
A small amount of sound energy is lost as a
result of the damping effect of the system
MASS
Determined by the mass
of the air molecules
If the mass of the membrane were
increased, it would be reasonable to also
expect the volume velocity generated by
the acoustic stimulus to decrease
The impedance of a mass increases with
frequency and dominates at high frequencies
When the acoustic impedance is at its
lowest point—that is, at the frequency
where the stiffness and mass
components of the acoustic impedance
cancel each other out-the system is
said to be in RESONANCE.

A simple mass–spring system:
The mass is set in motion and then
exchanges kinetic energy in its inertia with
the potential energy stored in the springs.
Vibration amplitude by
driving vibration
frequency:
• In a simple mass–spring
resonator, there will be a
simple resonance peak
frequency as shown above.
• Damping from friction will
lower this peak, broaden its
response, and lower the
peak response frequency.
• The solid line represents vibration velocity of a driven
system with a simple resonance, and the dot-dash gray
line the impedance, plotted by frequency.
• Also shown in the lower plot is the phase of the driving
force to the velocity of the driven body

The vibration transfer is not
as simple as it seems
because the acoustic
impedance of fluid in the
inner ear is much more than
the air in the middle ear (i.e.
IMPEDANCE MISMATCH).
Hence, greater force is required
to cause vibration in the fluid.
99.9% of sound energy is
reflected away from the surface
of the water when sound travels
from air into water.
To compensate this loss, the
tympanic membrane and the ear
ossicles together convert the sound
of greater amplitude but lesser force,
to sound of lesser amplitude but
greater force and serve as an
IMPEDANCE-MATCHING DEVICE.

• PHASE DIFFERENCE
• Middle ear couples sound energy to cochlea by preferential conduction of sound to
oval window and producing differential pressure between the oval and round
windows, which is required for movement of cochlear fluid.
If sound reaches simultaneously
no movement of Perilymph & no
hearing.
When oval window receive
compression, round window
receive rarefaction.
Sound don't reach both
windows simultaneously.
Tympano-
ossicular system
amplifies pressure
acting on oval
window
In case of
interrupted
ossicular chain,
magnitude of
sound pressure
on oval & round
window are
similar

INCUDOMALLEOLAR JOINT
INCUDOSTAPEDIAL JOINT
STAPES TO VESTIBULE INTERFACE
TM TO MANUBRIUM INTERFACE
VIBRATION LOSSES IN THE NORMAL MIDDLE EAR

MODES OF VIBRATIONS IN THE MIDDLE EAR STRUCTURES
TYMPANIC MEMBRANE
Protection of
middle ear &
inner ear
against loud
sounds
Transmission
of vibrations
to ossicles
Postero-
superior
part
moves
maximum
Movement is
mostly to &
fro like
piston & is
frequency-
dependent
OSSICLES
Axis of rotation of
ossicles and axis
of suspension by
their ligaments
nearly coincide
with their centre of
rotational inertia
Bones are able to
vibrate with very
little loss through
the suspending
ligaments
Stapes footplate
motion is like a
piston up to 1kHz
and more rotatory
movement (along
both long & short
axis of footplate)
for higher
frequencies (due
to asymmetrical
attachment of
annular ligament
as annular
ligament is more
thickened in the
posterior part)

Khanna and Tonndorf, did
not confirm this pattern of
movement at any
frequency; rather, there
were 2 maxima of
vibration, one on either
side of the manubrium.
Their results suggested
that as the TM moved to
and fro, it buckled in the
regions between the
manubrium of the
malleus and the anterior
and posterior edges.
At frequencies above
6 kHz the vibrating
pattern becomes more
complex and chaotic
The vibration breaks up into
many small zones with a
reduced the efficiency of
sound transfer mechanism.
Bekesy postulated that
the ear drum moved like a
stiff plate up to
frequencies of 2 kHz.
He also suggested
that the inferior edge
of the drum is flaccid
and moves the most.

Sound stimulus
enters the EAC
TM vibrates
Vibration of the
malleus
The entire
ossicular chain
vibrates
Sound
transmission to
the inner ear via
the stapes
footplate
OSSICULAR
COUPLING
Ear canal sound
pressure &
motion of the
tympanic
membrane
Middle ear sound
pressure
produced
Because the
cochlear windows
are spatially
separated, the sound
pressures within the
middle ear cavity
that act at the oval
and round windows,
respectively, are not
identical
The small differences
between the
magnitudes and
phases of the two
window pressures
result in a small but
measurable difference
in sound pressure
between the 2
windows
ACOUSTIC
COUPLING
MIDDLE EAR COUPLING

Acoustic coupling is about 60dB less
than the ossicular coupling and in normal
conditions one can easily ignore the
acoustic coupling as ossicular coupling
dominates normal middle ear function

Middle ear
mechanics in the
ear with TM
perforation and
ossicular chain
discontinuity
Acoustic coupling
with sound pressure
difference on oval
window & round
window is the main
mechanism for
transmitting sound
energy into the
cochlea
Sound energy
through ossicular
coupling will not
reach the cochlea
Middle ear
mechanics in
the ear with
intact TM and
ossicular chain
discontinuity
Sound energy will
neither reach the
cochlea through
ossicular coupling
nor through acoustic
coupling
Maximal
conductive hearing
loss

• Comparison of air-bone gaps with surgically confirmed
complete ossicular chain interruption with an
intact tympanic membrane to air-bone gaps
predicted on the basis of hearing resulting from acoustic
coupling.
• In this pathological state, there is no ossicular coupling.
• Since acoustic coupling is about 60 dB smaller than
ossicular coupling, the prediction is a 60 dB CHL, which is
consistent with the measured air-bone gaps. The standard
deviation for each of the measured points is about 10 dB.

A special type of
ossicular interruption
consists of resorption/
break in one of the
ossicles and its
replacement by
connective tissue
e.g. resorption of the long
process of incus and its
replacement by a band of
fibrous tissue in COM
Such “PARTIAL OSSICULAR
INTERRUPTIONS” are often
associated with an air-bone gap
that is greater at higher vs lower
frequencies
At lower frequencies, a
fibrous band seems to be
tense enough to allow near-
normal sound transmission
At higher frequencies, the
fibrous band flexes such that
motions of the TM are not
readily coupled to the stapes

• Comparison of air-bone gaps with missing tympanic
membrane (TM), malleus and incus to air-bone gaps
predicted on the basis of acoustic coupling.
• With loss of the tympanic membrane (shielding effect), there is
enhancement of acoustic coupling by about 10 to 20 dB
compared to the normal ear.
• The predicted and measured gaps are similar.

