This document discusses the anatomy and physiology of the inner ear and hearing. It begins by describing the gross anatomy of the inner ear, including the cochlea, vestibule, semicircular canals, utricle and saccule. It then discusses the microscopic anatomy of the cochlea, including the organ of Corti, basilar membrane, hair cells and stereocilia. Next, it covers the fluids of the inner ear, blood supply and the physiology of sound conduction through the external ear, middle ear and inner ear. It concludes by describing mechanotransduction in the hair cells and how this leads to neural transmission of sound.

1. INNER EAR ANATOMY
&
PHYSIOLOGY OF HEARING
MODERATOR - DR SHANTANU MANDAL
CO MODERATOR – DR ANKIT PARASHER
PRESENTER – DR PRASANN RASANIA

2. INNER EAR ANATOMY

3. INNER EAR Based on function
 AUDITORY –
 Cochlea - 2.5 turns around central axis
 VESTIBULAR –
 1. Vestibule – Central part of the labyrinth
 Utricle
 Saccule
 2. 3 Semicircular canals
 Superior
 Posterior
 Lateral
Direction of view

4. COCHLEA auditory labyrinth
 GROSS ANATOMY – Cross Section
 Formed of 3 parallel canals coiled in a spiral around a
central ‘stalk’ the modiolus
 Divided into three compartments :-
 Scala Vestibuli
 Scala Media
 Scala Tympani

5.  Basilar Membrane –
 Thin sheet extending from spiral lamina to spiral ligament of
cochlea.
 Movement of the basilar membrane occurs by pressure
changes induced by stapes footplate motion at the oval
window
 Reissner's membrane –
 From soft tissue ridge of spiral lamina to the spiral ligament,
lined by thin squamous peri lymphatic cells
 Tectorial membrane –
 Extracellular structure that overlies both the inner and outer
hair cells.
 Only the tallest stereocilia of the outer hair cells are directly
embedded into the underside of the tectorial membrane.
 Attached on its inner edge to the spiral limbus and is loosely
connected to the supporting cells such as the Heusen's cells by
means of microscopically visible projections called trabeculae.

6. Cochlea cross-section
 Scala Media
 The central canal is filled with endolymph
 Bind coiled tube, connected to the saccule via ductus reunions.
 Triangular in shape and bounded by three walls
 Floor – Organ of corti running along the basilar membrane
 Lateral wall – Formed by Stria Vascularis
 Roof – Reissner’s membrane

7. Cochlea cross-section
 Scala Tympani –
 lowermost channel
 Underneath the basilar membrane
 Terminates at the round window
 Connected with subarachnoid space through the aqueduct
of cochlea
 Scala Vestibuli –
 uppermost channel
 Located above the Reissner’s membrane
 Continuous with vestibule and closed at oval window by
stapes footplate

8. INNER EAR FLUIDS
 Perilymph –
 Rich in sodium ions and fills the space between
bony and membranous labyrinth
 It communicates with CSF (subarachnoid space)
through aqueduct of cochlea
 Two views regarding formation of perilymph.
 It is a filtrate of blood serum and is formed by
capillaries of spiral ligament.
 It is a direct continuation of CSF via aqueduct of
cochlea.

9. INNER EAR FLUIDS CONTD
 Endolymph –
 It fills the entire membranous labyrinth and is rich in K ions .
 It is secreted by secretory cells of stria vascularis of the
cochlea and the dark cells of macula
 Regarding its flow –
 Endolymph from cochlea reaches saccule via Ductus Reuniens ,
and endolymphatic duct and gets absorbed through
endolymphatic sac.

11. ORGAN OF CORTI
 End organ of hearing
 Ridge of cells resting on the basilar
membrane and overlaid by tectorial
membrane
 Has 2 special types of cells:
 Outer hair cells
 Inner hair cells
 Others:
 Deiter cells
 Cells of hensen
 Cells of claudius

12. Organ of Corti contd.
Outer hair cells
 More in number ( 12 – 14k )
 More in rows ( 3 – 5 )
 Late development
 Amplify sound
 More sensitive to Ototoxic drugs
 More sensitive to Acoustic trauma
 Produce Oto-acoustic Emissions
 Mainly efferent fibres supply
Inner hair cells
 Less in number ( 3500 )
 Less in rows ( single )
 Early to develop
 Mechanoelectrical transduction
 Less sensitive
 Less sensitive
 Doesn’t produce
OAE
 Mainly afferent fibres
supply

