Auditory Science
Peripheral Auditory System
Karolina Kluk
karolina.kluk@manchester.ac.uk
Coronal section
http://www.cartage.org.lb/
Horizontal section
Sagittal section
http://www.cartage.org.lb/
1
Auditory System
Central Auditory System
Peripheral Auditory System
•  outer ear
•  middle ear
Peripheral
Auditory
System
•  cochlea
•  auditory nerve
Outer ear
How the Ear Works
Inner ear
Middle ear
How the Ear Works
Sound waves enter the ear canal and cause the
eardrum to vibrate.
These vibrations are carried by three tiny bones, the
malleus, incus, and stapes, to the cochlea.
The cochlea is a thin tube filled with fluid that is
curled up into a spiral. In the cochlea, acoustic
vibrations are transduced into electrical neural
impulses.
2
How the Ear Works
Running the length of
the cochlea is the
basilar membrane.
Each place on the
membrane is tuned to
a particular frequency:
How the Ear Works
The sensory cells in the ear, the hair cells, are
arranged in rows along the length of the basilar
membrane:
Outer
Hair Cells
Inner
Hair Cell
3
Central
auditory system:
•  Cochlear Nucleus
•  Superior Olivary Complex
•  Lateral Lemniscus
•  Inferior Colliculus
•  Medial Geniculate Body
•  Auditory Cortex
FROM AIR TO EAR – OUTER EAR
Composed of elastic fibro-cartilage
Peripheral Auditory System
•  outer ear
•  middle ear
•  cochlea
•  auditory nerve
Outer ear
Inner ear
Middle ear
PINNA (Auricle)
Composed of elastic fibro-cartilage
4
S shaped
Functions of the Pinna
The pinna collects sound energy and funnels the energy
into the ear canal.
The pinna modifies sound entering ear depending on
direction, resolving ambiguities and providing
cues to source elevation:
15° Elevation
Ear Canal
(External Auditory
Meatus)
Bony
external auditory meatus
Cartilaginous
external auditory meatus
-15° Elevation
15°
10 dB!
Surrounded by bone, extremely thin
skin, hairless, very sensitive
-15°
3
4
5
6
7
8
9
10
Frequency (kHz)
Functions of the Ear Canal
The ear canal protects the tympanic membrane
(eardrum). The sticky nature of earwax helps to
prevent dust and foreign objects penetrating the
ear canal.
The ear canal acts as a resonator (like an organ pipe),
amplifying frequencies in the region of
3000-4000 Hz.
Surrounded by cartilage,
thick skin, hair and earwax glands
Ear Canal Resonance
RESONANCE is the tendency of a system
to oscillate at larger amplitude at
some frequencies than at others
5
Ear Canal Resonance
V
f 0 = sound
4 Ltube
Total
Canal
Pinna
Vsound = 344 m/s
EAM length = 2.5 – 3 cm
Wavelengths at
resonance = 10 – 12 cm
2.5 kHz
5.5 kHz
3 kHz
Resonance frequency =
3500 – 4000 Hz
Natural resonances of ear canal not heard – subject has grown up with them!
Sandlin (2000) Hearing Aid Amplification, Chapter 12
Hearing aid resonances and changed ear-canal acoustics combine
Sandlin (2000) Hearing Aid Amplification, Chapter 12
6
Babies!
•  As well as a higher resonant frequency due to the
shorter canal, babies also have a smaller canal
volume.
•  This means that for a given sound source, the intensity
is higher in the baby s ear than in an adult s.
•  Thus, fitting a baby s hearing aid needs to take this
into account – see hearing aids lectures & their
discussions on RECDs
Baby s ear-canal is smaller (shorter) than adult s – resonant frequency is higher
Modified from: Sandlin (2000) Hearing Aid Amplification, Chapter 12
Directional Effects – head and torso
Cues for localization
diffraction
1. Interaural Time Difference (ITD)
direc
t
reflection
2. Interaural Level Difference (ILD)
Owing to the physical nature of sounds, ITDs
and ILDs are not equally effective at all
frequencies.