PARTIAL/
COMPLETE
FIXATION OF
THE STAPES
FOOTPLATE
CHL ranging
from 15-60dB
depending on
the degree of
fixation
FIXATION AT
THE LEVEL OF
THE ANTERIOR
MALLEAL
LIGAMENT
CHL < 10dB
ANKYLOSIS OF
THE MALLEAL
HEAD
CHL of 15-25 dB
BOTH
MALLEUS &
INCUS
ANKYLOSES
CHL of 30-50 dB

THE PRIMARY MECHANISM OF CHL
DUE TO A PERFORATION IS A
REDUCTION IN OSSICULAR
COUPLING CAUSED BY A LOSS IN
SOUND-PRESSURE DIFFERENCE
ACROSS THE TM

TYMPANIC
CAVITY
VOLUME
MASTOID
AIR
VOLUME
THE
MIDDLE-
EAR AIR
SPACE
VOLUME
Important
parameter that
determines
the amount of
hearing loss
caused by a
perforation
Small middle-ear
air space volumes
result in larger air-
bone gaps

BAFFLE EFFECT
If there is a posterior TM perforation, round window is exposed resulting
in severe CHL as there is loss of shielding effect from sound waves

↑ Impedance
of the middle-
ear air space
↓ Middle-ear
air volume
At
frequencies
< 1kHz
REDUCTION IN
OSSICULAR COUPLING
CHL of up to
30-35 dB
At frequencies
> 1kHz
Mass loading
of TM by fluid
Ears with air in the
tympanic cavity
show a smaller
conductive loss
than ears with no
visible air bubbles.

If the atelectasis
results invagination
of the tympanic
membrane into the
round window niche
The protective effect
of the TM and middle-
ear air space on
round window motion
is lost
Larger 40 to 50
dB air-bone
gaps should
result
This prediction is
consistent with the amount
of acoustic coupling in
cases where there is loss
of the TM, malleus, and
incus.
As long as the area outside
the round window remains
aerated and is shielded
from the sound pressure in
the ear canal by the TM
CHL caused by the
atelectasis should not
exceed the amount of
middle-ear sound
pressure gain in normal
ears, i.e. air-bone gaps of
up to 25 dB
TM Atelectasis
(occurring without TM
perforation and in
presence of intact &
mobile ossicles)
Reduction of
ossicular coupling
CHL varying from
negligible to 50dB

Middle Ear Transformer Mechanism can be
divided into 3 stages
CATENARY
LEVER
That provided by
the eardrum.
OSSICULAR
LEVER
That provided by the
ossicles.
HYDRAULIC
LEVER
Provided by the difference in
surface area between the
tympanic membrane and the
stapes footplate.
The middle ear amplifies the sound before reaching the inner ear, and this amplification is frequency
dependent; it is 20 dB at 250–500 Hz, maximal of 28 dB at 1000 Hz, and decreases at high
frequencies about 6 dB for each additional 1 kHz above 1000Hz.

CATENARY LEVER: BUCKLING OF
THE EARDRUM:
• The power of sound in the ear canal is matched to
the outer rim of the TM, because the annular
ligament surrounding the tympanic membrane is
immobile and sound energy is directed away from
the edges of the drum and toward the center of the
drum via waves that travel on the TM surface.
• The attachment of the tympanic membrane at the
annulus amplifies the energy at the malleus
because of the elastic properties of the stretched
drumhead fibers and thus works as a catenary
lever (ratio of force acting on tympanic membrane
to that acting on the malleus), where large
displacements near the annular ring (the outer
edge) produce small displacements of the malleus,
so the ear drum itself can increase force when it
moves.
• This buckling effect increases pressure by a factor
of 2 = 6dB.

CATENARY LEVER (CONTINUED)…………..
• Helmholtz was first to propose a concept of a catenary lever to the action of
the tympanic membrane.
• A familiar example of this type of lever is a tennis net. The tighter a tennis
net is stretched, the greater the force exerted on the posts holding it.
• Because the bony annulus is immobile, sound energy applied to the
tympanic membrane is amplified at its central attachment, the malleus.
• It is estimated that even though the curvature of the tympanic membrane is
variable, the catenary lever provides at least a two times (2x) gain in sound
pressure at the malleus.
• Force exerted on the stretched curved fibers of the tympanic membrane are
amplified at its point of attachment, the annular bone and the malleus
handle.
• The annular bone is immobile, so that the malleus is the recipient of this
magnified energy, directing it into the ossicular chain for transmission to
the perilymphatic fluid.

TM
Middle layer- complex
arrangement of radial &
circular muscle fibres
Shape
Concave towards
EAC like a
loudspeaker cone
Convex in each
segment from
annulus to
malleus handle
Makes membrane flexible
Buckles in
response to sound
Buckling helps in
impedance matching
Sound energy
absorbed by fibres
of middle layer
Transferred to
malleus handle
Increases force
transferred to
inner ear
Improves
impedance
value by
factor of 4

OSSICULAR LEVER
 Handle of malleus is 1.3 times longer than long process of
the incus, providing a mechanical advantage of 1.3.
 Overall this produces a lever action that converts low
pressure with along lever action at malleus handle to high
pressure with a short lever action at tip of long process of
incus.
 The catenary and ossicular levers, acting in concert provide
an advantage of 2.3.

• The hydraulic lever acts because of the size difference between the
tympanic membrane and the stapes footplate.
• Sound pressure collected over the large area of the tympanic
membrane and transmitted to the area of the smaller footplate results
in an increase in force proportional to the ratio of the areas.
• Helmholtz's third concept of impedance matching which is referred
as areal ratio.
• According to some workers (Wever and Lawrence) out of a total of 90 mmsq area of the
human tympanic membrane, only 55 mmsq is functional.
• The effective vibratory area of tympanic membrane is 45 (55) sqmm whereas
foot plate area is 3.2 sqmm; Hence effective areal ratio is 14:1 (17:1) : This is
a mechanical advantage provided by tympanic membrane.
• The product of areal ratio into lever ratio is known as transformer ratio. i.e.,
14(17) × 1.3 = 18(21):1 and gain [+26 (28) dB SPL] is boosted by this area ratio
between eardrum and stapes footplate.
SCHEMATIC OF THE MIDDLE
EAR SYSTEM
(A) Motion of the ossicular
chain along its axis of rotation
is illustrated.
(B) Area of the tympanic
membrane (ATM ) divided by
area of the footplate (AFP)
represents the area ratio (ATM
/AFP ).
The length of the manubrium
(lm) divided by the length of
the incus long process (li) is
the lever ratio (lm/li) . PEC ,
External canal sound
pressure; PV , sound pressure
of the vestibule; TM, tympanic
membrane.
HYDRAULIC
LEVER

If all the force applied to the tympanic membrane were to be transferred
to the stapes footplate
The force per unit area would be 20 times larger (26 dB) on the
footplate than on the tympanic membrane
Second mechanism for impedance matching is
called the lever ratio, which refers to the
difference in length of the manubrium of the
malleus and the long process of the incus.
The manubrium is slightly longer than the
long process of the incus
A small force applied to the long arm of the
lever (manubrium) results in a larger force on
the short arm of the lever (incus long
process).

