Physiology of Hearing

1. 1
Dr Gauhar Hussain, MD
Assistant Professor of Physiology
College of Medicine, Dawadimi
Shaqra University
Hearing

2. 2
At the end of this session students should be
able to-:
• 34- Describe the mechanism of conduction of
sound, physical principles of sound waves and
auditory pathway from organ of Corti to cortex
Objective

3. 3
What properties of sound
does the ear detect?
Physical
Dimension
Perceptual
Dimension
INTENSITY Loudness
FREQUENCY Pitch
COMPLEXITY Timbre

4. 4
The Human ear is sensitive to sounds over a wide range of:
- Frequencies: 16 – 20,000 Hz
- Amplitudes: 0.0002 – 200 dyne/cm2
• > 85 dB can cause hearing loss, 120-160 painful
• Moves through elastic medium as particle oscillations
around equilibrium positions.
• Speed of sound - phase velocity (c) depends on physical
properties of medium, mainly on elasticity and temperature.

5. 5
Sound conduction in the ear
Main Components of Hearing Mechanism:
Divided into 4 parts (by function):
•Outer Ear
•Middle Ear
•Inner Ear
•Central Auditory
Nervous System
Main Components of Hearing Mechanism:
Divided into 4 parts (by function):
• Outer Ear
• Middle Ear
• Inner Ear
• Central Auditory
Nervous System

6. 6
Auricle (Pinna): Gathers sound waves
Aids in localization, Amplifies sound approx. 5-6 dB
External Auditory Canal:
• Approx. 1 inch long, “S” shaped
• Outer 1/3 surrounded by cartilage; inner 2/3 by
mastoid bone
• Allows air to warm before reaching TM
• Isolates TM from physical damage
• Cerumen glands moisten/soften skin
• Presence of some cerumen is normal
External ear

7. 7
Middle ear
• Air filled
• 3-ossicles
• 2-muscles
• Eustachian
tube

8. 8
• Due to the pressure changes produced by sound waves
tympanic membrane vibrates, i.e. it moves in and out
of middle ear. Thus, TM acts as a resonator.
• Changes acoustical energy into mechanical energy
• Middle ear contains air and inner ear contains fluid
(with inertia),so sound is not transmitted easily.
• It is transmitted by increasing the pressure in
middle ear.
Transmission of sound through
middle ear to inner ear

9. 9
• Ear ossicles move as a single unit –magnify the
sound intensity by 1.3 times.
• TM and oval window surface area ratio increase the
pressure 17 times.
• TM and auditory ossicles converts sound of greater
amplitude but lesser force to lesser amplitude and
greater force.
• Impedence Matching = 17 x 1.3 = 22 times
Transmission of sound through
middle ear to inner ear

10. 10
Impedence Matching
force increases about 1.3 times
17x1.3= 22
55/3.2=17
17

11. 11
• also called tympanic reflex or acoustic reflex,
• is a protective reflex which reduces sound pressure
amplitude by affecting the mobility and transmission
properties of the auditory ossicles.
• Stimulus for this reflex is loud sound.
• Latent period is 40–80 ms.
• Sound is attenuated or intensity of low frequency
sound is reduced by 30–40 decibel.
Attenuation reflex

12. 12
• Reflex activity. acoustic reflex, The two muscles of the middle
ear (tensor tympani and stapedius) contract reflexively in
response to the intense sound.
• Contraction of tensor tympani pulls the malleus inwards :
causes tensening of TM and decreases its vibration.
• Contraction of stapedius muscle pulls stapes footplate out of the
oval window.
• These two opposing forces make the ossicular system very rigid
and excessive sound is prevented from going to oval window.
Attenuation reflex

13. 13
• Advantages of attenuation reflex are:
• It protect the cochlea from damaging vibrations
caused by intense sounds- loud music, jet aircraft
• It attenuates and masks all the low frequency
sounds in loud environment and allows the person
to concentrate on the sound above 1000 Hz.
• Eustachian Tube :Connects middle ear cavity to
nasopharynx; Mucous-lined
• “Equalizes” air pressure in middle ear

14. 14
• Coiled tube 35 mm long : Snail
shaped cavity within mastoid bone
• 2 ½ turns, 3 fluid-filled chambers
• Basilar membrane and
Reissner's membrane divide it into
three chambers or scalae
1. Scala vestibuli starts at the oval
window.
2. Scala media having endolymph
3. Scala tympani ends at the round
window.
Perilymph – more proteins.
Inner ear & Cochlea

15. 15
Primary receptor of hearing
• Scala Media contains Organ of Corti
Converts mechanical energy to
electrical energy
• Hair cells are arranged in 4-5 rows:
• 3 or 4 rows of outer hair cells lateral
to the tunnel formed by the rods of
Corti, and supported by Deiter ‘s cells.
• Single row of inner hair cells medial
to the tunnel.
Organ of Corti

