The document discusses the physiology of hearing. It describes the requirements for normal hearing including adequate stimulus, conduction to the sensory organ, transduction at the organ, neural transmission, and central processing. It then discusses the anatomy and functions of the external ear, middle ear, and inner ear. The external ear collects and conducts sound waves. The middle ear acts as an impedance matcher and lever system to efficiently transmit vibrations from the eardrum to the inner ear.

1. PHYSIOLOGY OF HEARING
DR AYAN MAITI

2. HEARING IS IMPORTANT FOR
• Communication: hearing is essential to language
• Localisation: determination of location of unseen sound
sources
• REQUIREMENTS FOR NORMAL HEARING:
1. Adequate stimulus (sound)
2. Conduction of stimulus to sensory organ of hearing
3. Sensory transduction of stimulus at organ of hearing
4. Neural transmission of the signal
5. Central auditory processing of the signal at brain

3. SOUND
• Sound is vibratory energy that is transmitted from the source through surrounding
media in the form of pressure waves.
• The speed of sound depends on the medium through which the wave passes.
• In the context of otolaryngology, the most relevant physical medium is air since
most sounds reach the ear by the vibrations of air molecules
• Speed of sound in air is 343m/s in water is 1482m/sec

4. PROPAGATION OF SOUND WAVE
1. Sound wave is propagated from source through alternate compression and
rarefaction of air molecules
2. Relative to the ambient pressure, a localized pressure increase is produced and
this pressure increase is known as compression.
3. as the molecules move further to the left they are drawn increasingly further
apart from their adjacent air molecules resulting in pressure drop below the
ambient pressure and this is called rarefaction
4. Since air is a continuous and elastic medium, the pressure variations will
propagate away from the source such that sound generated by the source can be
detected in remote area: This is known as acoustic radiation.

5. TECHNICAL TERMS
Amplitude/loudness
• Strength of the sound
• Loudness denotes the
appreciation of sound
intensity
• Expressed in decibel
(dB)
• dB = 10 log10 J/Jr,
where J is the intensity
of the sound of
interest, and Jr is the
intensity of reference
Frequency/pitch /tone
• Number of cycles per
second
• • Pitch /Tone denotes
the appreciation of
frequency
• • Expressed in
Hertz(Hz)
Impedance •
• Resistance offered by a
medium to sound
waves

6. EAR ACTS AS A TRANSDUCER

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

8. THE PERIPHERAL EAR
• The peripheral ear consists of the pinna, the external auditory meatus and canal
• THE PINNA
• The pinna is vestigial in man, with non-functional auricular muscles
• The pinna and the external auditory meatus and the canal develop from similar
morphological structures in utero.
• In terms of acoustic function, although the pinna is vestigial, when coupled with
the external auditory meatus and canal, the unit acts to provide frequency specific
resonance of sound incident on the ear to make up for the impedance
encountered at the air–fluid interface between the cochlea and the middle ear

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

10. SOUND COLLECTION
• There are numerous anatomical ridges and convolutions in the pinna and
pinna acts like a funnel
• Sound waves from either the horizontal or the vertical direction are
reflected from these ridges and enter the ear canal with the original non-
reflected incident sound
• 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

11. • Pinna works best as a reflector above 1 kHz.
• Some of the reflected waves enter the canal and may interfere with the original
waves if they are out of phase, noted particularly between 7 kHz and 10 kHz
where they produce a pinna notch.
• PINNA NOTCH:
A. Due to its anatomy, the pinna largely eliminates a small segment of the frequency
spectrum; this band is called the pinna notch
B. For low frequencies, it behaves similarly to a reflector dish, directing sounds
toward the ear canal
C. For high frequency,some of the sounds travel directly to the canal, others reflect
off the contours of the pinna first: these enter the ear canal after a very slight
delay.
D. This delay causes phase cancellation, virtually eliminating the frequency
component whose wave period is twice the delay period.
E. In the affected frequency band – the pinna notch – the pinna creates a band-stop
or notch filtering effect
• The frequencies at which the reflected and incident waves boost each other are
2–3 kHz, roughly at frequencies which possess wavelengths of four times the
length of the pinna.

