PHYSIOLOGY OF HEARING BY DR. SHRISHTI JAIN

MECHANISM OF HEARING

1. PHYSIOLOGY OF HEARING
DR. SHRISHTI JAIN
ENT RESIDENT

2. SOUND WAVE
•It is a pressure wave.
•Has phases of rarefaction
followed by compression.
•It requires a medium to travel
in.
•Can't travel in vacuum.

3. Parts of a sound wave
•FREQUENCY-Frequency(f) is the number of cycles per second
(usually expressed in Hertz (Hz)).
•AMPLITUDE-The height of the sinusoidal wave
•VELOCITY-The velocity of sound waves radiating from a source at
sea level is about 344 metresper second.
•WAVELENGTH-Wavelength(λ) is the distance between
corresponding points of displacement in successive cycles.

4. •Phase relates to the temporal
relationship between two (or more)
oscillatory components that are
present simultaneously
•Impedancedepends on how stiff
and densea medium is.
•acoustic impedance at a particular
frequency will contribute to defining
how much sound pressure is
generated by a sound wave.
•If impedance is high, less sound
pressure is generated for the same
vibration
•Sound intensity= the amount of
energya sound wave carries or
transfers through a medium.

5. •Pitch→ how we perceive frequency
•Loudness→ how we perceive intensity
•When the frequency doubles, the pitch increases by one
octave.
•The human ear can hear about 10 octavesin total.
•The ear is most sensitive to 2–4 kHz, and less sensitiveto very
lowor high frequencies.
•A pure tone= sound with only one frequency.
•Used in hearing tests.
•It has three parts:
•Onset (rise time):sound intensity increases.
•Steady period:sound stays constant.
•Offset (fall time):sound intensity decreases.

6. THE INFLUENCE OF ASOUND’S DURATION
•In addition to frequency and intensity, a third dimension of sound is its
duration
•a sound must exceed a certain minimum duration before its pitch
characteristics become apparent
•Below this point,, about 200ms, and the sound will have a click-like
sensation regardless of its duration.
•loudness increases as a function of intensity over time, a phenomenon
known as temporal integration.
•As the onset and offset periods get shorter, the frequency spectrum of
the sound becomes progressively broader (the so-called ‘spectral
splatter’); sound duration and bandwidth are inversely proportional.

7. MEASUREMENT OF SOUND
•The amplitude of a sound (either in terms of the intensity or resulting
pressure fluctuations) can be described by the decibel scale
•the frequency composition of a sound can be shown by spectral
analysis of the signal.
•Theinfluence upon both of these quantities by the duration of the
signal will also be described.
•It is a ratio. This means that the bel is the logarithm of intensity of
sound divided by some agreed reference intensity.
•Zero dB therefore simply indicates that the measured sound is the
same as the reference; it doesn’t mean silence.
•A negative dB value means that the measured sound is less than the
reference, and a positive dB value means that the measured sound is
greater than the reference

8. PHYSIOLOGY OF HEARING
The peripheral ear consists of
•the pinna,
•the external auditory meatus and canal,
•the middle ear with the tympanic membrane,
•the ossicles,
•the middle ear ligaments
•the mucosal folds,
•the Eustachian tube opening
•the oval and the round windows
•the inner ear with the cochlea
•the auditory nerve.

9. •The pinnais vestigial in humans, with non-functional auricular
muscles.
•A dysmorphic pinnamight reflect a dysmorphic external auditory
canal and middle ear.
•The pinna helps with sound localization.
•When coupled with the external auditory meatus and canal, the unit
acts to provide frequency-specific resonanceof sound incident on
the ear.
•This resonance compensates for the impedanceencountered at the
air–fluid interfacebetween the cochlea and the middle ear.
•The head shadow effect—caused by the intervening head and pinna
on either side—is an important factor for auditory localization,
generating interaural timeand interaural intensity differencesfrom
the same acoustic signal reaching each ear at different times and
intensities.
•The postauricular myogenic response (PAMR)is a measurable
compound action potentialgenerated in response to an electrical or
acoustic stimulus.