16. 16
• Hairs of the outer hair cells
are embedded in the tectorial
membrane
• Hairs of the inner hair cells
are not attached to the
tectorial membrane, but they
are bent by fluid moving
below tectorial membrane
Organ of Corti

17. 17
ROLE OF HAIR CELLS
Inner Hair Cells (3500): primary sensory cells
• Signal transduction is basic function of inner hair cells
• Generate action potential in auditory nerve fibers.
• about 90-95 percent of the auditory nerve fibers are stimulated by the inner
hair cells.
Outer Hair Cells (12000): facilitates the movement of basilar membrane
• control the sensitivity of the inner hair cells at different sound pitches,
“tuning” of the receptor system.(cochlear amplifier)
• responsible for otoacoustic emission, and are most commonly damaged
in presbycusis.

18. 18
• sensory hairs (cilia) - stereocilia, deformed by tectorial membrane.
• Bending of hairs towards lamina spiralis causes depolarisation
• Bending away lamina spiralis causes hyperpolarisation
• Tip links =depolarisation when stereocilia moves towards Kinocillium.
• Potassium ion is responsible for depolarisation of hair cells
Sensory Transduction in ear

19. 19
• The inner ear has 2 main functions:
1.Mechanical frequency analysis:
 travelling wave and
 resonant point (place principle)
2. Sensory transduction:
 Acoustically generated pressure waves are transformed into
neural impulses
VIIIth Cranial Nerve or “Auditory Nerve”
• Bundle of nerve fibers (25-30K)
• Travels from cochlea through internal auditory meatus to
skull cavity & brain stem
Sound processing in inner ear:

20. 20
• PLACE THEORY (which fibres, labelled lines)
• Von Békésy (Nobel prize 1961)
• 1 - Travelling wave; stiffness varies
• 2 - one place most active for a given frequency
• 3 - Tonotopic code; coded as place
• PERIODICITY THEORY
• (how they are firing, temporal code)
• Sound coded as pattern
Pitch discrimination

21. 21
• Movement of footplate of stapes against oval window causes
movement of perilymph in scala vestibuli.
• This fluid does not move all the way from oval window to round
window through helicotrema.
• It immediately hits the flexible vestibular membrane near oval
window. This causes movement of fluid in scala media, which causes
bulging of basal portion of basilar membrane.
Travelling wave

22. 22
Frequency analysis in inner ear:
Variation in thickness of membrane is reason
for basilar membrane to be frequency specific

23. 23
Resonance point is the part of basilar
membrane, which is activated by
traveling wave.
Where the wave has its highest
amplitude depends on its frequency
Frequency analysis in inner ear:

24. 24
• Sound Travels Through Ear tympanic membrane →
vibrate, like a drum → changing it into mechanical
energy → Malleus (attached to tympanic
membrane, starts motion → stapes moves in & out
of oval window of cochlea → creating a fluid
motion, or hydraulic energy → membranes in the
Organ of Corti to shear against hair cells →
electrical signal which is sent up Auditory Nerve to
brain → brain interprets it as Sound.
Summary

25. 25
1. Fibers from the spiral ganglion of
Corti enter the dorsal and ventral
cochlear nuclei located in the upper
part of the medulla
2. second-order neurons pass mainly to
the opposite side of the brain stem to
terminate in the superior olivary
nucleus (SON).
• A few second-order fibers also pass to
the superior olivary nucleus on the
same side.
Central
Auditory Pathway

26. 26
3. From the superior olivary
nucleus (SON), the auditory
pathway passes upward through
the lateral lemniscus.
4. Some of the fibers terminate in
the nucleus of the lateral
lemniscus, but many fibers bypass
this nucleus and travel on to the
inferior colliculus (IC)
Central
Auditory Pathway

27. 27
5. From there, the pathway passes
to the thalamus-medial
geniculate nucleus, where all
the fibers do synapse (Relay).
6. Finally, the pathway proceeds
by way of the auditory radiation
to the auditory cortex, located
mainly in the superior gyrus of
the temporal lobe: Wernicke’s
Area.
Central
Auditory Pathway

28. 28
7. Signals from both ears are transmitted through the
pathways of both sides of the brain, with a preponderance of
transmission in the contralateral pathway.
8. In at least three places in the brain stem, crossing over occurs
between the two pathways: (1) in the trapezoid body, (2) in the
commissure between the two nuclei of the lateral lemnisci, and
(3) in the commissure connecting the two inferior colliculi.
9.Many collateral fibers from the auditory tracts pass directly into
the reticular activating system of the brain stem.
10. Other collaterals go to the vermis of the cerebellum, which is
also activated instantaneously in the event of a sudden noise.
Central
Auditory Pathway

29. 29
 Guyton & Hall - Text book of Medical Physiology.
 Ganong’s - Review of Medical Physiology.
References
Thank you