7
Outer ear – summary:
Outer ear & localisation
•  See later lecture:
–  Differences between time of arrival at the two ears –
interaural time difference (ITD – effective cue at low
frequencies)
–  Differences in interaural level (ILD) – effective cue at
high frequencies
–  Monaural cues (different reflections from pinna
depending on direction of source) – high frequency
cue for location in vertical plane
Peripheral Auditory System
1.  The outer ear consists of pinna and the ear canal.
2.  The skin of the ear canal is innervated by four cranial nerves
3.  Functions of the outer ear are:
•  protection of tympanic membrane
•  acoustic transmission path to the tympanic membrane
•  sound collection
•  acoustic amplification by resonance
•  sound localization
4. Interaural Time Difference (ITD) is useful for sound localization at
low frequencies, Interaural Level Difference (ILD) is useful at high
frequencies.
The Middle Ear
malleus
incus
stapes
Composed of:
•  Air spaces
•  outer ear
•  Tympanic
membrane
•  middle ear
•  cochlea
•  auditory nerve
Outer ear
Inner ear
Middle ear
c
ni e
pa ran
m b
ty em
m
e
ow
ub
t
d
in
an
w
hi
d
c
a
un
st
ro
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E
•  Auditory
ossicles
•  Middle ear
muscles
8
Tympanic Membrane
Tympanic Membrane
The tympanic membrane (TM) has three layers:
1.  An outer layer of very thin skin, continuous with that of
the external ear canal;
2. A middle layer of radially and concentrically arranged
connective tissue fibres;
3. An inner layer, a single layer of cells, continuous with
the lining of the middle ear cavity.
Posterosuperior
Pars flaccida
quadrant
Posteroinferior
quadrant
AnteriorParssuperior
tensa
quadrant
Anteriorinferior
quadrant
The TM is shaped like a blunt cone,
with the apex (umbo) extending
toward the middle ear space.
The connective tissue layer makes
the TM a stiff structure (pars tensa),
the posteriosuperior quadrant with
very little connective tissue is less
stiff (pars flaccida).
Otoscopic appearance of
normal right tympanic membrane
Ossicles
Incus
Malleus
Stapes
9
The Middle Ear – Auditory Ossicles
The axis of rotation of the ossicular chain
Anterior ligament of
the malleus
Posterior ligament of
the incus
Functions of the Middle Ear (1): Impedance
Matching
The middle ear ensures that sound is transmitted
efficiently between the air and the fluids in the
cochlea.
The middle ear acts as an impedance-matching
transformer.
Middle Ear Impedance Matching
z
Acoustical impedance ( ) of any substance (be it gas, fluid,
or solid) is, by definition, equal to the sound pressure
p
vibrating the substance ( ) divided by the velocity (speed)
v
of vibration ( ). The closer the impedances of two media
(such as air and fluid) are to each other, the more acoustical
energy will be transmitted from one to the other.
Z = p/v
Impedance Matching
If the sounds were to arrive directly onto the oval
window, most of the energy would be reflected
back.
The middle ear acts as impedance-matching device
that improves sound transmission and reduces
the amount of reflected sound.
Transmission of sound through the middle ear is
most efficient at middle frequencies (500-4000 Hz).
10
1. Areal ratio mechanism:
AIR
WATER
incident sound
reflected sound
transmitted sound
Water is denser than air and has a higher speed of
sound, giving it a much higher impedance.
Only 0.1% of sound energy is transmitted
across the air/water boundary. 99.9% is
reflected (like throwing a ping-pong ball
against a basketball).
