PHYSIOLOGY OF HEARING

3. Mechanism of Hearing
1. Conduction of sound. Pinna, Ext. Auditory canal,
tympanic Membrane & Ossicles.
2. Transduction of mechanical energy to electrical
impulses.(Cochlea) Stapes Footplate.
3. Neural pathways (Conduction of electrical impulses to
the brain).

5. Conduction of sound
Pinna
External auditory canal
Tympanic membrane
Ossicular chain

6. Pinna - collects sound & directs them to external
auditory canal.
External auditory canal – transmits the sound to
tympanic membrane.

7. Sound wave from EAC
Tympanic membrane set into vibration
Ossicles vibrate – vibration of footplate of stapes
Oval window vibrates – sound energy reaches inner
ear

8.  Impedance matching mechanism
Middle ear
Sound conducted through air goes to cochlear fluid
Change of medium causes loss of energy which is
compensated by middle ear
1. Hydraulic action of TM:
Vibrating area of TM : vibrating area of stapes footplate
17 : 1
2. Curved membrane effect: Curved surface of TM makes
TM vibrate more at periphery and less at the center.
3. Lever action of ossicles: Handle of malleus 1.3 longer
than long process of incus
Thus sound energy arriving at the cochlea is 17 x 1.3 =
22.1 times.

9. Lever action of the ossicles – handle of malleus is
1.3 times longer than long process of incus.
Hydraulic action of tympanic membrane –
area of TM(21): area of footplate(1)
Vibratory area 14:1
Increase in sound force – 18:1
55sqmm(90):3.2sqmm – 17:1
Increase in sound force – 22 times

11.  Phase difference:
Sound waves do not reach oval window and round
window simultaneously
When oval window receives wave of compression, round
window receives a wave of rarefaction.
Preferential pathway – TM n ossicular chain
Cushion of air in the middle ear.

12. Natural resonance
 EAC : 3000-4000 Hz
 TM : 800-1600 Hz
 Ossicular chain : 500 – 2000 Hz
 Middle ear : 800 Hz

13. Bone conduction
Transmision of sound to inner ear by vibrating the
skull bones.
Theories
1. Inertial Theory
2. Compressional Theory
3. Osseotympanic Theory

14.  Inertial Theory
Sound stimulus
Skull vibrates but ossicles lag behind because of
inertia
Ossicles & skull vibrate out of phase to each
other
Piston like movement of stapes footplate in oval
window

15. Compressional theory: skull bones including bony
labyrinth vibrate & set inner ear fluid into vibration.
Osseotympanic theory : vibration of skull bones;
mandible lags behind because inertia.
EAC air vibration
TM vibration

16. Transduction mechanism
 Movements of the stapes footplate, transmitted to the
cochlear fluids, move the basilar membrane, setting
up shearing force between the tectorial membrane &
the hair cells.
The distortion of hair cells gives rise to cochlear
microphonics which trigger the nerve impulse.

18. Perceptive mechanism
Auditory signals from perilymph in cochlea
Organ of corti
Nerve ending
VIII cranial nerve
Cerebral cortex

19. Auditory pathway
Vestibulocochlear nerve
Ventral & Dorsal cochlear nuclei
Superior olivary complex
Lateral lemniscus
Inferior colliculus
Medial Geniculate body
Auditory Cortex

20. Theories of hearing
 RUTHERFORD’S TELEPHONE THEORY: Sound
stimulus causes basilar membrane of cochlea to
vibrate uniformly in all parts.
 Amplitude of vibrations = sound intensity.
 Rate of firing of auditory nerve fibres determines
frequency of sound.
 Helmholth’s place theory: Each pitch causes
vibration of its own particular “place” on basilar
membrane.

21.  Wever’s Volley theory:
 Higher frequencies(>5000Hz) by Place mech.
 Lower frequencies(<400Hz) by telephone
mechanism.
 400-5000Hz - by both mechanisms.
 Bekesy’s Travelling wave theory: sound stimulus at
oval window causes vibration of BM from basal turn
to apex.

22. THANK YOU