10. •THE EXTERNAL AUDITORY CANAL
•The external auditory canal (EAC)commences at the conchalopening
called the external auditory meatus (EAM).
•The ear canalcan be considered as a resonating tubefollowing the
quarter-wave principle for resonance; i.e., at a frequency with a
wavelength four times the length of the canal, the canal and the pinna
togetherwill resonate and vibratewith the incoming signal to augment
or magnifythe incident acoustic energy.
•The resonant frequencyremains more or less constant from the age of
2 years, though the resonance amplitude changes in old age.
•A standing waveis created when two sounds of the same frequency
travel in opposite directionsin the canal (e.g., when the same sound is
reflected by the tympanic membrane) and cancel each other out.

11. •THE MIDDLE EAR
•The middle earin humans acts as an efficient passive and linear
transformerto conduct acoustic energyfrom the tympanic membraneto
the stapes footplateat the oval windowand then to the cochlea.
•Acoustic energyis transferred from a low-impedance, high-velocity
medium(air) to a high-impedance, low-velocity medium(cochlear fluid)
through ossicular coupling.
•The impedance differenceis mainly matched by:
•the surface area ratioof the tympanic membrane to the stapes footplate,
and
•the lever actionof the ossicles and the membrane.
•The Eustachian tubeconnects the nasal cavityto the middle ear, supplying
airfor vibration of air particles and equalizing pressureacross the tympanic
membrane.
•The greatest transfer functionof the middle ear occurs at its resonant
frequency (1–3 kHz), determined by the mass and stiffnessof the system.

12. •THE TYMPANIC MEMBRANE
•The tympanic membrane transmits acoustic energyfrom a relatively large
areaof the membrane to the much smaller area of the stapes footplate.
•The area ratio is approximately 18:1, helping to overcome the acoustic
impedance mismatchat the air–fluid interfacebetween the middle and
inner ear.
•The incident soundfrom the external auditory canal (EAC)strikes the
membrane and sets up a travelling wave, mainly collected at the rimof the
membrane.
•The wave is conducted to the umboand coupled to the manubrium of the
malleus.
•At lower frequencies, the membrane transfers all its energy uniformlyto
the malleus.
•At higher frequencies, the movement becomes more complex, and part of
the vibration is shuntedby the middle ear.
•The middle ear limits sound wave propagationat higher frequencies.

13. •Structural integrityis crucial for the middle ear transformer
mechanism.
•When the rim of the membrane is intact(i.e., central perforations),
hearing is better preservedthan when the margins are affected.
•The degree of hearing lossis proportional to the sizeof the
perforationand inversely proportional to frequency—the greatest
loss occurs at lower frequencies.
•Reflectance of the membrane, an important factor for the pressure
difference across the membrane, is lost or diminished at lower
frequencies, reducing complianceand the transfer of energyto the
oval window.

14. •EUSTACHIAN TUBE
•The tubal openingis best demonstrated at 6–8 kHz, i.e., high-frequency sound.
•Since high-frequency soundsare the most important consonant-containing
sounds, the middle earmust act in the most efficient wayto conduct these
sounds.
•MIDDLE EAR MUSCLES
•The middle ear musclesare important for eliciting a prevocalization reflex, i.e.,
a reflex that occurs just before speaking.
•Due to the bony continuity of the human skeleton, vocalizationleads to
cochlear stimulationvia the bony external auditory canal (EAC), the ossicles,
and the skull bones.
•The middle ear musclesmodulate this acoustic signal, protecting the ear from
the intensity of one’s own voice.
•The stapedius muscle, supplied by the VIIth(facial) nerve, attaches to the
stapes neck, and its contraction pulls the annular ligamentin the footplate.
•The stapediusresponds primarily to high-intensity, low-frequency sounds
(around 0.8 kHz), which tend to overpower high frequencies, thereby preserving
high-frequency consonant-containing speech sounds.

15. •STAPEDIUS MUSCLE –LATENCY AND REFLEX
•The stapedius tendontakes about 100–200 msto contract fully,
which means it cannot protect the cochleafrom intense short-
impulse noisessuch as a gunshot.
•A noise primerjust before the gunshot may address this latent
periodand could be otoprotective.
•FUNCTIONS OF THE STAPEDIUS
•Protection of the cochleafrom high-intensity sounds.
•Reduction of middle ear outputfrom high-intensity, low-
frequency sounds.
•Modulation of self-monitoring of voice, preserving high-
definition, high-frequency ambient soundsduring speaking or
self-generated noise.
•The reflexcan be elicited by both ipsilateral and contralateral
stimulidelivered to one ear.