3. TM buckle:
2. Lever mechanism:
(depends on the conical shape of the TM)
d1
d2
11
Impedance Matching
Middle Ear Impedance Matching
1.  The difference between the areas of TM and the footplate of the stapes
increases pressure
Z2 = 17 ⋅ Z1
2. The difference between the lengths of the two lever arms
both increases pressure and decreases velocity at the oval
window
Z 2 = 1.7 ⋅ Z1
3. The tympanic membrane buckle increases pressure and
decreases velocity
Impedance is greater at the stapes footplate (oval
window) than at the tympanic membrane
Z 2 = (17 ⋅1.7 ⋅ 2)Z1 = 58⋅ Z1
20 x log (58/1) = 35 dB total advantage
Z 2 = 2 ⋅ Z1
Impedance Matching
Taken together, the
three mechanisms
result in at most 74%
of energy being
transmitted to the
cochlea.
Functions of the Middle Ear (2): Reducing Bone
Conducted Sound
The middle ear reduces the transmission of boneconducted sound to the cochlea.
Best transmission
Chewing, blood flow, air flow, creaking joints, and
contraction of muscles attached to the head,
cause the skull to vibrate.
The arrangement of the ossicles reduces the
transmission of skull vibration because there is
little relative movement of skull and ossicles when
the skull vibrates.
12
Middle Ear Muscles
Stapedius
1. Stapedius
2. Tensor Tympani
Tensor Tympani
Stapedius Muscle
When stapedius muscle contracts it stiffens ossicular
chain and reduces transmission of low
frequencies (< 1000 Hz).
Involved in two reflexes: stapedius reflex, which is
triggered by high sound levels and prevents
damage and masking by low-frequency sounds
(because of delay, useless for impulsive sounds).
Vocalisation reflex: stapedius contracts just before and
relaxes just after vocalisations, reducing masking
by self-generated speech.
http://www.iurc.montp.inserm.fr
Tensor Tympani
When tensor tympani contracts it pulls on malleus,
stiffens TM and pulls it into middle ear cavity,
reducing the volume of the cavity.
The muscle contracts (with tensor veli palatini) when
we swallow or yawn, pushing fluid in the middle
ear out of the Eustachian tube.
Also helps equilibrate pressure in the middle ear
with atmospheric pressure.
Middle ear – summary:
1.  Two muscles are attached to the ossicles, the tensor tympani to
the manubrium of malleus and the stapedius to the stapes.
2.  Function of the middle ear is:
•  to ensure the efficient transfer of sound from the air to the
fluids in the cochlea (impedance matching)
•  to reduce the transmission of bone-conducted sound to the
cochlea.
3.  There are 3 mechanisms involved in middle ear impedance
matching:
•  areal ratio mechanism
•  lever mechanism
•  tympanic membrane buckle
13
Peripheral Auditory System
•  outer ear
•  middle ear
•  cochlea
•  auditory nerve
Outer ear
Inner ear
Middle ear
Basilar Membrane
Running the length of
the cochlea is the
basilar membrane.
Each place on the
membrane is tuned to
a particular frequency:
Travelling Wave
High frequency sound
Low frequency sound
http://www.iurc.montp.inserm.fr
14
Travelling Wave
Travelling Wave
When pressure waves enter the cochlea, the basilar
membrane starts to vibrate.
The vibration takes the form of a “travelling wave”,
that appears to travel from the base of the
cochlea to the apex.
The wave peaks at the place tuned to the frequency
of the sound, then dies away rapidly.
Travelling Wave
Von Békésy (1947)
observed travelling waves
in the apex of the cochlea
from human cadavar ears,
using a microscope and
stroboscopic illumination.
Travelling Wave
Basilar
membrane
“Envelope” of
travelling wave
“Envelope” of
travelling wave
15
Low frequency tone – 250 Hz
Mid frequency tone – 1000 Hz
BM
displacement
BM
displacement
Base
Distance
Apex
Base
Distance
Apex
High frequency tone – 12500 Hz
BM
displacement
Base
Distance
Apex
16
Passive Mechanism
Passive Mechanism
Von Békésy observed travelling waves in cadavar
ears, in poor physiological condition and using
very high levels (140 dB SPL).
He observed quite broad tuning: Each tone excited a
large region of the basilar membrane.
Passive Mechanism - Linear Growth
High"
FREQUENCY"
Low"
Bekesy"
Stimulus"
B!a!s!e!