At higher frequencies, it vibrates in a complex manner,
with multiple areas that vibrate differently.
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.
The combined effects of the area ratio and the lever ratio
give the middle ear output a 28-dB gain theoretically.
In humans, the lever ratio is about 1.31:1

Middle Ear Transformer Mechanism- change the low pressure high displacement
vibrations of the air into high pressure low displacement vibrations suitable for driving the
cochlear fluids
CATENARY LEVER
That provided
by the eardrum.
Improves
impedance by a
factor of 4
OSSICULAR LEVER
That provided by
the ossicles.
•Length of malleus : incus = 2.1 : 1
•↑forces by 2.1 & ↓velocity by 2.1
•Net mechanical or impedance
advantage = 4.4 times
HYDRAULIC LEVER
AREAL ADVANTAGE
Area of TM : Stapes
footplate = 60:3.2 = 18
times increased pressure
on footplate/ oval window
AS PER SCOTT-BROWN

• T.M. Catenary lever (curved membrane effect):
• Sound waves focused on malleus. Magnifies 2 times
• Ossicular Lever ratio:
• Length of handle of malleus > long process of incus.
• Magnifies 1.3 times
• Surface area ratio (Hydraulic lever):
• T.M. = 55 mm2 ; Stapes foot plate = 3.2 mm2
• Magnifies 17 times
• Total Mechanical advantage:
• 2 X 17 X 1.3 = 45 times = 30 – 35 dB

MIDDLE EAR MUSCLES
TENSOR TYMPANI
Inserts on to the top of
the manubrium of
malleus
Contraction pulls the
malleus medially &
anteriorly
Detected as an inward
movement of TM
STAPEDIUS
Inserts on the posterior
aspect of the stapes
Pulls the stapes posteriorly
Rocking the stapes in the
oval window
Increases the inward tension on the
posterior edge of the annular
ligament and outward tension on the
anterior edge
• Contraction of both the muscles →→ exerts
force perpendicular to stapes and malleus to
increase impedance of ossicular chain
• Damps out unwanted resonance in middle
ear at higher frequency- STAPEDIAL/
AUDITORY REFLEX

RECONSTRUCTION OF THE SOUND
CONDUCTION MECHANISMS
AERATION OF THE
MIDDLE EAR
TYPE 3
TYMPANOPLASTY
TYPE 4 vs TYPE 5
TYMPANOPLASTY
OSSICULAR
RECONSTRUCTION

AERATION OF THE MIDDLE EAR
• Aeration of the middle ear (including the round
window) is critical to the success of any
tympanoplasty procedure.
• Aeration allows the tympanic membrane, ossicles and
round window to move.
• Clinical experience has shown that nonaerated ears
often demonstrate 40-to 60-dB air-bone gaps.
• The large gap in nonaerated ears occurs because----
1. Ossicular coupling is greatly reduced and
2. Stapes motion is reduced because the round
window membrane (which is coupled to the
stapes by incompressible cochlear fluids) cannot
move freely.
 The normal baseline volume is taken to be 6 cc.
 Note that reduction of the volume to 0.4 cc is
predicted to result in an air-bone gap < 10 dB.
 Volumes smaller than 0.4 cc are predicted to lead
to progressively larger gaps.

A pressure gain is attempted to the
round window by reconstructing the
hydraulic lever, by connecting the TM to
the remaining ossicles
There is no attempt to reconstruct the
hydraulic lever, but rather to allow
unimpeded access to the OW (oval
window) and to shield the RW (round
window) as much as possible

TYPE 4
TYMPANOPLASTY
TYPE 5
TYMPANOPLASTY
vs
• Surgical option in cases where the TM and
ossicles are missing
• The stapes footplate is mobile and there is
a canal wall-down mastoid cavity
• Incoming sound from the ear canal
impinges directly on the stapes footplate
while the round window is acoustically
shielded from the sound in the ear canal
by a tissue graft such as temporalis fascia
• With no ossicular coupling cochlear
stimulation depends on acoustic coupling
• Air space between shield, round window &
tympanic orifice of eustachian tube is
termed as “CAVUM MINOR”
• If the stapes footplate is ankylosed,
it is removed and replaced by a fat
graft and this arrangement
constitutes a type V tympanoplasty
 In a type V tympanoplasty, it is
reasonable to assume that the
mobility of the fat used to replace
the footplate will be greater than that
of the normal footplate
↓
 Hence, one would predict that the
average hearing results for a type V
would be better than those for a type
IV, especially for low frequencies

Air-bone gaps after type IV tympanoplasty: the best
surgical results are compared with a prediction based
on "maximum" acoustic coupling. The predicted and
measured results are similar, with an air-bone gap of
approximately 20 dB.
In both type IV and type V procedures, there is
no ossicular coupling and residual hearing
depends on acoustic coupling.
The introduction of a tissue graft to shield the
round window from sound enhances acoustic
coupling by increasing the sound pressure
difference between the oval and round
windows.
TYPE 4 & 5 TYMPANOPLASTY

TYPE 3 TYMPANOPLASTY
• Classical type III or stapes columella
tympanoplasty involves placement of a
tympanic membrane graft such as
temporalis fascia directly onto the
stapes head, i.e. the ossicular chain is
replaced by the single columella of the
stapes.
• Typically performed in conjunction with
a canal-wall-down mastoidectomy
• The hearing results after this procedure vary widely
with air-bone gaps ranging from 10 to 60dB
• Large air-bone gaps (40-60dB) occur as a result of
stapes fixation, non-aeration of the middle ear, or both
• When the stapes is mobile and the middle ear is
aerated, the average postoperative air-bone gap is on
the order of 20 to 25 dB, suggesting that there is little
middle-ear sound pressure gain occurring through the
reconstruction
• Interposing a thin disk of cartilage between
the graft and the stapes head improves
hearing in the lower frequencies by 5 to 10 dB
• The cartilage acts to increase the "effective"
area of the graft that is coupled to the stapes,
which leads to an increase in the middle-ear
gain of the reconstructed ear

OSSICULAR RECONSTRUCTION
 The positioning of the prosthesis appears to be important
to its function
 Measurements in human temporal bone preparations
suggest that the angle between the stapes and a prosthesis
should be less than 45° for optimal sound transmission
 "Coupling" refers to how well a prosthesis adheres to the
footplate or tympanic membrane, and the degree of
coupling will determine whether or not there is slippage in
sound transmission at the ends of a prosthesis
⸫ a prosthesis transmits sound effectively only if
there is good coupling at both ends
⸫ Inadequate coupling at the prosthesis-footplate
joint may be an important cause of a persistent
postoperative air-bone gap
 Effects of increasing ossicular mass: The mass of an
ossicular strut is increased as shown. These increases
are relative to the stapes mass which is 3 mg.
Increases up to 16 times are predicted to cause less
than 10 dB conductive loss and only at frequencies
greater than 1,000 Hz.

