Physiology of ear. Basic definition related to sound -hearing,sound,sound wave. mechanism of hearing mechanical conduction of sound transfer action of middle ear impedence areal ratio/ hydraulic lever lever ratio of ossicles catenary lever transduction of mechanical energy travelling wave theory of Bekesy sound propagation in cochlea electrical conduction of sound central auditory pathway acoustic reflex

1. Physiology of Ear
-Dr. Alka Kapil
( Assistant Professor, Dept. of ENT , MKCG Govt.
Medical College)

2. Sound is a form of energy produced by a vibrating object
Hearing is the perception of the sound vibrations in the environment
A sound wave consists of compression and rarefaction of molecules of the medium
(air, liquid or solid) in which it travels

3. Mechanism of hearing can be broadly divided into three steps :
1. Mechanical conduction of sound (conductive apparatus)
2. Transduction of mechanical energy to electrical impulses (sensory system of cochlea)
3. Conduction of electrical impulses to the brain (neural pathways)

4.  The outer ear is considered the “sound collector” due to its shape and location on the head,
the outer ear structure captures and transmits sound to be processed by the more medial
structures of the auditory system
 Because the peripheral auditory system is encased in bone, any stimulus that can initiate bone
conduction in the body has the potential to elicit an auditory percept
1 Mechanical conduction of sound

5.  For lower to mid frequencies, the tympanic membrane tends to vibrate like a disk with
maximum displacement in the inferior edge of the membrane
 For higher frequencies, the tympanic membrane vibrates in a more segmented fashion, which
likely facilitates the transmission of higher-frequency information by reducing the amount of
membrane that needs to vibrate
The tympanic membrane is the first point of energy transduction whereby sound
(i.e., acoustic energy) is converted to mechanical signal

6.  Whenever a sound passes from the external & middle ear to the inner ear there is a
change in medium from air to liquid.
 Due to this change 99.9% of the sound gets reflected back.

7.  To overcome this and match the impedance (resistance to flow of sound) of the middle ear
(low impedance) with the inner ear (high impedance), the middle ear acts as a transformer.
 By this transformer action of middle ear there occurs an increase in the force of any
sound that enters the middle ear, so that more of sound can now enter the inner ear rather than
most of it getting reflected back.

8. Areal/ Hydraulic lever
 The average ratio between the total tympanic membrane is
21 times larger than the oval window
 This means that sound pressure collected at the large TM
(45mm2) will be transmitted in an amplified manner to the
small oval window (3.2mm2)
 As the effective vibratory area of tympanic membrane is
only 2/3rd (45mm2), the effective areal ratio is reduced to
14:1, and this is the mechanical advantage provided by
the tympanic membrane.
 A gain (+26 dB SPL) is boosted by this area ratio between
TM and stapes footplate .
Schematic drawing to show the hydraulic lever of middle ear
14

10. Lever ratio of ossicles
 Handle of malleus is 1.3 times longer than long process
of the incus
 This provides a mechanical advantage of 1.3 because
of which the incus moves 1.3 times more than the
handle of malleus.
 A gain (+2 dB SPL) at the oval window is boosted by
this lever ratio of ossicles.

11. Catenary lever
 Sound energy is directed away from the edges of the TM and
toward the center of the TM via waves that travel on the TM
surface.
 The attachment of the tympanic membrane at the annulus
amplifies the energy at the malleus because of the elastic
properties of the stretched drumhead fibers and thus works as a
catenary lever (ratio of force acting on tympanic membrane to that
acting on the malleus) where large displacements near the annular
ring (the outer edge) produce small displacements of the malleus,
so the ear drum itself can increase force when it moves.
 This catenary lever (aka buckling effect of TM) increases pressure
by a factor of 2 = 6 dB SPL
Schematic
drawing to show the
catenary lever of the
tympanic membrane. The
attachment of the
tympanic membrane (TM)
at the annulus amplifies
the energy at the malleus
where large displacements
near the periphery (d1)
produce small
displacements of the
malleus (d2)

12. 55mm2
17.1
17 X 1.3 =22.1
22 : 1
Transformer action of the middle ear : Hydraulic effect of tympanic membrane and lever action of ossicles combine to compensate the sound energy lost during its
transmission from air to liquid medium.

