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1. PHYSIOLOGY OF HEARING
DR MESHWA OZA
ENT RESIDENT

2. ORGAN OF CORTI
• Organ of Corti is the sense organ of hearing and is situated on the
basilar membrane. Important components of the organ of Corti are:
• 1. Tunnel of Corti
• 2. Hair cells
• 3. Supporting cells
• 4. Tectorial membrane

5. PATHWAY OF SOUND
• Pinna- collects sound signal
• External auditory canal
• Tympanic membrane and ossicles
• Stapes footplate- Oval window
• Scala vestibuli- Vibration of perilymph
• Basilar membrane- Organ of corti
• Electrical impulse
• Cochlear nucleus
• Brain

6. 3 PARTS
1. CONDUCTION OF SOUND (External ear, Middle ear)
2. TRANSDUCTION INTO ELECTRICAL IMPULSES (Inner ear)
3. CONDUCTION OF IMPULSE TO BRAIN (Neural pathway)

7. CONDUCTION OF
SOUND

8. PINNA
• Function:-
• 1. Collection of sound
• 2. Localisation of sound
• Shadowing effect of head
• Increased intensity at nearer ear
• Decreased time at nearer ear
• 3. Concentration of sound
Causes 6 dB increase in sound

9. EXTERNAL AUDITORY CANAL
• Transmits sound to from pinna to tympanic membrane.
• Acts as resonating tube
• Increases sound by 15-22 dB in frequency range of 2-7 KHz

10. TYMPANIC MEMBRANE
• Transmits sound from external ear to middle ear and then to inner
ear.
• When sound moves from Middle ear containing air, to inner ear
containing water, there is increased impedence/resistance.
• This causes IMPEDENCE MISMATCH.

11. IMPEDENCE MATCHING/TRANSFORMING
ACTION
• Middle ear converts
↑Amplitude, ↓ force
↓
↓Amplitude, ↑ force

12. 1. HYDRAULIC ACTION OF TM
• Total surface area of TM- 60 mm2
• Effective vibratory area- 45 mm2
• Surface area of stapes footplate- 3.2 mm2
• Effective areal ratio=45/3.2= 14:1
• Small movement in larger area
↓
Larger movement in smaller area

13. 2. LEVER ACTION OF OSSICLES
• Axis of rotation- Imaginary line passing through Anterior malleolar
ligament and Incudal ligament.
• Handle of Malleus- 1.3 times longer than long process of Incus.
• Mechanical advantage of 1.3
• This causes less displacement of stapes and production of more force.
• Total transformer ratio- 14*1.3= 18:1
• 25 to 30 dB increase when it reaches cochlea.

15. CURVED MEMBRANE EFFECT
• Movements of tympanic membrane are more at the periphery than at
the centre where malleus handle is attached.
• This too provides some leverage.

16. PHASE DIFFERENTIAL BETWEEN OVAL
AND ROUND WINDOWS
Stapes footplate vibrates Oval window
↓
Scala vestibuli
↓
Scala tympani
↓
Round window bulges
• When oval window is receiving wave of compression, the round window is at
the phase of rarefaction.
• Phase differential between the windows contributes 4 dB when tympanic
membrane is intact.

17. NATURAL RESONANCE OF EXTERNAL AND
MIDDLE EAR
• Certain frequencies will cross certain areas of ear more easily.
• EAC- 3000-6000 Hz
• TM- 800-1600 Hz
• Ossicles- 500-2000 Hz
• Middle ear- 800 Hz
• Average 500 to 3000 Hz pass through external and middle ear
• This is called Speech frequency.

18. TRANSDUCTION OF
SOUND

19. • Wave of motion passes through perilymph, Basilar
membrane moves.
• Organ of corti also moves.
• Shearing action between tectorial membrane and hair cells.
• Movement of cilia of outer hair cells.
• Opening of channels (Cilia tight junction).

20. • Endolymph- More K+, More Ca+ i.e., Positive potential
• Hair cells- Negative potential
• Outer hair cells- -75 mV, Inner hair cells- -45 mV
• So positive current flows inside hair cells.
• This current flows through auditory nerve.

21. • Inner hair cells- Transfer information about movement of basilar
membrane to auditory nerve
• Outer hair cells- Have a protein called PRESTIN
• Micro cilia contract and elongate by sound
• Which causes-
• 1. Amplification of sound
• 2. Sharpening of sound
• 3. Cochlear microphonics production
I. Dependent on oxygen, absent in death, decreased in oxygen
deprivation
II. Independent of oxygen

22. TRAVELLING THEORY OF VON BEKESY
• Sound wave travels from base of cochlea to apex.
• Each part of cochlea has its specific frequency.
• Called natural resonant frequency.
• A sound wave, depending on its frequency, reaches maximum
amplitude on a particular place on the basilar membrane and
stimulates that segment.
• Base- Higher frequency
• Apex- Lower frequency

24. NEURAL PATHWAY
• 8TH
nerve
• Cochlear nucleus
• Superior olivary complex
• Lateral lemniscus
• Inferior colliculus
• Medial geniculate body
• Auditory cortex(Superior temporal gyrus- Brodmann area 41)

26. ELECTRICAL POTENTIALS OF COCHLEA AND
CN VIII
• Four types of potentials have been recorded; three from the
cochlea and one from CN VIII fibres. They are:
• 1. Endocochlear potential
• 2. Cochlear microphonic
• 3. Summating potential
• 4. Compound action potential
• 1,2,3- from cochlea
• 4- from nerve fibres

27. • 1. ENDOCOCHLEAR POTENTIAL
• Direct current (DC) potential recorded from scala media.
• It is +80 mV and is gener ated from the stria vascularis by Na+/K+-
ATPase pump and provides source of energy for cochlear
transduction.
• It is present at rest and does not require sound stimulus.
• This potential provides a sort of “battery” to drive the current through
hair cells when they move in response to a sound stimulus.
• 2. COMPOUND ACTION POTENTIAL
• It is an all or none response of auditory nerve fibres

28. • 3. COCHLEAR MICROPHONIC (CM)
• When basilar membrane moves in response to sound stimulus,
electrical resistance at the tips of hair cells changes allowing flow of
K+ through hair cells and produces voltage fluctuations called
cochlear microphonic.
• It is an alternating current (AC) potential.
• 4. SUMMATING POTENTIAL (SP)
• It is a DC potential and follows “envelope” of stimulating sound. It is
produced by hair cells.
• It may be negative or positive. SP has been used in diagnosis of
Ménière’s disease.
• It is superimposed on VIII nerve action potential.

29. • Both CM and SP are receptor potentials as seen in other sensory end-
organs.
• They differ from action potentials in that:
• (i) they are graded rather than all or none phenomenon
• (ii) have no latency
• (iii) are not propagated
• (iv) have no postresponse refractory period.