Courtesy of Richard Baker
Passive Mechanism
1. Broad tuning and therefore poor frequency
selectivity.
2. Insensitivity to low-level sounds.
INTENSITY"
BASILAR MEMBRANE DISPLACEMENT"
17000
PLACE"
3. Linear response growth.
So how do we explain the excellent frequency
selectivity and high sensitivity exhibited by
healthy human listeners?
A!p!e!x!
17
Recent Measurements
Recent Measurements
(Data from Sellick et al., 1982)
Recent physiological measurements of tuning in
healthy ears show much sharper tuning.
Also, the tuning depends critically on the
physiological condition of the animal: the better
the condition, the sharper the tuning.
Sellick et al. (1982) measured tuning
curves at a single place on the
basilar membrane.
Figure shows the input sound level,
in dB SPL, required to produce a
constant velocity of motion at a
particular place on the BM, as a
function of frequency.
CF
Response Functions
Active Mechanism
The shallow response growth, and sensitivity, goes away if the cochlea is
in a poor physiological condition:
50
40
Gain ≈ 50 dB
30
Active!
Gain!
BM Velocity (dB)!
60
BM Velocity (dB)
Alive
Post-mortem
Linear
70
BM Velocity (dB)
The frequency selectivity of the basilar membrane is enhanced by the
active amplification of frequencies close to the CF of each place:
Gain!
Passive!
20
0
10
20
30
40
50
60
70
80
90
100 110
Frequency
Input Level (dB)
Level (dB SPL)
Ruggero et al. (1997)
18
OHC Transduction
As the basilar membrane vibrates up and down, the
hairs (stereocilia) on the hair cells are bent from
side to side:
Outer Hair Cells
Stereocilia
Cuticular plate
Tight junction
Deiters cell
phalangeal process
Mitochondrion
Motile apparatus
Nucleus
Efferent
nerve
Fettiplace & Hackney (2006) Nature Reviews – Neuroscience, 7, 19-29
Good animations available at http://www.cochlea.org
Outer Hair Cells
Afferent
nerve
http://www.iurc.montp.inserm.fr
Outer Hair Cells
1.  12 000 OHCs
2.  The upper surface of each OHC contains:
-  Cuticular plate
-  Three rows of stereocilia
-  A noncuticular area that contains the
basal body of a rudimentary kinocilium
(at the bottom of the W or V ,
towards the stria vascularis)
3.  On each OHC are about 150 stereocilia,
arranged in two or more rows that form a
shape of a V
4.  The tips of the tallest raw of stereocilia are in
contact with a tectorial membrane
19
OHC
When the stereocilia are
bent in one direction, tip
links stretch and open ion
channels.
Positively-charged
potassium ions enter the
cell causing it to become
“depolarised.”
http://www.iurc.montp.inserm.fr/cric/audition/english/start2.htm
OHC Electromotility
OHC Electromotility
OHC membrane
depolarisation
changes the
conformation of
the motor
protein PRESTIN
Fettiplace & Hackney (2006) Nature Reviews – Neuroscience, 7, 19-29
http://www.iurc.montp.inserm.fr
20
Reverse, or electro-mechanical transduction
OHC damage
Cochlear trauma causes OHC dysfunction and loss or
reduction of active amplification. Causes include:
Exposure to loud sounds
Ototoxic drugs
Hypoxia
Dancing OHC, courtesy of Jonathan Ashmore, UCL
Santos-Sacchi, J. (2003) New tunes from Corti s organ: the outer hair cell boogie rules. Current Opinions in
Neurobiology, 13, 1-10
http://130.132.220.165/publications/santos-sacchi%202003.pdf
Inner Hair Cells
Stereocilia
Cuticular
plate
Rudimentary
kinocilium
Mitochondrion
Nucleus
Rough
endoplasmic
reticulum
Afferent nerve ending
Efferent nerve ending
Death
Inner Hair Cells
1.  3500 IHCs
2.  The upper surface of each IHC
contains:
- Cuticular plate
- Three rows of stereocilia
- A noncuticular area that
contains the basal body of a
rudimentary kinocilium
3.  On each IHC are about 40
stereocilia, arranged in two or
more parallel rows that form a
very shallow U
http://www.iurc.montp.inserm.fr
21
Inner Hair Cell Activation
Mechanoelectric Transduction
-40mV
Excitation ! stereocilia
bend towards the basal body
+40mV
Inhibition ! stereocilia
bend away from the
basal body
-60mV
http://www.physiology.wisc.edu/ftp/pub/aud-tour/
http://www.iurc.montp.inserm.fr/cric/audition/english/start2.htm
Mechanoelectric Transduction
Mechanoelectric Transduction
When the stereocilia are
bent in one direction, tip
links stretch and open ion
channels.
