1.
MECHANISM OF HEARING

2.
MECHANISM OF HEARING
Sound waves travel
through external
auditory Meatus
Vibration of tympanic
membrane
Vibrations travel
through ossicles
Moves stapes which
causes vibration in
fluid of cochlea
Stimulate hair cells of
organ of Corti
Produce AP in auditory
nerve
Reach cerebral cortex
Perception of hearing
occurs.

3.
 Role of external ear- directs sound waves towards
tympanic membrane.
 Role of middle ear
 Role of inner ear
FUNCTIONS OF MIDDLE EAR:
1. Role of tympanic membrane:
Sound waves produce pressure changes – tympanic
membrane vibrates in and out of middle ear. (acts like a
resonator)
2. Role of auditory ossicles:
-vibrations in tympanic membrane are transmitted to
malleus and incus and reach the stapes
-this causes to and fro movement of stapes against
oval window and against perilymph present in scala
vestibuli of cochlea.
3. Role of eustachian tube - responsible for equalizing
pressure on either side of tympanic membrane.

4.
FUNCTION OF MIDDLE EAR -
IMPEDANCE MATCHING
 Tympanic membrane and the auditory ossicles
effectively reduce the sound impedance which is
called impedance matching.
 It is the process by which tympanic membrane and
auditory ossicles convert sound energy into the
mechanical vibrations in the fluid of internal ear with
minimum loss of energy by matching the impedance
offered by the fluid.

5.
 External ear- 40% , ossicles – 60%
 Ossicles act like lever system
 Force exerted by stapes on cochlear fluid is 17-
22 times greater than force exerted by tympanic
membrane because:
i. Head of malleus is longer than long process of
incus
ii. Surface area of tympanic membrane (55 sq.mm)
is larger compared to that of foot plate of stapes
(3.2sq.mm)
Significance of impedance matching:
 Sound waves are transmitted to cochlea with
minimum loss of intensity.
 Without impedance matching conduction

6.
TYPES OF CONDUCTION
1. Ossicular conduction – conduction of sound
waves through middle ear by auditory ossicles
2. Air conduction –conduction of sound waves
through air in middle ear. It occurs when
auditory ossicles are diseased
3. Bone conduction –conduction of sound waves
by bones. When middle ear is affected bone
conduction occurs.

7.
ROLE OF INNER EAR
TRAVELLING WAVE:
 Movement of foot plate of stapes against oval
window causes movement of perilymph in scala
vestibuli
 It immediately hits the vestibular membrane near
oval window and displaces the fluid in scala
media
 This causes bulging of basal portion of basilar
membrane towards scala tympani.
 This elastic tension developed in bulged portion
of basilar membrane initiates a wave called
travelling wave.

8.
EXCITATION OF HAIR CELLS
 Stereocilia of hair cells are embedded in tectorial
membrane
 When travelling wave produces vibration of
basilar membrane, it moves stereocilia
 This excites hair cells and generates receptor
potential.

9.
ELECTRICAL EVENTS DURING
THE PROCESS OF HEARING
 SOUND TRANSDUCTION- energy (movement of
cilia in hair cell) caused by sound is converted
into action potentials in the auditory nerve fiber.
 RECEPTOR POTENTIAL OR COCHLEAR
MICROPHONIC POTENTIAL – mild
depolarisation in the hair cells of cochlea when
sound waves are transmitted to internal ear. The
resting membrane potential is about -60mV
 Receptor potential in hair cells causes generation
of action potential in auditory nerve fibers.

10.
RECEPTOR POTENTIAL
Sound waves in inner
ear
Travelling wave is
produced
Vibration of basilar
membrane
Moves stereocilia
toward kinocilium
Opening of mechanical
Influx of potassium ions
Mild depolarisation upto -50mV
Receptor potential
Release neurotransmitter-
glutamate
Generates action potential in

11.
ROLE OF HAIR CELLS
 Inner hair cells
– responsible for sound
transduction
- primary sensory cells
 Outer hair cells
– electromotility (shortened
during depolarisation and elongated
during repolarisation)
- contractile protein prestin
- cochlear amplifier

12.
ROLE OF EFFERENT NERVE
FIBERS
 INNER HAIR CELL:
- Efferent fiber terminates on afferent
nerve fiber
- Release Ach
- Inhibits glutamate release from inner
hair cell. Hence controls generation of
action potential.
 OUTER HAIR CELL:
- Efferent fiber terminates on cell body
- Inhibits electromotility of this cell

13.
ENDOCOCHLEAR POTENTIAL
 Electrical potential developed in fluids
outside the hair cells.
 The difference in potassium
concentration is responsible for
development of electrical potential
difference between endolymph and
perilymph.
 +80mV
 Lower portion of hair cells bathed in
perilymph

14.
1. THEORIES OF FIRST GROUP –
Analysis of sound frequency is the
function of cerebral cortex, cochlea
merely transmits sound.
2. THEORIES OF SECOND GROUP-
frequency analysis is done by
cochlea which later sends
information to cerebral cortex.

15.
THEORIES OF FIRST GROUP
1. Telephone theory of Rutherford
2. Volley theory

16.
THEORIES OF SECOND
GROUP
1. RESONANCE THEORY OF HELMHOLTZ
2. PLACE THEORY: The nerve fibres from
different portions of Organ of Corti on basilar
membrane give response to sounds of different
frequency. Accordingly, corresponding nerve
fiber gives information to brain regarding the
portion of organ of corti that is stimulated.
3. TRAVELLING WAVE THEORY: explains how
travelling wave is generated in the basilar
membrane.

17.
 Appreciation of loudness of sound
 Localisation of sound - Cerebral
cortex and medial geniculate body
are responsible for localisation of
sound.

18.
AUDITORY DEFECTS
1. Conduction deafness –occurs due
to impairment in the transmission
of sound waves in external ear or
middle ear
Causes: obstruction of external
auditory meatus, thickening of
tympanic membrane due to inequality
of pressure on either side,
inflammation of middle ear (otitis
media), fixation of footplate of stapes

19.
2. Nerve deafness –caused by
damage of hair cell, organ of corti,
basilar membrane or cochlear duct
or lesion in auditory pathway.
causes- degeneration of hair cells,
damage of cochlea by prolonged
exposure to loud noise, tumor
affecting VIII cranial nerve.

20.
TESTS FOR HEARING
RINNE TEST
 Normal person hears vibration in air even after
the bone conduction ceases
 Air conduction via ossicles is better than bone
conduction.
 But in conduction deafness, the vibrations in air
are not heard after cessation of bone conduction.
 Thus in conduction deafness, the bone
conduction is better than air conduction.
 In nerve deafness, both air conduction and bone
conduction are diminished or lost.

21.
„WEBER TEST
 In unilateral conduction deafness
(deafness in one ear), the sound is
heard louder in diseased ear.
 In unaffected ear, there is a masking
effect of environmental noise.
 In affected side, the sound is louder due
to the absence of masking effect of
environmental noise.
 During unilateral nerve deafness, sound
is heard louder in the normal ear.