13. Hair Bundles
 Consists of Actin-filled stereocilia
 Arranged asymmetrically in a staircase fashion of increasing height
 With a single ‘kinocilium’ located behind tallest stereocilia
 Hair bundles are linked to each other by ‘Tip links’
 They are Mechano-sensitive organelles of Hair cells
 Every hair cell sits over a phalangeal supporting cell,
i.e. Deiters Cells for OHC’s

14. Hair cells
 Tip links - the tops of the shorter stereocilia are attached
by thin filaments to the back sides of their adjacent longer
stereocilia
 Made up of Cadherin 23 and protocadherin 15
 The hair bundles of IHC’s are organized as a smooth
curved line of 2-3 rows of stereocilia & OHC’s stereocilia
bundles are arranged in a shallow V-shape.

15. Inner Ear contd.
Ampulla – Opening of the semicircular
canals (5)
CRISTAE AMPULLARIS - Ampulla
consists of hair bundles enclosed in a
gelatinous material ( Cupula ) and
further attached to 8th nerve.

16. CRISTA AMPULLARIS
Function :-
Detect Rotational motion in all the
3 dimensions by stimulation of all
the 3 semicircular canals.

17. UTRICLE & SACCULE
 Macula – flat sheet of epithelium that are oriented at
right angles to one another
 Overlaid by an ‘Otoconial Membrane’ which has large
numbers of Otoconia aka CaCO3 crystals

18. Macula in Utricle – In antero-posterior plane
Macula in Saccule – In superior-inferior plane
Utricle – Horizontal linear acceleration
Saccule – Vertical linear acceleration

19. BLOOD SUPPLY OF LABYRINTH
Mainly through labyrinthine artery which is a
branch of anterior inferior cerebellar artery.
VENOUS DRAINAGE:
Internal Auditory Vein
Vein of Cochlear Duct
Vein of vestibular Aqueduct
All 3 drain into inferior and petrosal sinuses.

20. PHYSIOLOGY OF HEARING

21. SOUND WAVE CONDUCTION
Air
conduction
Mechanical
Conduction
Fluid
Conduction

22. ACOUSTICS
 Sound – Form of energy produced by
vibrating object
 Consists of compression and rarefaction of
molecules of media
 Frequency – Number of cycles per second
 Pure tone – Single frequency sound
 Complex sound - Sound with more than one
frequency
 Pitch – Subjective sensation produced by
frequency of sound
 Intensity – The power transmitted by sound
wave through a unit area.

23. Speech Intensity
 Its measured in decibels (dB)
 Sound of whisper – 30 dB
 Normal conversation – 45-60 dB
 Noisy market – 60 dB
 >80dB sound is termed dangerous
 As per WHO Guidelines
 Anything above >140 dB can lead to deafness

24. EXTERNAL EAR Functions
 Sound collection
 Sound localization
 Each auditory cortex receives sound from both the
ears ( Horizontal Sound Localization )

25. Head Shadow Effect
 Sound reaches left ear first
 Right ear has become a shadow site
 Left ear receives sound early and high in intensity
 Right ear receives sound late and is low in intensity
 Each auditory cortex is able to compare right and left side
sounds and tell about the location of the sound
 Process of sound localisation starts at the level of brain stem
in the auditory pathway

26. IMPEDANCE
 Impedance is defined as resistance offered by a medium for transmission
of sound.
 IMPEDANCE MATCHING
 To compensate for the sound loss
 Converts sound of greater amplitude and less force to that of less amplitude
and greater force

27. IMPEDANCE MATCHING
 Hydraulic action of tympanic membrane
 Lever action of the ossicles
 Curved membrane effect

28. MIDDLE EAR Mechanics
 Surface area of tympanic membrane is 20 times Surface area
of stapes footplate
 Thus amplifying pressure over oval window
 Thus if the transformer action of the area ratio is "ideal," the
sound pressure applied to the inner ear by the stapes
footplate should be 20 times or 26 dB larger than the sound
pressure at the tympanic membrane.