13. 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
• THE HEAD SHADOW: Cues for sound localization from right/left
1. Sound wave reaches the ear closer to sound source before it arise in
farthest ear
2. Sound is less intense as it reaches the farthest ear because head act as
barrier
The different changes of transfer function from the pinna to the external
auditory canal ultimately delivered to the cochlea through the middle ear &
encoded accordingly to be finally perceived by the auditory cortex as
directionality of sound or a spatial representation of sound
A pathology in the pinna may therefore interfere with this localization and
directionality process

14. THE EXTERNAL AUDITORY CANAL
• The external auditory canal (EAC) commences at the conchal opening called the
external auditory meatus (EAM),includes the ear canal itself and ends at the
tympanic membrane lateral surface, which separates it from the middle ear
• It is made up of an outer cartilaginous part and an inner bony part with two
constrictions, one at the junction of the distinct anatomical parts and one 5 mm
before the tympanic membrane in the bony part
• It can be considered as a tube which is open at one end
• The EAC has both acoustic and non-acoustic functions

15. ACOUSTIC PHYSIOLOGY OF EAC
• The most important physiological function of the EAM and the EAC is to ‘transfer’
the acoustic signal from a sound field to a much smaller area, i.e. the tympanic
membrane.
• “If a tube is closed at one end and opened at the other end, then sound
introduced at the open end will show distribution of sound pressures along the
tube depending upon the frequency of the sound and length and diameter of the
tube, there will be no amplification only redistribution of energy in the form of
resonant peaks”
• The ear canal can be considered as a resonating tube following the quarter-wave
principle for resonance

16. • At a frequency with a wavelength four times the length of the canal, the canal and
the pinna together will resonate and vibrate with the incoming signal to augment
or magnify the incident acoustic signal given by the equation:
f = c/4l.
[where f is the frequency, c is the speed of sound and I is the length of the canal]
• TOTAL GAIN:The resonant frequency which enhances the incident sound by 10–20
dB is generally 2.7 kHz with a range of 2–7 kHz
• Signals below 1 kHz are generally resonated by the head and the torso whereas
frequencies above 3 kHz are augmented by the pinna

17. CLINICAL ASPECT
• The EAC is subject to variation in size and may grow from the neonate until the age
of 7–9 years.The transfer function may change during this period
• During fitting hearing aids, it must be calibrated accordingly by referring to the
normal range
• the resonance amplitude changes at old age. Since prescribed hearing aid function
is significantly dependent on external ear canal function, this change of resonance
must be taken into account to prevent inappropriate amplification
• In disorders of the EAC ranging from dysplasia to occlusive pathologies (e.g. wax or
tumour), hampers the external canal resonance gain and causes some conductive
hearing loss
• This hearing loss is usually concentrated around the area of maximum resonance,
i.e. at about 2.7 kHz
• In cases of occlusive hearing aid moulds, this loss must be incorporated in the
prescription
• Complete occlusion of the ear canal by impacted wax can lead to a hearing loss of
40 dB

18. • EAC and the pinna acting as a unit are also involved in providing
monoaural and binaural cues for localization of sound
• The EAC pressure gain and transfer factor changes with directionality of
the incident signal
• It happens especially above its resonant frequency and can be as much as
30 dB in both the horizontal and vertical planes
• standing wave phenomenon:
1. A standing wave is created when two sounds of the same frequency
travel in opposite directions in the canal (e.g. when the same sound is
reflected by the tympanic membrane) and cancel each other out.
2. For sounds of high frequencies {8kHz}, a standing wave may be produced
which might generate a spurious hearing deficit

19. THE MIDDLE EAR
• The middle ear in the human acts as an efficient passive and linear transformer to
conduct acoustic energy from the tympanic membrane to the stapes footplate at
the oval window and to the cochlea
• consists of the tympanic membrane, the ossicular chain of the malleus– incus–
stapes complex along with the middle ear muscles.