13. Energy transduction of the peripheral auditory
system. The system is simplified to include only the
anatomic sites of energy transduction. The active process
(red) represents activation of outer hair cells (OHC) during
low-intensity stimulation. The passive process (blue)
represents activation
of inner hair cells during high-intensity stimulation.

14. 2 Transduction of mechanical energy
into electrical impulses
 Movements of the stapes footplate, transmitted to the cochlear fluids, move the basilar membrane
and set up shearing force between the tectorial membrane and the hair cells. The distortion of hair
cells gives rise to cochlear microphonics, which trigger the nerve impulse.
 A sound wave, depending on its frequency, reaches maximum amplitude on a particular place on
the basilar membrane and stimulates that segment (travelling wave theory of von Bekesy)
 Higher frequencies are represented in the basal turn of the cochlea and the progressively lower
ones towards the apex

15. Travelling wave
theory
“Traveling waves” along the basilar membrane for high- (A),
medium- (B), and low-frequency (C) sounds
 Proposed by George Von Bekesy
 The theory states that a sound impulse sends a
wave sweeping along the basilar membrane. As the
wave moves along the membrane, its amplitude
increases until it reaches a maximum, then falls off
sharply until the wave dies out. That point at which
the wave reaches its greatest amplitude is the point
at which the frequency of the sound is detected by
the ear.

16. Uncoiled cochlea showing the direction of the traveling wave along the basilar membrane to seven
different frequency inputs. For each subsequent frequency input (from 1,600 to 25 Hz), the peak of
maximum vibration moves from base to apex, highlighting the tonotopic gradient of the cochlear partition.

17. Sound Propagation in the cochlea
Vibratory wave: oval window → scala vestibuli → helicotrema → scala tympani → round window

18. As sound energy travels through the external and middle ears, it causes the stapes footplate to vibrate
The vibration of this footplate results in a compressional wave on the inner ear fluid
Because the pressure in the scala vestibuli is higher than that in the scala tympani, this sets up a
pressure gradient that causes the cochlear partition to vibrate as a traveling wave
Because the basilar membrane varies in its stiffness and mass along its length, it is able to act as a
series of filters that respond to specific sound frequencies at specific locations along its length

19.  Basilar membrane and organ of corti move up and down with sound stimulus
 This causes shearing action between
tectorial membrane and hair cells
leading to bending of stereocilia sideways
3 Conduction of electrical impulses to brain

20. Vibratory wave → deflection of hair cell stereocilia → potassium influx → depolarization (resting potential in
endolymph +60-100 mV relative to perilymph) → action potential at first level neurons of spiral ganglion

21. When the stereocilia are bent in the direction of the
longer ones, K+ channels are opened, causing
depolarization, which in turn opens voltage-gated Ca2+
channels.
The influx of Ca2+ augments the depolarization and
elicits release of the excitatory transmitter glutamate
This in turn depolarizes the sensory nerve generating
the auditory impulse which is carried to the
auditory cortex of brain

22. Central auditory pathway
E. Auditory ( Eighth ) nerve
C. Cochlear nuclei
a. Dorsal cochlear nucleus
b. Anterior ventral cochlear nucleus
c. Posterior ventral cochlear nucleus (the
majority of auditory fibers cross the midline)
O. Superior Olivary complex
L. Lateral lemniscus
I. Inferior colliculus
M. Medial geniculate body
A. Auditory cortex
DAS, Dorsal acoustic stria; IAS, intermediate acoustic stria;
VAS, ventral acoustic stria

23.  The primary auditory cortex is located on the superior
surface of the temporal lobe (Heschl gyrus); this is
also known as area A1,which corresponds to Brodmann
area 41
 The auditory association cortex is also known as area A2
and corresponds to Brodmann areas 22 and 42 of the
temporal lobe

24. Acoustic Reflex
 Definition: When Loud sound of 70–100 dB above the hearing threshold is presented to
one ear, the stapedius muscle of both the ears contract decreasing the mobility of stapes.
 Function: protecting the inner ear from noise trauma

25. Flowchart of mechanism of hearing

27. Thank You