Introduction
Frequency
selectivity of the
basilar membrane
Role of OHCs Role of IHCs
Role of
supporting cells
Role of tectorial
membrane
Role of stria
vascularis
Genes that alter cochlear
K+ homeostasis when
mutated
Role of spiral
ganglia
Sound evoked
electrical potentials in
the cochlea
Pathophysiology of
cochlear hearing
loss
INNER EAR

INTRODUCTION
COCHLEAR ENDOLYMPH HAS A POSITIVE EP OF
+85 mV → THIS DIFFERENCE IN ION COMPOSITION
AND THE EP DIFFERENCE PROVIDE THE ENERGY
REQUIRED FOR THE COCHLEA’S WORK

Sensory cells in the
cochlea
Transform sound into a
code
Auditory nerve
Conveys the
information to the brain
Cochlea separates sounds according to their
spectrum (frequency) so that different
populations of hair cells become activated by
sounds of different frequency
Cochlea compresses the amplitude of sounds
and thereby makes it possible to
accommodate the large dynamic range of
natural sounds
INTRODUCTION (contd….)

FREQUENCY SELECTIVITY OF THE BASILAR MEMBRANE

Sound
Strikes
the ear
drum
Ossicles
Movement
of stapes
Vibration
transmitted to
inner ear
Displacement of
the cochlear fluid
in scala vestibuli
The incompressibility of
perilymph causes a pressure
gradient b/w the scala
vestibuli & tympani
Movement of
basilar membrane
& organ of corti
FREQUENCY SELECTIVITY OF THE BASILAR MEMBRANE (contd….)

HELMHOLTZ’S
PLACE/
RESONANCE
THEORY (1863)
The basilar membrane was
constructed of different
segments that resonated in
response to different
frequencies
These segments are
arranged according to the
location along the length of
basement membrane
For this tuning process
to occur the segments in
different locations would
have to be under different
degrees of tension
According to this
theory, a sound
entering the cochlea
causes the vibration
of the segments that
are tuned to
resonate at that
frequency
DRAWBACK:
• Sharply tuned resonators
dampen rather slowly.
• This could lead to constant
after ringing long after the
stimulus has ceased.
• This theory also fails to explain
why a stream of clicks of
frequencies ranging from 1220,
1300 and 1400 Hz is heard as
100 Hz pitch.

HELMHOLTZ’S PLACE/ RESONANCE THEORY (1863)

TELEPHONE/
FREQUENCY
THEORY OF
RUTHERFORD (1886)
Sound Wave Cochlea
Electrical
Impulse
Cortex
Sound wave & Analysis
DRAWBACK:
• Single nerve fibre transmit- 1000 impulse/sec
• Fails to explain- single nerve fibre > 1000Hz/sec
• Proposed that all frequencies activate the entire length of the
basilar membrane along with the hair cells.
• He postulated that the frequency of the signal is represented by the
rate of firing of the auditory nerve fibers.
• He believed that all vibrations are portrayed by the nerve impulses
to the brain without complex vibrations in the cochlea.

WEVER’S
VOLLEY
RESONANCE
THEORY (1949)
Combines both
the place and
telephone
theories
High frequencies
(5000 Hz) are
perceived in the
basal turn
Low frequencies
(1000 Hz) stimulate
nerve action
potential equal to
frequency
stimulation
Intermediate frequencies (1000-5000 Hz) are
represented in the nerve by asynchronous discharges
which then combine (“BUNCH”/ “VOLLEY”)
actively to represent the frequency of stimulus.
DRAWBACK:
• No evidence
This could be possible if
one neuron could fire in
response to one cycle and
another neuron fires in
response to the next cycle,
while the first neuron is still
in its refractory period

VON BEKESY’S
TRAVELLING WAVE
THEORY (1960)
High pitched
sounds
cause a
short
travelling
wave not
beyond the
basal turn
High pitched
sounds
cause a
short
travelling
wave not
beyond the
basal turn
Low
frequency
stimuli
cause
maximum
displaceme
nt near the
apex
Middle
frequency
changes
occur in
between
these two
It is now known
that the basilar
membrane is
much more
sharply tuned for
frequency
filtering
It is now known
that the basilar
membrane is
much more
sharply tuned for
frequency
filtering
The basilar
membrane
becomes less
selective in
tuning at high
stimulating
intensities due
to non linearity
of its response
The basilar
membrane
becomes less
selective in tuning
at high stimulating
intensities due to
non linearity of its
response
The basilar
membrane
becomes less
selective in tuning
at high stimulating
intensities due to
non linearity of its
response
The sharp tuning and
non linearity is due to
an active mechanical
amplifier which uses
biological energy to
boost the membrane
vibration.
The sharp tuning
and non linearity
is due to an active
mechanical
amplifier which
uses biological
energy to boost
the membrane
vibration
The sharp tuning
and non linearity is
due to an active
mechanical
boost the membrane
vibration
The sharp tuning
and non linearity is
due to an active
mechanical
boost the membrane
vibration
This Wave
begins from the
base and
moves towards
the apex
This Wave
begins from the
base and
moves towards
the apex
Travelling wave
is independent of
frequency
Travelling wave
is independent of
frequency
The region of
maximum
displacement
varies according
to frequency

VON BEKESY’S TRAVELLING WAVE THEORY (1960)

VON BEKESY’S TRAVELLING WAVE THEORY (1960)
• EVIDENCE:
 NIHL- deafness to certain
frequency
 Cochlear microphonics
 High frequency at the base
 Low frequency at the apex
 Experimental animals-
destruction of particular place-
deaf to that particular
frequency

High frequency sounds produce peak displacement
towards the base of the cochlea whereas the peak
moves progressively towards the apex as sound
frequency decreases.
There is therefore a clear relationship between
frequency and displacement in the BM.
The frequency at which the maximum displacement
occurs is also called the CHARACTERISTIC FREQUENCY
at a specific place in the BM making it highly
frequency-specific or tonotopic.
A cochlear tuning curve is the response of the cochlear BM to changing
intensities to achieve a maximum amplitude response and is plotted as a
function of intensity with frequency
A psychophysical tuning curve is the plotting of the amplitude of a
narrow band masker required to mask a fixed pure tone as a function of
the masker signal.