20. THE MIDDLE EAR SPACE
• The middle ear is a closed cavity.
• The greatest transfer function of the middle ear is at the resonant frequency
between 1 and 3 kHz and is determined by the mass and the stiffness of the
system.
• The stiffness or elasticity of the middle ear is determined by
1. the tympanic membrane,
2. the ossicular ligaments and
3. the middle ear cavity compression of the air column
• the mass is governed by the ossicles
• The stiffness factor limits passage of low-frequency sounds while the opposite is
true for the mass factor.

21. THE TYMPANIC MEMBRANE
• middle ear mechanism delivers acoustic energy from a relatively large area of the
tympanic membrane to the much smaller area of the stapes footplate. The ratio is
about 14 : 1
• The canal side of the membrane exhibits a uniphasic response with little pressure
variation and is relatively uniform
• The middle ear side is multiphasic with significant pressure variations.
• There is also a destructive interference due to reflection of the sound waves by the
membrane that creates a pressure difference across the membrane
• The reflectance of the membrane is high at the lower frequencies below 1 kHz
while it is lowest between 1 kHz and 4 kHz, which means that between these
frequencies maximum energy is delivered to the cochlea and the middle ear is
most efficient

22. • The incident sound from the EAC hits
the membrane and sets up a
travelling wave, which is mainly
collected at the rim of the
membrane.
• This is then conducted to the umbo
and coupled to the manubrium of
the malleus
• The middle layer of TM has a
complex arrangement of radial and
circular fibres
• Shape of TM:
1. although it is concave towards the
EAC, it is actually like a loudspeaker
cone i.e convex in each segment
from annulus to malleus handle.
2. TM buckles as it moves to and fro
3. This buckling factor helps in
Impedance Matching as the sound
energy absorbed by the middle
fibrous layer, in turn, transferred to
the malleus handle

23. • In disorders of the tympanic membrane (e.g. tympanic atelectasis or perforations)
the multiple lever action is compromised, leading to a hearing loss, and a
myringoplasty cannot restore this feature of the membrane in all cases
• Tympanosclerosis : hearing is preserved in spite of a loss of action of the
membrane, presumably because the preferential distribution of the acoustic
energy to the oval window from the middle ear is intact due to preservation of
some structural integrity
• when the rim of the membrane is intact (i.e. central perforations), hearing is
better preserved than when the margins are affected as energy transfer from the
canal to the membrane is via the rim of the membrane
• PERFORATED TM:
1. the volume velocity of the sound waves (the volume of vibrating air particles
conducting thesound per unit time) is not preferentially distributed to the stapes–
oval window interface
2. a part of it is absorbed or shunted by the middle ear cavity
3. The main reason for a hearing loss is the reduction in pressure across the
tympanic membrane due to this shunt and preclusion of the multilever action
4. The degree of the loss is proportional to the size of the perforation and inversely
proportional to the frequency, i.e. the greatest loss is at lower frequency.
5. because reflectance of the membrane, which is an important factor for the
pressure difference across the membrane, is lost or diminished at lower
frequencies,

24. THE EUSTACHIAN TUBE
• The Eustachian tube opens to supply air into the middle ear cavity
• The tubal opening is best demonstrated at 6–8 kHz, i.e. high frequency sounds
• high-frequency sounds are the most important consonant-containing sounds, the
middle ear must act in the most efficient way to conduct these sounds, which
should be close to its resonant frequency and unaffected by the damping effect
of reduced air supply
• In Eustachian tube dysfunction where the middle ear cavity is deprived of air, as
in early stages of otitis media with effusion (OME), the stiffness of the system is
augmented, which leads to a low-frequency hearing loss.
• With further progression, fluid collection in the middle ear cavity increases the
mass of the system, which generates a high-frequency hearing loss

25. THE MIDDLE EAR CAVITY
• The reflections of the acoustic signal from the walls of the middle ear cavity may
contribute to pressure differences across the tympanic membrane
• may provide an additional drive to the membrane function.