16. •MIDDLE EAR WINDOWS
•The pressure differencebetween the ovaland round windowsis
fundamental for the cochlear travelling wave.
•The round windowalso participates in absorption and secretion of
perilymphdue to its semipermeable membrane.
•Intratympanic drug deliverycan be done through the round windowto
achieve effective intralabyrinthineconcentrations.
•Virtual third windowsinclude the vestibular aqueductand the bony
skull.

17. •BONE CONDUCTION
•The cochlea can be stimulated directlythrough vibrations of the bony
skull.
•Mechanism:
•Airborne or internal soundsmake the skull vibrate.
•Vibrations move endolymphin the cochlear basilar membrane (BM).
•This bypasses the external and middle ear.
•Non-osseous pathway:
•Acoustic energy can also travel through brain tissue and
cerebrospinal fluid (CSF).
•Energy is then transmitted to the cochlea via its fluid channels.

18. •The three ossicles(malleus, incus, stapes) are connected by synovial
joints.
•Malleus length ≈ 2.1 ×incus length→ gives a mechanical advantage /
impedance ratio ≈ 4.4.
•Lever actionis frequency-dependent, most efficient around 2 kHz.
•Malleus–incus rotation:
•Rotates around an anterior–posterior axisthrough the ossicles’ center of
gravity.
•Low frequencies:
•Simple motion, less relative movement between malleus and incus.
•High frequencies:
•Complex motion, some slippage between malleus and incus→ conducts
high-frequency sounds more efficiently.
•Stapes movement:
•Low frequencies:piston-like motion.
•High frequencies:complex multidirectional movements along long and
short axesof the footplate

19. •MIDDLE EAR IMPEDENCE MATCHING
•The middle earmatches sound energy between air (outer ear)and fluid (inner ear)
to prevent loss of energy at the stapes–oval window.
•This is achieved by two main mechanisms:
•Area Ratio Effect:
•The tympanic membrane (TM)is much larger than the stapes footplate.
•This size difference concentrates pressure, increasing sound energy delivered to the cochlea.
•Lever Action:
•The malleus and incusact as a lever system, amplifying the force transmitted to the stapes.
•Together, these mechanisms provide about 20–26 dB pressure gain.
•~20 dB gain between 0.25–0.5 kHz
•Maximum gain (~26.6 dB)at 1 kHz
•Decreases by ~8.6 dB per octaveabove 1 kHz → nearly zero gain above 7 kHz.
•Ossicular couplingis frequency-dependent—more effective at low and mid
frequencies.
•Cochlear impedance(resistance to sound transmission) is lower at higher
frequenciesthan at lower ones.

20. THE COCHLEA
•When sound enters the cochlea, it undergoes two main
processes:
•Active mechanical process (by OHCs):
•Outer Hair Cellscompress and amplify the sound vibrations.
•This enhances sensitivity and sharpens frequency tuning.
•Electrochemical process (by IHCs):
•Inner Hair Cellsconvert mechanical vibrations into electrical signals
(nerve impulses).

21. COCHLEAR TRAVELLING WAVE
•three different travelling waves
generated:
othe wave as a result of the pressure
difference of the two compartments,
othe wave as a result of the
mechanical displacement of the BM
othe acoustic energy wave which
displaces the cochlear fluids.
The displacement wave is by far the
most important wave for cochlear
function

22. •The travelling wavemoves along the basilar membrane (BM) because the BM
becomes thinner and less stifffrom base to apex.
•The BM is influenced by critical oscillators, each tuned to a specific
frequency at its region.
•These oscillators work with outer hair cells (OHCs), adding active energy to
boost BM movement.
•As the wave travels from base to apex, signal strength must stay stable
despite gradual BM changes.
•Oscillators can switch roles:
•Active→ amplify/modify the signal.
•Passive→ let the signal pass.
•At a certain point, oscillators may cancel each other’s effect (called a Hopf
bifurcation).
•To prevent this, oscillators use self-regulation (self-tuning).
•This property is essential for producing the cochlear tuning curve, which
allows sharp frequency selectivity.