29. MIDDLE EAR Mechanics
 Ossicular Lever Ratio = 1.3:1
 Handle of malleus is 1.3 times longer than long
process of incus.
 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
 Thus further amplifying the sound 1.3 times

30. MIDDLE EAR Mechanics
 So Theoretically Middle Ear Gain is 28 dB
 Middle ear transformer ratio = 20:1
(Ossicular coupling)
 CURVED MEMBRANE EFFECT
 Movement of Tympanic membrane is more at Periphery
than at Center where Malleus is attached.
 Buckling motion of tympanic membrane result in an
increased force and decreased velocity to produce a
increase in effectiveness of energy transfer

31. ACOUSTIC COUPLING
Amplification of sound by creating a phase difference between
oval and round window
Occurs in middle ear
Sound don’t reach both windows simultaneously.
When oval window receive compression, round window receive
rarefaction
In the normal ear, the magnitude of this acoustically-coupled
window pressure difference is small, on the order of 60 dB less
than ossicular coupling
Impedance matching
Ossicular coupling
Acoustic coupling

32. Other Pathways of Sound Conduction BONE
CONDUCTION
 Environmental sound can also reach the inner ear by producing vibrations of the whole body and
head, so-called whole body sound conduction
 Sound-induced vibrations of the whole body and head can stimulate the inner ear by
 (1) generating external ear or middle-ear sound pressures via compressions of the ear canal and middle-ear
walls,
 (2) producing relative motions between the ossicles and inner ear, and
 (3) direct compression of the inner ear and its contents by compression of the surrounding fluid and bone.
 However, measurements of hearing loss due to pathology such as congenital aural atresia suggest
that the whole body route can provide a stimulus to the inner ear which is about 60 dB smaller than
that provided by normal ossicular coupling.

33. Acoustics & Mechanics Of diseased middle ear
 Ossicular Interruption with Intact TM :
 When there is ossicular interruption in the presence of an intact drum, ossicular coupling is lost and sound input
to the cochlea via the middle ear occurs as a result of acoustic coupling.
 Since acoustic coupling is about 60 dB smaller than ossicular coupling,
 Complete ossicular interruption would result in a 60 dB conductive hearing loss
 Loss of the Tympanic Membrane, Malleus, and lncus :
 Loss of ossicular coupling together with an enhancement of acoustic coupling by about 10 to 20 dB, as
compared to the normal ear
 The conductive hearing loss is of 40 to 50 dB

34. Acoustics & Mechanics Of diseased middle ear Contd.
 Ossicular Fixation :
 Partial or complete fixation of the stapes footplate results in conductive hearing losses that range from 5 dB to
60 dB depending on the degree of fixation.
 Fixation of the footplate reduces ossicular coupling by hindering stapes motion, resulting in a conductive
hearing loss.
 The conductive hearing loss resulting from fixation of the malleus is determined by the location, extent, and
type of pathology causing the fixation
 Tympanic Membrane Perforation :
 Perforations of the tympanic membrane cause a conductive hearing loss that can range from negligible to 50
dB
 The primary mechanism of conductive loss due to a perforation is a reduction in ossicular coupling caused by a
loss in the sound-pressure difference across the tympanic membrane

35.  Middle Ear Effusion :
 Fluid in the middle ear, is associated with a conductive hearing loss of up to 30 to 35 dB,
 Reason - reduction in ossicular coupling due to several mechanisms.
 At frequencies greater than 1,000 Hz, the loss is caused primarily by mass loading of the tympanic
by fluid, with decreases in sound transmission of up to 20 to 30 dB.
 At frequencies below 1,000 Hz, the hearing loss is due to an increase in impedance of the middle-ear air
resulting from reduced middle-ear air volume
 Tympanic Membrane Atelectasis :
 Atelectasis of the tympanic membrane occurring without a tympanic membrane perforation (and in the
presence of intact and mobile ossicles) can result in conductive hearing losses that vary in severity from
negligible to 50 dB.
 Reason - reduction in ossicular coupling due to the tympanic membrane abnormality
Acoustics & Mechanics Of diseased middle ear Contd.

36.  Transduction of sound from Mechanical to Electrical occur in
ORGAN OF CORTI in inner ear.
 Vibration of Basilar membrane.
 Stimulation of hair cells
 Membrane potential change in hair cells
 Neural transmission of signals

37. VIBRATION OF BASILAR MEMBRANE
 Sound waves from middle ear pass to inner ear
through Oval window by in & out movement of
stapes.
 Movement of stapes causes displacement of
cochlear fluid in scala vestibuli
 The incompressibility of perilymph causes a
pressure gradient between SV & ST, leading to
movement of Basilar membrane & Organ of
Corti
 Movement of basilar membrane causes organ of
corti to move up & down.
 Hair of the outer hair cells are embedded in
Tectorial Membrane.
 As both Tectorial membrane & basilar
membrane moves, they slide each other with
movement

38.  As organ of Corti Moves up, tectorial membrane
slide foreward moving stereocilia away from limbus.
 As organ of Corti Moves down, tectorial membrane
slide backward moving stereocilia towards limbus.
 These tip links are attached to a cation channel
 Opens because of a mechanical force hence also
called Mechanoelectrical transduction channel (MET
Channel)
 USHER SYNDROME
 Mutation of gene encoding Cadherin 23 or
Protocadherin 15

39. Stimulation of hair cells contd.
 Bending of stereocilia stimulate hair cells
 Depolarization – as stereocilia bend away from
limbus.
 Hyperpolarization – as stereocilia bends towards
limbus.