Tuningcurvesof the basilarmembrane(BM)and
theinnerhaircells(IHCs) andouter haircells
(OHCs) atabasallocationinthe guineapigcochlea
• Thetuningcurveplotsthesound-pressure level(SPL)
requiredtoproduceafixedlevelof responseatagiven
locationalongthecochlearpartition.
• Therequiredsoundlevelislowestwhenthesound
stimulusisatitscharacteristicfrequency.
• ThetuningcurvesfortheBMandtheIHCsandOHCsat
thesamelocationonthecochlearpartitionarevery
similar(similarcharacteristicfrequency).

The frequency tuning curves of
auditory nerve fibers superimposed
and aligned with their
approximate relative points of
innervations along the basilar
membrane

EFFERENT INNERVATION OF THE COCHLEA
2 groups of efferent fibers originate in the brain stem
Myelinated medial olivo-
cochlear (MOC) efferents
Unmyelinated lateral olivo-
cochlear (LOC) efferents
Protective effects against acoustic injury
and such a feedback could be important in
loud noise environments

Myelinated medial olivo-cochlear
(MOC) efferents
Arise from neurons located around the medial
superior olivary nucleus
Project to the contralateral and ipsilateral cochleae
Form cholinergic synapses with outer hair cells
Stimulation leads to increased thresholds, which is
due to a decrease in the degree of cochlear
amplification by outer hair cells
This sound-evoked feed back, therefore, decreases
sensitivity of the hearing apparatus in situations
when the metabolically expensive amplification
mechanisms are not needed
TYPICAL TUNING CURVES OF AUDITORY NERVE FIBERS
• (Thick lines) Tuning curves of auditory nerve fibers
with characteristic frequencies of 2kHz (blue) and
5kHz (red).
• (Thin lines) Substantial decrease of the auditory
threshold in response to stimulation of the MOC
system.
• Changes in the specific shapes of tuning curves
depend on the characteristic frequencies (CF) of the
individual fibers.

Unmyelinated lateral olivo-cochlear (LOC) efferents
Originate from neurons with small somata located in and around the lateral superior olivary nucleus
Project predominantly to the ipsilateral cochlea
Terminate on the dendrites of afferent type I neurons beneath inner hair cells
LOC efferent synapses are neurochemically complex and utilize cholinergic, GABAergic, and
dopaminergic transmission as well as various neuropeptides
Their direct input on the afferent neurons suggests that they
regulate afferent activity, thereby affecting the dynamic range
Loss of specific neurotransmitters or destruction of cell
bodies in the brainstem leads to either enhancement or
suppression of auditory nerve response
These LOC feedback effects are slow and usually require
minutes to become effective
Perform slow integration and adjustment of
binaural inputs needed for accurate binaural
function and sound localization

LOSS OF
COCHLEAR
AMPLIFICATION
e.g. OHC
LOSS
BROADENING OF
TUNING CURVE (U-
SHAPED)

Discharge rates
within the auditory
nerve fibres are not
only determined by
the frequency but
also by the intensity
of the stimulus
As intensity
increases, discharge
rate within a single
auditory nerve fibre
increases
The number of auditory
nerve fibres activated at
a given characteristic
frequency increases with
intensifying stimuli
With increasing
stimulus intensity,
other afferent nerve
fibres of nearby
characteristic
frequencies are also
activated
Nerve fibres possess
spontaneous firing
activity without sound
stimulation
Fibres with high
spontaneous firing rates
have a low threshold for
intensity while fibres
with intermediate and
low spontaneous firing
rates have a high
threshold

Frequency is coded by the
auditory nerve fibres discharge
characteristics known as
“PHASE LOCKING”
When auditory nerve neurons fire action
potentials, they tend to respond at times
corresponding to a peak in the sound
pressure waveform, i.e., when the basilar
membrane moves up
The result of this is that there are a bunch of
neurons firing near the peak of each and
every cycle of a pure tone
No individual neuron can respond
to every cycle of a sound signal, so different
neurons fire on successive cycles.
Nonetheless, when they do respond they
tend to fire together.
• The response (across the whole population of hair cells/8th nerve fibers) must follow each rise and fall of
sound pressure level in the sound signal
• Phase locking only happens at low frequencies
• Above 5kHz, spike responses of auditory nerve fibres occur at random intervals

Artificial amplification by hearing aids can’t correct this deficiency by the cochlea
TEMPORAL
SUMMATION
The phenomenon by which an increase
of stimulus duration increases
sensitivity (loudness of the sound) is
known as TEMPORAL SUMMATION and
it is a normal cochlear function
In a subject with normal cochlear
function, there is a 10-15dB
improvement of hearing threshold, if the
duration of sound stimulus is increased
from 10 mSecs to 500 mSecs
Helps in
our natural
hearing
Lost in
cochlear
damage

Basilar
membrane
frequency
tuning is
non-linear
The frequency
selectivity of
the basilar
membrane was
greater at low
sound levels
than at higher
levels

J. J. Zwislocki
& E. J. Kletsky
• Resonators (Tectorial
Membrane And the Stereocilia
of the OHCs) together with the
travelling wave motion are the
bases for the frequency
selectivity of the cochlea
• The tectorial membrane may
sharpen the cochlear
frequency selectivity

DAVID
KEMP,
1978
 Echoes from the cochlea → The sound
arose from the cochlea → OHCs → Oto-
Acoustic Emissions (OAE)
 Mechanical activity of the cochlear
amplifier used to enhance Basilar
membrane movement spilled back out of
the cochlea and could be detected as an
OAE

OHCs act as “MOTORS” that
compensate for the energy losses in
the propagation of the travelling
wave on the basilar membrane
Increases the sensitivity and the
frequency selectivity of the ear
Plays a key role in the
amplification & sharp tuning
OHC Loss
Elimination of the auditory nerve’s low threshold sensitivity & its sharp
tuning (NOT affecting its high threshold characteristics)
ROLE OF OHCs IN BASILAR MEMBRANE MOTION

ROLE OF OHCs IN BASILAR MEMBRANE MOTION (contd….)