26. 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

27. IMPEDANCE TRANSFORMER
1. Impedance is defined as the resistance offered by a medium for transmission of
sound
2. middle ear acts as impedance transformer
3. Cochlear fluids have an impedance equal that of sea water (1.5X10 N.sec/m3)
4. Middle ear ossicles are suspended by ligaments
5. Axis of rotation of ossicles and axis of suspension by ligaments virtually coincides
with their centre of inertia
6. At low frequencies the ligaments play an important role in maintaining ossicular
positions(elastic effect)
7. 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

28. 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

29. 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
1. Middle ear efficiency is the best at 1kHz
2. There is transmission loss of low frequency sounds due to elastic
stiffness of middle ear ligaments(annular ligament is the most
important)
3. Air inside middle ear cavity also dampens low frequency sound
transmission
4. Grommet insertion improves transmission of low frequency sounds

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

31. 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

32. 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

33. C) SHAPE OF THE TYMPANIC MEMBRANE
• TM acts as a mechanical lever by buckling
• 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 but Functional area of tympanic
membrane is two third (69mm2).Area of stapes footplate is 3.4mm2.
• So , Effective areal ratio is 14:1
A. The combined effects of the area ratio and the lever ratio give the middle ear
output a 28-dB gain theoretically.
B. 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
C. Total transformer ratio=14x1.3=18.2:1

34. THE MIDDLE EAR WINDOWS & PHASE DIFFERENTIAL
EFFECT
• The middle ear is characterized by
two natural windows or real
connections between the middle and
the inner ear.
• The oval window articulates with the
stapes footplate while the round
window is covered by the secondary
tympanic membrane.
• The pressure difference between the
oval and the round windows is
fundamental for the cochlear
travelling wave which drives cochlear
function

35. • 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
• Disorders of the oval window or in the
vicinity (e.g. stapedial or cochlear
otosclerosis), congenital absence of the
oval window will lead to a loss of this
preferential distribution and have a
significant effect on auditory sensitivity.

36. THE THIRD WINDOW
• In addition to the two natural windows, virtual third windows may exist
for some absorption of the sound from the middle ear thereby affecting
the preferential stapes-oval window sound distribution.
• This includes the vestibular aqueduct and the bony skull itself as well as
the ossicular inertia, all of which may shunt away some of the energy
• A pathological third window which can be a real one (perilymph fistula,
X-linked gusher syndrome i.e., real connection between the inner and
middle ear) or a virtual one (dilated vestibular aqueduct or semicircular
canal dehiscence i.e. no direct communication between the inner and
middle ear) also absorbs or shunts part of this preferential distribution.
• The cochlea might in part be directly stimulated by the vibrations in the
third window itself.
• Measured air-conduction thresholds therefore decrease and bone-
conduction (BC) thresholds increase, generating a spurious conductive
hearing loss or ‘false’ or air–bone gap
• improved BC can also be explained by the fact that the third window
generates a pressure drop across the two natural wind

37. 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

38. 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

39. ATTENUATION 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.
• thereby cause the entire ossicular system to develop increased rigidity, thus
greatly reducing the ossicular conduction of low frequency sound
• function of the middle ear muscle reflex pathway appears to be protective;
• electromyographic recordings of tensor tympani muscles have shown minimal
electrical activity in response to sound presentation
• Patients who have a paralyzed stapedius muscle from facial palsy or stapes surgery
but intact tensor tympani function are absent middle ear muscle reflexes

40. STAPEDIAL REFLEX
• Intense, low-frequency sound, or broadband noise, presented to the
ipsilateral (black pathways) or contralateral ears (gold pathways) or to
both ears can activate contraction of the ipsilateral stapedius muscle.
• Following transduction of the auditory signal by the hair cells of the
cochlea, the action potential is propagated along the auditory nerve
(AN) and activates unidentified interneurons located in the ventral
cochlear nucleus (CN).
• Interneurons, either directly or indirectly, project from the cochlear
nuclei to the stapedius motoneurons (black and gold arrows).
• Interneurons from the ipsilateral or contralateral CN synapse on
stapedius motoneurons (black and gold terminals).
• Efferent motor projections that originate in stapedius motoneurons
terminate on the stapedius muscle.