Positively-charged
potassium ions enter the
cell causing it to become
“depolarised”: increase in
voltage from -40 mV to +40
+ 80 mV
- 40 mV
mV.
1.
Stereocillia bend
2.
K+ flows into the IHC
3.
IHC gets depolarised
4.
Voltage-gated Ca2+ channels
open; Ca2+ flows into the IHC
5.
Neurotransmitter – glutamate – is
released into the synaptic clefts at
the base of the IHC
http://www.iurc.montp.inserm.fr
22
Mechanoelectric Transduction
+ 80 mV
6.
The neurotransmitter causes
depolarisation of the dendrite of
the auditory nerve.
7.
Action potentials are generated in
the auditory nerve fibre.
8.
Transduction complete.
- 40 mV
Transduction
The inner hair cells
depolarise, and
release
neurotransmitter, only
when the stereocilia
are bent in one
direction:
Displacement of
Basilar
Membrane!
0
5
10
15
20
Inner Hair
Cell!
Auditory
Nerve Fiber!
http://www.iurc.montp.inserm.fr
Cochlear Electrical Potentials
Vibration of Basilar Membrane!
Auditory Nerve Spikes!
Electric Potential
Auditory nerve fibres
produce spikes that
are phase-locked
(synchronised) to the
vibration of the
basilar membrane:
Displacement
Phase Locking
The electro-chemical properties of the cochlea
determine how the transduction process
occurs.
Single Fiber!
Information about the cochlear transduction process
can be obtained from the cochlear potentials.
Many Fibers!
Time!
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Resting Potentials
Scala Vestibuli
Perilymph
(+) 2-5 mV
Scala Media
Endolymph
+ 80 mV
- 70 mV
Scala Tympani
Perilymph
0 mV
The voltage
gradients in
different fluid-filled
compartments can
be assessed by
passing an
electrode from the
scala tympani
through the organ
of Corti and into the
scala media.
http://www.iurc.montp.inserm.fr
Von Békésy
(1952)
Endocochlear Potential
A large +80 mV endocochlear potential is recorded
when the electrode is in the scala media.
The endocochlear potential is the main driving force
that is responsible for moving positively charged
ions through the transduction channels of the
hair cell stereocillia.
The endocochlear potential is generated by active
metabolic processes in cells within the stria
vascularis.
Resting Potentials
The endolymph has a resting potential of
approximately + 80 mV at all times.
The OHCs have intracellular resting potentials of about
-70 mV.
There is approximately a 150 mV difference between
the endolymph and the inside of OHCs.
The IHCs have intracellular resting potentials of about
-40 mV.
24
OHC
IHC
Radial afferent
Lateral efferent
Afferent Innervation (To the Brainstem)
All cochlear hair cells are
innervated by peripheral
processes of spiral ganglion cells.
Medial efferent
Spiral afferent
The central processes comprise
the cochlear division of the VIIIth
cranial nerve, which terminates in
the cochlear nucleus.
http://www.iurc.montp.inserm.fr
Afferent Innervation (To the Brainstem)
Although there are 3 times as many
OHCs as IHCs, 95% of the spiral
ganglion neurons of the cochlea
innervate only IHCs.