Acoustic stimuli Movement of
stereocilia
Basilar membrane
displacements
Towards tallest
stereocilia
Towards smallest
stereocilia
Depolarisation Hyperpolarisation
OHCs contract OHCs elongate
OHCs exert
mechanical
force on BM
Displacement
opens up the
cation-selective
ionic gates
Mechanical
deformation
causes ionic
conduction to
change
Subsequently changes the
membrane potential of the hair
cells
This specialized membrane
(on the stereocilia) and
displacement of the cilia
Opens specific ionic
channels located
near or at the tips of
the stereocilia
Cell membrane becomes “leaky”

Frequency-specific movement of
the basement membrane (peak
amplitude)
Bending of stereocilia located
at the specific point
K+ flow into the hair cells
Depolarisation
Ca2+ channel opens
Neurotransmitter (Glutamate)
release from the vesicle in
synaptic cleft
Initiates AP in auditory nerve fibres

STEREOCILIA
ACTIN
PRESTIN
Found in
muscles
Increases sensitivity of
the ear
Membrane
protein
Performs a direct
voltage-to-force
conversion
Plays an important role in
the motility of OHCS
Forms a specialized membrane on the
stereocilia and displacement of the cilia
opens specific ionic channels that are
located at or near the tips of the
stereocilia

Highest level of actin turnover, regulated by proteins such as
myosin XVa, whirlin, epidermal growth factor receptor pathway
substrate 8 (EPS8), myosin III and espin-1
• Shafts comprise the
majority of the length
• Has extreme stability
of actin filaments,
which are cross-
linked into a parallel
bundle by espin,
plastin and fascin
isoforms as well as
Xin actin binding
repeat containing 2
(XIRP2)
• Stereocilia taper at their base before joining the hair cell
body
• A rootlet comprised of actin filaments bundled by TRIOBP
spans the joint and stabilizes the stereocilia
Tip links composed of cadherin 23 (CDH23) and protocadherin
15 (PCDH15) connect the tips of shorter row stereocilia to the
sides of adjacent taller stereocilia
The tips of mechanotransducing stereocilia
(in the shorter rows of the bundle) are
specialized and house mechanoelectrical
transduction channels, which are associated
with the base of tip links, as well as a
different set of actin regulatory proteins such
as twinfilin-2 (TWF2), Eps8-like 2 (EPS8L2)
and myosin XVa.

Actin filaments in the stereocilia are closely
packed
Cross-linked by different proteins like Epsin,
Fimbrin, Fascin & Plastin 1
Plastin → 2nd most abundant protein in stereocilia after
actin → Loss of plastin 1 results in progressive hearing
loss and balance dysfunction and progressive thinning
of stereocilia
Actin filaments descend from the stereocilium into
the cuticular plate as rootlet → rootlet is formed of
densely packed actin filaments
TRIOBP → actin bundling protein, helps
in formation & maintenance of Rootlet

ACTIN SPECTRIN
An actin cross-linking
protein, has elastic,
deformation-resisting
properties
TROPOMYOSIN
Protein that binds
around actin, stiffens
actin
Provides support for
cuticular plate, so that
stereocilia are
supported on a rigid
platform, enhancing
their ability to respond
to small displacement
forces
MYOSIN
Types- 1c, 6, 7a, 15- in cuticular
plate and stereocilia
Immunolabelling of myosin- 6, 7a helps
to differentiate adult HC from HC during
development
Mutation of myosin- 6, 7a or 15 show deafness
and balance disorders & abnormalities in their
stereociliary bundles
Myosin 6 mutation- stereocilia are fused & increased
in Iength. In humans causes age related hearing
loss& balance dysfunction in elderly
Myosin 15 mutations- stereocilia
are decreased in height
Myosin 7a mutations-
Usher syndrome type1b
CUTICULAR PLATE
OF HAIR CELLS

Myosin-1C
Essential for the
adaptation process
Controls the set point of
mechanosensitivity
Cadherin 23 and
protocadherin 15
Components of
the tip link
Mutation
Usher Syndrome (congenital
hearing loss with retinitis
pigmentosa)
This mechanoelectrical
transduction apparatus is present
in all hair cells
Consists of one or more
mechanically gated cation
channels, closely associated
elastic structures, and a tip link
that connects the tip of one
stereocilium to the side of the
next tallest stereocilium

TIP-LINK COMPLEX
Sans protein → localized to the tip-link lower and upper
insertion points → While dispensable for the initial
formation of the tip-link, sans is required for its
maintenance and/or renewal.
Cadherin-23 and protocadherin-15- make up the upper and
lower part of this link, respectively
Harmonin-b- scaffolding and actin-bundling protein, which
binds with high affinity to the cytoplasmic region of
cadherin-23
Cadherin-23 + (tail of) Myosin VIIa + Harmonin-b → ternary
complex → anchors the tip-link to the actin filaments of
the stereocilium
Myosin 1C → putative adaptation motor → localized to the
tip-link upper insertion point
Myosin VIIa → maintains the tip-link under tension at rest
→ play a role in MET (mechanoelectrical transduction)
adaptation together with myosin 1C

Two distinct processes are
responsible for this
adaptation
Rapid channel
reclosure or "fast
adaptation": by Ca2+
binding to a proposed
intracellular site near
the channel's gate.
Slow adaptation:
sliding of a myosin-
based motor that is
associated with the
transduction
apparatus; i.e. when
the upper insertion
point of the tip-link
slides down the
stereocilium.

MECHANOELECTRICAL
TRANSDUCTION:
A. At rest, approximately 90% of the transduction
channels are closed. Myosin-based molecular
motors climb toward the stereociliary tips and
adjust the tension in the tip link and associated
structures to assure that the transduction
apparatus operates at the highest sensitivity.
B. Increased mechanical tension in the tip link and
associated structures leads to opening of the
transduction channels and incoming cations
depolarize the cell. Local increase of the Ca2+
concentration affect the myosin motors and
result in slippage of the transduction apparatus,
thereby decreasing the mechanical tension and
open probability of the transduction channels.

• Ankle link: connect stereocilia at their proximal ends
• Shaft connectors: present along the mid-region of the stereociliary shaft
• Top connectors: located just below the level of tip link
• Lateral links connect the shaft of one stereocilium to all its neighbors—
• may have a role in holding the bundle together and stabilizes it
• helps stereocilia in a hair bundle to move as a single unit
• Mutations leads to splaying of stereocilia (loss of effective MET) &
causes both Hearing impairment & vestibular disturbances

• Cochlear hair cells are susceptible
to insults from overstimulation,
chemicals, aging and unknown
factors.
• In most situations it is outer hair
calls that degenerate or become
injured.
• These changes are irreversible but
measures have been identified that
can prevent or reduce the
destruction of hair cells.
OTOPROTECTIVE ENDOGENOUS
COMPOUNDS:
• Heat Shock Factors (HSFs) and
Heat Shock Proteins (HSPs)- can
modify the apoptosis response;
• Neurotrophin (glial cell-derived
neurotrophic factor);
• Co-enzymes Q9 and Q10.