41. • Acoustic impedance measurements have proven that the stapedius is the
primary sound-evoked middle ear muscle
• Two major functions of the stapedius reflexes
• 1) modulation of middle ear impedance and attenuation of acoustic
energy that reaches the cochlea.
• 2) high-pass filtration of low-frequency sound (background noise) to
prevent masking of speech frequencies
• The stapedius muscle also contracts in response to internally or self-
generated vocalization and thus it may serve to prevent self-stimulation.

42. 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.

43. 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

44. • 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

45. ENDOLYMPH
• Actively pumped by stria vascularis in the scala media
• It is rich in potassium, low in sodium and has negligible calcium.
• Endolymphatic sac maintains homeostsis of endolymph
• Endolymph has positive potential gradient +50-120mv(endocochlear potential)
• Na-k ATPase is responsible for this gradient

46. PERILYMPH:
• lt is ultrafiltrate of blood plasma and the CSF, rich in sodium and low
in potassium and calcium
• Occupies perilymphatic space. continuous between vestibular &
cochlear divisions
• Ionic concentration resembles extracellular fluid
• Electric potential of s.tymapani is +7mv and s. vestibuli is +5mv

47. BASILIAR MEMBRANE
• Separates s.media from s .tympani
• Length 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.

48. 1. 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.
2. 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.
3. Hair cells contain stereocilia along the apical surface and are connected by tip
links.
4. 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

49. 1. The scala vestibuli and the scala tympani are filled with perilymph, which has a low
potassium concentration.
2. The scala media is filled with endolymph, which has a high potassium concentration.
3. 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.
4. The maintenance of such a large electrochemical gradient is performed by the stria
vascularis

50. 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.
• furthermore the hair cells have a
negative intracellular potential of
-70mv wrt the perilymph but -150mv
wrt endolymph at their upper
surfaces where the hair cells project
through the reticular lamina and into
the endolymph

51. THE POTASSIUM CYCLE
• The main driving ion for cochlear function as furnished by different cochlear
processes is potassium.
• The way it is recycled is called a potassium cycle, responsible for the endocochlear
potential.
• The stria vascularis has 3 cell types:
• the marginal cells: related to the medial scala media, responsible for maintaining a
low potassium composition in the intrastrial space by continuous active uptake of
the ion
• the intermediate cells which have the marginal cells medially and the basal cells,
laterally, are connected to the basal cells by gap junctions regulated by the
connexin
• the basal cells: with the intermediate cells in their medial end and laterally
connected to the spiral ligament in the lateral wall of the cochlea by gap junctions
as well

52. • potassium from the blood is actively taken up by the fibrocyte of the spiral ligament
and pumped to the basal cells which in turn deliver the ion to the interstitial cells
• These cells present the ion to the intrastrial space from where they are taken up by
the marginal cells.
• The scala media receives its ions from the marginal cells.
• Regulated by various enzymatically driven potassium channels and Na–K ATP
systems; the end result is maintenance of a high potassium ionic composition in the
scala media
• Hyperacoustic stimulation depresses the potassium cycle for protecting the cochlea
and actually leads to a drop in the endolymphatic potential and stimulation of the
P2X by the ATP pathway, which inhibits OHC motility
• Genetic mutations in the connexin family or the potassium transport family interfere
with maintaining endocochlear potential
• connexin 26/30 hearing loss, the commonest genetic autosomal non-syndromic
prelingual genetic hearing loss, and Jervall–Lange–Nielsen syndrome with long QT
interval where the KCNQ1 ionic transport gene in the stria vascularis and cardiac
conductive system is deficient

53. 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
starts putting energy into the
wave .the amplitude raises
rapidly only to fall rapidly

54. 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

55. • 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.
• There are therefore three different travelling waves generated: the wave as a result of the
pressure difference of the two compartments, the wave as a result of the mechanical
displacement of the BM, and the acoustic energy wave which displaces the cochlear fluid
• 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.