40. Depolarisation
 when the cilia are bent in the direction of the
longer ones
 the tips of the smaller stereocilia are tugged
outward.
 This causes a mechanical transduction that opens
200 to 300 cation- conducting channels
 Thus allowing rapid movement of potassium ions
from the surrounding scala media fluid into the
stereocilia,
 Thus causes depolarization of the hair cell
membrane

41. Depolarisation contd.
 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 there
is generation of 8th nerve action potential

42. OUTER HAIR CELLS & AMPLIFICATION
 OHCs have a key role in amplification of Basilar membrane. Amplification is
necessary for detection of sounds at low pressure levels
 Mechanism of Amplification – Somatic Electromotility
 The OHC’s change their length by 3-5% in response to electrical
stimulation
 When Depolarized, they Contract & when Hyperpolarized they Elongate.
 The OHC’s exert mechanical force causing movements of basilar
membrane motion caused by the travelling wave. Prestin – motor protein
responsible for somatic electromotility in outer hair cells

43. ELECTRIC POTENTIALS OF COCHLEA AND CN VIII
 Cochlear Microphonics
 Endocochlear potential
 Summating potential
 8th nerve potential

44. Cochlear Microphonics
 Electric potential in the inner ear
 Produced when the basilar membrane moves in response to sound
stimulus
 Due to Influx of K+ ions
 Its an AC Potential and depends on sound intensity

45. Endocochlear Potential
 Endolymph in scala media secreted by stria
vascularis has high conc of Na-K-ATPase & unique
electrogenic K pump
 So it has high K conc & electrically positive to
perilymph.
 Potential developed between Endolymph &
Perilymph is Endolymphatic potential or
Endocochlear potential
 + 80 mV
 It’s a resting potential and doesn’t depend on
sound intensity

46. 8th Nerve Action Potential
 Follows All or none principle
 8th nerve stimulation is majorly by
Inner Hair Cells
 Outer hair cells mainly amplify the
sound

47. Frequency Localization in cochlea
 20 – 20000 Hz
 Each frequency of sound is heard at a
different place in the basilar
membrane
 20000Hz – at base of cochlea
 20 Hz – at apex of cochlea
 As we increase the frequency, the
peak shifts towards the base.

48. Theories of hearing
 Place theory of Helmholtz
 Temporal theory of Rutherford
 Volley theory of Wever
 Bekesy or travelling wave theory

49.  Place theory - According to Helmholtz, basilar membrane has different
segments that respond to different frequencies
 Frequency theory – According to Rutherford, there is synchronization
between frequency of sound & rate of discharge through cochlear nerve.
So Frequency of Action potential in auditory nerve determine Loudness
than pitch.

50. Travelling wave theory
 Von Bekesy
 Basilar Membrane is Mechanical Analyzer of sound
frequency.
 Pattern of movement of basilar membrane is that of
Travelling Wave.
 Basilar membrane Near oval window vibrate in response
to sound of Higher frequency & Near Apex respond to
Lower Frequency.
 The tonotopic gradient is a continuous gradient in
basilar membrane width and also with changes in hair
cell height and length of cellular structures i.e.
stereocilliary hair bundles
 So Higher frequencies are represented in Basal turn &
Lower Near Apex

51. NEURAL TRANSMISSION OF SIGNALS
Cochlear nuclei
Superior Oliviary
Complex
Nucleus of
lateral lemniscus
Inferior
Colliculus
Medial
Geniculate Body
Auditory Cortex

52. Auditory Pathway Contd.

53. Auditory Pathway Contd.
 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). Area A1
 Auditory association cortex –
 Area A2, corresponds to Brodmann's areas 22 and 42.
 The primary auditory cortex is tonotopically tuned, with high
frequencies being represented more medially, and low
frequencies being represented more laterally

54. SUMMARY