ROLE OF IHCs
• IHCs are the sensory cells that convert mechanical stimulation into electrical signals and synaptic activity
transmitted to the brain
• ANSD can be inherited as an AR disease, now called DFNB9 due to the mutation in the gene otoferlin, and
presents with profound behavioural neural hearing loss
• A second AR gene pejvakin- DFNB59

ROLE OF IHCs (contd…)
The mature ribbon synapse between the
sensory IHCs and post-synaptic SGNs
involves the spatial confinement of
several molecular components:
 pre-synaptic density merge to one
single ribbon anchor and,
 post-synaptically, one continuous
elongated post-synaptic density
composed by functional synaptic
AMPA-preferring glutamate
receptors, but also some silent
NMDA receptors.
 Font color indicates association with
the correspondingly colored pre-
/postsynaptic localization.
• IHC-spiral ganglion synaptic
complex
• IHC is connected to all type I
spiral ganglion neurons (SGNs)
forming the radial afferent system
going to the cochlear nuclei
• The OHC synapses with small
endings from type II SGNs,
forming the spiral afferent system
• Molecular composition of a
mature ribbon of the IHC-synaptic
complex ensuring the temporal
precision of peripheral sound
encoding

Genetic syndromic auditory neuropathy affects multiple
nerves, while non-syndromic auditory synaptopathy is
limited to IHC ribbon synapses.
These auditory phenotypes may result from loss of
function, dominant negative effects, or gain-of-function
expression mechanisms.
HEARING RESCUE STRATEGIES may be designed to
target the different levels of disease mechanisms, i.e.,
(i) WT gene addition to rescue loss function phenotype;
(ii) Gene or mRNA editing to correct dominant-negative,
gain-of-function, or loss of function mutations;
(iii) Gene silencing to correct gain-of-function or
expression; and
(iv) NT3 gene or protein addition to enhance the repair
and/or regeneration of auditory synapses and nerve
fibers. The mutated protein shown here is the OPA1
protein involved in dominant optic atrophy (DOA).

A. Schematic illustration of type I SGNs,
bipolar neurons innervating IHCs via
myelinated peripheral projections
Diagram of model Spiral Ganglion
Neuron (SGN) fiber illustrating two
mechanisms of hidden hearing loss
B. Synaptopathy, modeled by
removing IHC-AN synapses
2 mechanisms of hidden hearing loss
are simulated
C. Myelinopathy, modeled by varying the
lengths of the unmyelinated segment (Lu)
or the heminode (Lh)
(B,C) Model peripheral fibers of type I SGNs
(SGN fiber) consist of an unmyelinated segment
at the peripheral end adjacent to the site of IHC
synapses, followed by a heminode and 5 myelin
sheaths with 4 nodes between them.

 Hansen cells, Deiter’s cells,
Pillar cells, Interphalangeal
cells and Border cells
ROLE OF
SUPPORTING
CELLS

GENERAL CHARACTERISTICS OF SUPPORTING CELLS
• The supporting cells provide mechanical support to the
epithelium(hair cells)
• Cell bodies of these cells contact each other & rest on the
basement membrane that underlies the sensory epithelium
• Functionally coupled to each other by large gap junctions
→ act as functional syncytium
• Gap junction not present in HC
Pore forming channel proteins of one cell are in direct contact
with pore forming channel proteins of neighborhood cells
Supporting cells
causes the removal of
excess K+ ions from
the intercellular
spaces of the sensory
epithelium during hair
cell repolarization
The protein subunits that form gap junctions are connexin
family → 6 connexins form a hemichannel / CONNEXON
CONNEXON of two adjacent cells form the communication
pathway between two adjacent cells → allows the passage of
small metabolites, second messenger, ions → connects the
cells both electrically & chemically
2 CONNEXIN ISOFORMS →
CX26, CX30
• Cx26 & Cx30 mutation →
hereditary SNHL
• CX26 mutation → most
common cause of non
syndromic hereditary
deafness
• CX26 is important for
development of organ of corti
• Connexin mutations has less
effect on vestibular system
ROLE OF SUPPORTING CELLS (contd…)

ROLE OF TECTORIAL MEMBRANE
The importance of the tectorial membrane
is illustrated by the fact that mutations of
tectorial membrane genes such as alpha-
and beta-tectorin and otoglein cause
profound hearing loss.

ROLE OF STRIA VASCULARIS
• The stria vascularis is a highly vascularized, multilayered tissue that is
part of the lateral wall of the scala media.
• 3 distinct cell types: marginal, intermediate, and basal cells.
• Tight junctions provide the ionic barriers:
a. At the level of the marginal cells
b. At the level of the basal cells.

ROLE OF STRIA VASCULARIS (contd…)
• The extracellular space in between these two barriers is called the intra-strial
compartment
• The MARGINAL CELLS separate the scala media from the intra-strial
compartment that is filled with the intra-strial fluid
• The BASAL CELLS separate the intra-strial space from the perilymph that
surrounds the fibrocytes of the spiral ligament
• Malfunctions in several K+ channels lead to perturbation of cochlear K*
homeostasis, resulting in hearing impairment

K enters the hair cell via the
MET channels
On its basolateral side K+
released into the
perilymphatic space via K+
channels (KCNQ4)
K+ travel toward the spiral
ligament via the perilymphatic
space and, intracellularly, via
the epithelial gap junctions
Type II fibrocytes in the spiral
ligament take up K+ and
provide a path to the stria
vascularis via the connective
tissue gap junctions

K+ enters basal and
intermediate cells
through gap
junctions with type I
and type II fibrocytes
KCNJ10 (K+ channel)
has been identified
as important for
releasing K+ into the
intra-strial space
Gene encoding
KCNJ10 is
consequently
essential for proper
generation of the
endo-cochlear
potential
K+ is efficiently
removed from the
intra-strial space by
marginal cells, which
actively take up K+
via NKCC1 (Na+/K+/
2Cl-) cotransporters
and by Na+/K+-
ATPases
Marginal cells
secrete K+ into the
scala media via the
KCNQ1/KCNE1 (K+
channel) maintaining
the high K+
concentration of the
endolymph essential
for MET

INNER EAR VASCULAR NETWORK:
(A) Arterial blood supply and venous drainage of the cochlear and vestibular systems.
(B) Partial corrosion casts of the mouse cochlea; The capillaries of the stria vascularis run parallel along
the length of the cochlea.

GENES THAT ALTER COCHLEAR K+
HOMEOSTASIS WHEN MUTATED

Regulation of endolymphatic Ca2+ concentration is also of great
Importance—
a) tip links break at very low Ca2+ concentrations.
b) at high Ca2+ concentrations-the mechanoelectrical transduction channels are blocked.
Cochlear fluid volume regulation is equally important for cochlear
function—
a) LONGITUDINAL FLOW: secretion along the membranous labyrinth with
reabsorption in the endolymphatic duct and sac.
b) RADIAL FLOW: local secretion and reabsorption-via the stria vascularis.

• Under pathological conditions such as an increase or decrease of endolymph
volume, longitudinal flow may be relevant.
• On a cellular level, transmembraneous water movement largely depends on pore-
like water-permeable channels such as AQUAPORINS.
• Lack of aquaporin 4 results in hearing impairment.
• Aquaporin 2 found in the endolymph-lining epithelium of the endolymphatic sac
and is regulated by the hormone vasopressin → Modulation of these channels
→ ↑K+ secretion into the endolymph → ↑osmotic volume movement → typical
hydrops.
• Similar findings with aldosterone → ↑activity of both epithelial Na+ channels
and Na+/K+ ATPase.