56. • The travelling wave propagates aided by the gradual diminution of the
thickness and stiffness of the basement membrane from base to apex
• As it propagates, it is acted upon by numerous critical oscillators, the
characteristic frequency of which is specific to a particular region of the
BM
• These oscillators move the BM in addition to the travelling wave by
expending active energy and are coupled with OHCs in the organ of Corti.
• The oscillators become active when they compress or modify this signal
and passive when they allow the signal to pass.
• There is a critical point at which these may cancel each other out called
the Hopf bifurcation
• In order to prevent this, the oscillators must possess an autoregulation
process or a self-tuning property
• the critical oscillation function and the compressive function of the OHC
are responsible for tuning the BM in response to an acoustic signal, which
is variable along the length of the BM and is spatially represented,
• the acoustic output, which is the end result of the BM function, is a non-
linear output.
• In disease processes, the non-linearity may become linear and can be
measured in the growth function of distortion-product otoacoustic
emissions.

57. COCHLEAR TUNING CURVE
• The cochlear travelling wave reaches a maximum
displacement somewhere along the BM following
which it starts to dissipate.
• 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 .
• The human ear has a dynamic range up to 120 dB
which inherently dictates that very high intensity
signal must undergo modification at the cochlear
level without damaging the cochlea and need to be
compressed.
• Compression is achieved by the OHCs to generate the
tuning curve which is essential for maintaining the
integrity of the BM and increasing its stability in the
presence of high-intensity stimuli
• For lower-intensity signals, the OHCs amplify the BM
response by mechanical elongation/compression of
their cell bodies which sharpen the tuning curve

58. • Another property of the cochlea is its filtering action which is
indirectly dependent on the tonotopicity
• the characteristic frequency is where other neighbouring
frequencies are filtered so as not to interfere with the frequency
selectivity
• This is essentially a central auditory function contributed by the
ability of the cochlea to discern a rapid sequence of sounds coming
from a single or a multiple source
• When the sounds are close together in frequency, fusion may
occur and the perception will be of a single sound as the filtering
action becomes less.
• The tuning curve loses its sharpness and frequency selectivity is
compromised in cochlear pathologies
• some genetic losses involve the lower-frequency sensitivity
• while ototoxicity affects the high-frequency sensitivity.

59. 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.

60. INNER HAIR CELLS
• The IHCs are the true sensory end organs for
hearing
• generates the action potential conducted to
the type 1 spiral ganglion cells(myelinated
and well developed synapses)
• 95% of afferent auditory nerves make
contact with inner hair cells
• Detects basilar membrane movement
• Inner hair cells respond to the velocity
rather than displacement of basilar
membrane
• The IHCs receive the output from the OHCs
through the modified movement of the BM.
• it delivers the eventual output of cochlear
function,in the form of coding to the
cochlear nerve: namely frequency coding
and intensity coding

61. OUTER HAIR CELLS
• The OHCs are cylindrical with
bundles of stereocilia composed of
actin filaments which project at their
apical ends in the scala media
• The action potential is transmitted to
the type 2 spiral ganglion cells, which
are non-myelinated and smaller
• They show reciprocal synapses with
their OHC counterpart,i.e. the type 2
cells feed back to the OHC providing
a closed loop neuronal circuit for
bidirectional signalling and reverse
transduction
• Very few outer hair cells synapse with
auditory nerves
• Its job is to provide a motor for
altering OHC physical dimensions for
further displacement of the BM.They
serve to amplify basilar membrane
vibration

62. • The OHC exhibits the special feature of electromotility,
which is a highly sensitive & is responsible for fine-tuning of
the acoustic sign
• this motility is driven by two forces: a voltage-dependent
mechanotransduction that moves the hair bundle with an
active movement and a somatic non-linear capacitance
prestin-mediated motility which modulates the stiffness of
the stereocilia and alter their size
• The action of prestin is voltage dependent and results in
either contraction or elongation of the OHC necessary for
augmenting the acoustic signal incident on the BM.

63. 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.
• The tops of the shorter
stereocilia are attached by thin
filaments to the back sides of
their adjacent longer stereocilia
TIP LINK

64. • 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

65. 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 cationic channels,
allowing rapid movement of potassium ions from the surrounding scala media
fluid into the stereocilia, which causes depolarization of the hair cells

66. • 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.