23. •OHCs (Outer Hair Cells)provide two key functions:
•Critical oscillation→ helps sharpen frequency tuning of the BM.
•Compressive function→ reduces large input amplitudes, preventing
damage and keeping sensitivity.
•These functions vary along the BM because each region is tuned to a
different frequency (spatial representation).
•As a result, the basilar membrane’s acoustic responseis non-linear—
it doesn’t scale proportionally with the input sound.

24. •Cochlear tuning curve
•High-frequency sounds→ cause maximum vibration near the baseof the
cochlea.
•Low-frequency sounds→ cause maximum vibration near the apexof the
cochlea.
•The point of maximum displacementon the basilar membrane is called the
characteristic frequency.
•The human ear can detect sounds across a wide dynamic range (~120 dB).
•To protect the cochleafrom damage, loud sounds are compressedat the
cochlear level.
•For soft (low-intensity) sounds, Outer Hair Cells (OHCs)actively amplifythe
basilar membrane movement by changing their length (elongation and
contraction).
•This OHC activity sharpens the tuning curve, allowing precise frequency
discrimination.
•The process remains non-linear—meaning the response does not increase
proportionally with sound intensity.

25. •COCHLEARFLUID
•Thescala media (cochlear
duct)containsendolymph.
•Endolymphisrich in potassium (K⁺),low in sodium
(Na⁺), and hasvery little calcium (Ca²⁺).
•Stria vascularisin the scala mediaactively
pumpsthese ions to maintain thebalance.
•Hair cell stereociliaare bathed inendolymph, while
theircell bodiesare surrounded byperilymph(rich in
Na⁺, low in K⁺).
•Cochlear microcirculationmaintains fluid integrity
and is protected by theblood–labyrinthbarrier.
•Themain ion driving cochlear function is
potassium (K⁺)→ it isrecycledthrough
thepotassiumcycle, creating
theendocochlearpotential.

26. •This endocochlear potentialis a constant
resting electrical potentialin the scala
media, essential for hearing.
•Sodium (Na⁺)also plays a role —Na⁺/Cl⁻
ATP cotransportersin marginal cellsand
spiral ligament fibrocyteshelp regulate K⁺
movement.
•Overall, there is a dynamic ion exchange
(mainly K⁺and Na⁺) that maintains cochlear
function and electrical balancein the scala
media.
•COCHLEAR FILTERING ACTION
•The cochleahas a filtering action, which is
indirectly dependent on tonotopicity.
•When sounds are close together in
frequency, fusion may occur, and the
perceptionwill be of a single sound, as the
filtering action becomes less effectivefor
neighboring frequencies.

27. •Cells of the Stria Vascularis
•Marginal Cells
•Face the scala media.
•Actively pump K⁺from the endolymph to keep low K⁺
levelsin the intrastrialspace.
•Help maintain ionic balance in the endolymph.
•Intermediate Cells
•Located between marginal and basal cells.
•Connected to basal cells via gap junctions(regulated by
connexin genes).
•Site where the endocochlearpotential is generated.
•Basal Cells
•Found beneath intermediate cells.
•Connected laterally to the spiral ligamentby gap
junctions.
•Receive K⁺from spiral ligament fibrocytesand pass it to
intermediate cells → part of the potassium recycling
pathway.
•Fibrocytes of the Spiral Ligament
•Take up K⁺from the bloodand deliver it to basal cells.
•Support continuous K⁺cyclingbetween blood and
endolymph.

28. •Cochlear sensory epithelia andsupporting
cells
•OHCs-12000, in 3 rows
•cochlear tuning, amplification, compression and
frequency selectivity
•innervated by type2 spiral ganglion afferents
•cochlear efferentsfrom the medial olivocochlear
bundle
•IHC-3500, 1 row
ocochlear transduction of the incoming acoustic
signal
olargely generate the cochlear afferents to the
spiral ganglion type1 cells
oreceive efferents from the lateral olivocochlear
bundle
Undulations or mechanical vibrations from the BM lead to a
shearing force in the tectorial membrane which in turn moves the
stereocilia of the hair cells in the endolymph.