FIGURE: INNERVATION OF THE ORGAN OF
CORTI:
• Schematic drawing of afferent and efferent
innervation of IHCs and OHCs.
• Shown from top to bottom are unmyelinated type Il
afferent and myelinated type I afferent fibers,
unmyelinated LOC efferent fibers, and myelinated
MOC efferent fibers.
• The majority (approximately 95%) of the afferent nerve
fibers (ANFs)— thick and myelinated— originate from type
I ganglion neurons— exclusively innervate IHCs.
• The remaining (5%) ANFs— thin, unmyelinated— emanate
from type II ganglion neurons— contact OHCs.
• About a dozen type I ganglion neurons innervate each IHC
(converging innervation pattern), whereas the type II
ANFs divide into multiple branches and contact multiple
outer hair cells (diverging innervation pattern).
ROLE OF SPIRAL GANGLIA

ROLE OF SPIRAL GANGLIA (contd…)
All auditory information is carried to the brain stem by the afferent system
The auditory and the vestibular nerves join each other to form the eighth
cranial nerve (the vestibulo-cochlear nerve)
Efferent fibers originate in the brain stem from neurons located in the
superior olivary complex and send information to the cochlea by synapsing
with OHCs as well as with afferent fibers beneath IHCs
The efferent system allows the central nervous system to modulate the
operation of the cochlea
The innervation pattern of the organ of Corti clearly underlines the
functional differences of the two types of cochlear hair cells

Finally, activity of the
MOC and the LOC
efferent systems seem to
have protective effects
against acoustic injury
and such a feedback
could be important in
loud noise
environments

SOUND EVOKED ELECTRICAL POTENTIALS IN THE COCHLEA
ENDOCOCHLEAR
POTENTIAL
This resting
potential of
+80 mV
Direct current (DC)
is recorded from
scala media
Acts as a battery and helps in
driving the current through the
hair cells when they move after
exposure to any sound stimulus
COCHLEAR
MICROPHONICS
(CM) (AC CURRENT)
Flow of K+ through
the OHCs produces
voltage fluctuations
Absent in the part of
cochlea where the
OHCs are damaged
SUMMATING
POTENTIAL (SP)
(DC)
Either negative or
positive
Probably arises from HCs
with a small contribution
from OHCs
Helps in the
diagnosis of
Meniere's disease
COMPOUND
(AUDITORY NERVE)
ACTION POTENTIAL
(AP)
Neural discharge
of auditory nerve
Follows all or
none phenomena
 CM and SP differ from APs:
(i) they are graded rather than all or
none phenomenon,
(ii) have no latency,
(iii)are not propagated, and
(iv)have no post response refractory
period.
Cochlear potentials
recorded from the human
ear are known as the
electrocochleogram
(ECochG) that comprises
all the sound evoked
cochlear potentials (SP, CM,
and AP).

PATHOPHYSIOLOGY OF COCHLEAR HEARING LOSS
LOSS OF
COCHLEAR
FUNCTION
Drop in auditory
sensitivity
Loss of frequency
selectivity
Drop in complex sound
appreciation and analysis
Loss of
perceptual
streaming
Loss of fine-
tuning of the
acoustic
Loss of non-linearity amplification and
compressive properties
Loss of temporal pattern of sounds due to the loss of spatial
discrimination and the ability of the cochlea to distinguish b/w
closely following frequencies and intensities
STRUCTURAL
ABNORMALITY
Dysplasia/ aplasia (congenital maldevelopment,
trauma or a space occupying lesion)
ABNORMAL
METABOLIC ACTIVITY
Cochlear ionic transport as a result of either a
genetic syndrome or an acquired condition (e.g.
problems with glucose or thyroid metabolism)

PATHOPHYSIOLOGY OF COCHLEAR HEARING LOSS (contd…)
VASCULAR CHANGES
Cochlear vascular afflictions
Compromise in stria
vascularis function
A diminution in cochlear
nutrition and hypoxia
Sluggish vascular flow or
complete obstruction of the
cochlear arterial blood flow
Endolymphatic hydrops will
lead to a compromise in
function of the stria
vascularis
OVERLOADING THE BM
Due to local or systemic causes (e.g.
a metabolic syndrome such as
diabetes, hyperlipidemias, iron
overload, inflammation or
autoimmune conditions)
Prevent OHC motility and
IHC loss of transduction.

INFECTION AND
INFLAMMATION
Disruption of the
biochemical pathway
of cochlear function
Accumulation of toxic
lipid-derived
substrates
Hamper cochlear
function
GENETIC
MUTATIONS
Genes encoding for numerous proteins
involved in cochlear function
Cochlear hearing loss
BIOCHEMICAL
PATHWAY
ABNORMALITIES
Noise Trauma,
Ototoxicity and
cumulative wear and
tear action in
Presbycusis
Biochemical cascade
loses its balance
Apoptosis and cell
death

BIOCHEMICAL PATHWAY/
ENZYMATIC CASCADE
FOR HAIR CELL DAMAGE

Eighth Nerve
Cochlear Nucleus
Olivary Complex
Lateral Lemniscus
Inferior Colliculus
Medial Geniculate Body
Auditory Cortex

Pathway begins from hair cells of the organ of corti and terminated at auditory cortex
The area of cortex, concerned with hearing is situated in the superior temporal gyrus
(Brodmann's area 41)
1st order neuron
Mitral cells of Spiral
Ganglion
2nd order neuron
Cochlear nuclei (Dorsal,
Ventral)
3rd order neurons
Superior Olivary Nucleus +
Nucleus of Lateral Lemniscus

3rd order neurons
do not cross the
midline and
terminate at
Internal Capsule
Some fibres from
midline to terminate
at the MGB in
thalamus and some
terminate at
ipsilateral MGB
Fibres from MGB
go to auditory
cortex through
auditory radiation
Fibres from MGB
go to auditory
cortex through
auditory radiation
Ventral cochlear
nucleus of 2nd
order neurons has
4 groups of fibres
1st group of fibres
C/L SON through
trapezoid body
2nd group of fibres I/L SON
3rd group of fibres C/L nucleus of LL
4th group of fibres I/L nucleus of LL
Dorsal cochlear
nucleus has 2 sets
of fibres
1st group of fibres C/L nucleus of LL
2nd group of fibres I/L nucleus of LL
1st order neurons
arise from two sets
of fibres
enter MO
enter dorsal and
ventral cochlear
nucleus

PHYSIOLOGY OF HEARING: SOUND TRANSMISSION AND PROCESSING

PHYSIOLOGY OF HEARING: SOUND TRANSMISSION AND PROCESSING

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PHYSIOLOGY OF HEARING: SOUND TRANSMISSION AND PROCESSING