In humans, about 18 spiral ganglion
neurons synapse on each IHC.
Type I Spiral
Ganglion Cells
Both their cell bodies and
their peripheral and central
processes are well
myelinated.
These are the Type I spiral ganglion
cells.
Schematic representation of the hair cell afferent innervation (from
Liberman)
25
Type II Spiral
Ganglion Cells
Are only 5% of sensory neurons
to the cochlea.
They are small, unmyelinated
cells whose physiological
properties are unknown.
Go to OHCs, with each fibre
making small afferent sensory
synapses with many OHCs.
Efferent Innervation (From the Brainstem)
Organ of Corti receives efferent innervation from the olivocochlear
bundle (OCB).
Neurons with cell bodies in the superior olivary complex of the
brainstem.
Carry information from the brain to the ear instead of the reverse, i.e.
brainstem is controlling some activities of the cochlea.
There are two types of olivocochlear bundle (OCB) neurons:
Those that terminate just under IHCs (lateral OCB)
Those that go to OHCs (medial OCB)
Schematic representation of the hair cell
afferent innervation (from Liberman)
Lateral Olivocochlear Bundle Neurons
Cell bodies lie in the lateral part of the superior olivary complex.
Small with unmyelinated (slow) axons that stay mostly on the
same side (don t cross over).
Lateral Olivocochlear Bundle Neurons
They synapse just under the IHCs, on the peripheral processes of
Type I spiral ganglion neurones of the cochlea.
Lateral OCB Neurons
Lateral OCB neurons
26
Neurotransmitters at IHC Synapses
Medial Olivocochlear Bundle Neurons
From medial olivocochlear bundle to the OHCs.
Cell bodies in the medial portion of the superior olivary complex.
Large neurons, well-myelinated axons (fast!).
afferent
efferent
Medial OCB neurons
http://www.iurc.montp.inserm.fr
Medial Olivocochlear Bundle Neurons
Neurotransmitters at OHC Synapses
Most of the axons cross the midline to the opposite side of the
brain and leave the brain with the vestibular division of the
VIIIth cranial nerve.
Synapse directly onto base of OHC.
Medial OCB neurons
efferent
afferent
http://www.iurc.montp.inserm.fr
27
Innervation of the IHCs and OHCs - Summary
IHCs
1. Afferent Innervation
- Type I fibres (95% of all afferent fibres),
well-myelinated
- many-to-one connection to IHCs (in
humans, about 18 synapse on each
IHC)
2. Efferent Innervation
Lateral olivocochlear bundle fibres
- synapse on the afferent nerves leaving the
IHCs
- unmyelinated
OHCs
1.  Afferent Innervation
- Type II fibres (5% of all afferent fibres),
unmyelinated
- one-to-many connection to OHCs (in
humans, each innervates about 10
OHCs)
2. Efferent Innervation
Medial olivocochlear bundle fibres
- synapse directly on the OHCs
- well-myelinated
Reading List
Core reading:
Musiek, F.E., Baran, J.A. The Auditory System; Anatomy,
Physilogy, and Clinical Correlates (1st Edition) Allyn &
Bacon, 2006. ISBN-10: 0205335535
Plack CJ. The Sense of Hearing. Routledge, 2005. ISBN:
0805848843
Yost WA. Fundamentals of Hearing: An Introduction (5th
Edition), Academic Press, 2006. ISBN-10: 0123704731
Additional reading:
Moore B.C.J. Introduction to the Psychology of Hearing (4th ed). Academic Press,
1997
Webster, B.D. Neuroscience of Communication (2nd Ed.) Singular, San Diego, 1999
Pickles, J.O. An Introduction to the Physiology of Hearing , 3nd Ed. Academic
Press, 2008
Thank you for your attention !
Durrant JD and Lovrinic JH. Bases of Hearing Science (4rd Ed). Williams and
Wilkins, 2008
Geisler DC. From Sound to Synapse: physiology of the mammalian ear. Oxford
University Press, 1998
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