67. • AUDITORY NERVE FIBRES
• originates from the joining of the spiral
ganglion cells in the cochlea where
their cell bodies lie.
• There are two types of nerve fibres :
• type 1: large diameters, innervate the
IHCs & constituting 95% of the nerve
fibre population and are myelinated,
• type I spiral ganglion cells are three
categories:
1. high spontaneous activity: >18
spikes/sec
2. medium spontaneous activity:0.5 to 18
spikes/sec
3. low spontaneous activity :<0.5
spikes/sec
• high spontaneous neurons have a
larger diameter, and low spontaneous
neurons have a smaller diameter.
• type 2 fibres: smaller diameter,
innervate the OHCs and are
unmyelinated
• FREQUENCY CODING AT AUDITORY
NERVE
• Phase locking:
1. The discharges of the fibres to low-
frequency sounds occur at times, in
other words, there is a phase locking
mechanism which occurs up to 5 kHz
2. Sound stimulus, transmittor release
and action potential generation
occur in synchrony (phase locking)
3. It is important to convey temporal
information of the incoming signal.

68. 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
superior
olivary
complex
nucleus of
lateral
lemniscus
cochlear
nuclei
Inferior
colliculus
medial
geniculate
body
auditory
cortex
Cochlear nucleus
1. The critical first relay station for all
ascending auditory information
2. located in the pontomedullary
junction.
3. major subdivisions: the dorsal
cochlear nucleus, the anterior
ventral cochlear nucleus, and the
posterior ventral cochlear nucleus.
4. Each subdivision has a restricted
population of cell types.
5. The second-order neurons of the
cochlear nucleus are tonotopically
organized

69. • 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 brainstem to terminate in the
superior olivary nucleus
• From the superior olivary
nucleus,three fiber tracts project
auditory information to the
contralateral inferior colliculus:
the dorsal stria, also called the
stria of Monaco; the
intermediate stria,also called the
stria of Held; and the ventral
stria, also known as the
trapezoid body. These fiber tracts
collectively form the lateral
lemniscus

70. • 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.

71. • 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.

72. • THE MEDIAL GENICULATE BODY
• It 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
corresponds to Brodmann's area 41

73. • 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

74. FUNCTIONS OF AUDITORY CORTEX
• Perception of sound
• Judging the intensity of the sound
• Analysis of different properties of sound

75. 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.

76. 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 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.

77. • THEORIES OF HEARING
• Place theory of Helmholtz
• Temporal theory of Rutherford
• Volley theory of Wever
• Place theory of Lawrence
• Travelling wave theory of Bekesy

78. 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.

79. TELEPHONIC THEORY
• Rutherford proposed that entire cochlea responds as a whole to all
frequencies instead of being activated on a place 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

80. 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

81. PLACE VOLLEY THEORY
• Proposed by Lawrence
• Combines both volley and place theory
• This theory thus attempts 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

82. 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

83. CENTRIFUGAL INNERVATION OF COCHLEA
• The cochlear efferent system consists
of projections from both the lateral
olive and the medial olivary complex
which synapse mostly with type 1
spiral ganglion cells and type 2 spiral
ganglion cells respectively
• The efferent fibres are carried by the
inferior vestibular nerve
• The ratio of efferent to afferent fibres
in the OHC is 1 : 2 whereas those in
the IHCs is 1 :7
• The medial system innervates both
ears while the lateral system supplies
only the ipsilateral cochlea

84. • The cochlear efferents serve an important function by virtue
of their modulation of inhibitory and excitatory
neurotransmitter release thus helps in cochlear protection
from loud noise.
• The activation of the efferent system modifies frequency
specific gain at the tonotopic BM by acting on the voltage-
dependent OHC motility and attempts to linearize the
signal with a damping effect
• OTHER FUNCTIONS:
1. fine perception of the acoustic signal for localization,
2. improving the signal-to-noise ratio and
3. supporting adaptation and frequency selectivity

85. THANK YOU