29. •Cochlear outer hair cells
• Cylindrical cells with stereocilia bundles made of actin filaments
projecting into the scala media.
• Mechanical amplification and fine-tuning of sound; not primary
sensory transducers.
•Stereocilia movement:
•Toward tallest stereocilia → depolarization
•Toward shortest stereocilia → hyperpolarization/repolarization
•OHCs are more negative than endolymph, allowing K⁺ to move
passively (no energy needed).
•OHCs change length in response to voltage changes.
•Amplifies basilar membrane motion → sharpens frequency
selectivity and enhances tuning curve.
•OHCs receive efferent input from the medial olivocochlear (MOC)
bundle.
•Synapse with type II spiral ganglion neurons → modulates reverse
transduction and amplification.
OHCs do not send auditory signals directly to the auditory nerve.

30. •PRESTIN
•Theaction ofprestinis voltage dependent and results in either
contraction or elongation of the OHC necessary for augmenting the
acoustic signal incident on the BM
•Prestin belongs to the SLC26A family
•participates in selective anion transport and binder, in this case
chloride and carbonate. T

31. •Inner Hair cells
•Primary sensory cells for hearing.
• Generate action potentials → sent to
type I spiral ganglion cells → auditory
nerve.
•Motility: Non-active; respond to basilar
membrane (BM) motion modified by
OHC activity.
•Coding role: Convert cochlear activity
into neural code.Have 2 codingroles
•Frequency coding: Which
pitch/frequency is heard.
•Intensity coding: Number/rate of action
potentials increases with sound
intensity.

32. •Cochlear supporting cells
•Hansen cells, Deiters cells, pillar cells, interphalangeal cells and
border cells
•contribute to the development, differentiation, patterning and
synaptogenesis of the hair cell sensory epithelia
•Loss of cochlear function causes:
•↓ Hearing sensitivity
•↓ Frequency selectivity → poor sound analysis and clarity
•↓ Non-linearity, amplification & compression → narrow dynamic
range
•↓ Perceptual streaming → difficulty separating sounds
•↓ Temporal pattern recognition → poor timing and pitch
discrimination

33. •Causes of cochlear dysfunction:
•structural abnormality
•abnormal metabolic activity
•vascular changes
•overloading of the basilar membrane (bm)
odiabetes, hyperlipidemia, iron overload, autoimmune or inflammatory
diseases
•infection and inflammation
•genetic mutations
•biochemical pathway abnormalities
oNoise trauma, ototoxicity, or aging (presbycusis)

34. Cochlear nerve

35. 1. Origin and Structure
The auditory (cochlear) nerve is part of cranial nerve VIII.
It arises from spiral ganglion cells in the cochlea.
2. Types of Cochlear Nerve Fibres
Type I: Large, myelinated fibres (95%) → innervate IHCs.
Type II: Small, unmyelinated fibres → innervate OHCs.
3. Pathway and Termination
Nerve passes through internal auditory canal, joins
vestibular nerve → forms vestibulocochlear nerve →
terminates at cochlear nuclei in the medulla.
4. Tonotopic Organization
Frequency encoding from cochlea maintained in
cochlear nucleus (spatially arranged by frequency).
5. Action Potentials & Frequency Encoding
Sound stimulus triggers spike potentials; firing rate
varies with intensity and frequency. Each fibre has a
characteristic frequency.
6. Phase-Locking Mechanism
Fibres fire at specific phases of low-frequency sounds
(up to ~5 kHz), helping encode timing of sound.

36. 7. Two-Tone Suppression
A second tone near the first frequency can suppress the
first — aids complex sound processing (non-linear
function).
8. Intensity Coding
Fibres have spontaneous firing rates. High-rate fibres →
low threshold; low-rate fibres → high threshold. Firing
rate increases with intensity until saturation.
9. Auditory Nerve Adaptation
Spike frequency adapts to continuous stimulus;
adaptation more marked in high-frequency fibres —
helps with timing cues.
10. Disorders Affecting Auditory Nerve
Pressure from tumors (e.g. vestibular schwannoma),
demyelination (MS), trauma, infection, autoimmune or
congenital causes → cause neural deafness.
11. Auditory Neuropathy Spectrum Disorder (ANSD)
Normal OHC function but abnormal nerve transmission.
Causes include hypoxia, prematurity,
hyperbilirubinemia, genetic mutations (otoferlin –
DFNB9, pejvakin – DFNB59). PTA may show variable
hearing; cochlear implants may help if lesion is pre-
neural.

38. THANK YOU