The Peripheral Auditory System
George Pollak
Section of Neurobiology
1
• Hair cells, the transducers of the auditory system, and how they work.
Qu ickT i m e™ an d a T I FF (Un co m pr es se d) de co mp re ss or ar e n ee d ed to se e thi s p ic tu re.
Organ of Corti
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stereocillia of
inner hair cells
stereocillia of
outer hair cells
Basilar
membrane
stereocilia on one hair cell
9
A
B
C
ti p link
s tere ocil ia
tra p door
of
ioni c
cha nnel
E
F
G
A m odel for m ec hoele ct rica l t rans ducti on by ha ir ce ll s. In the abs ence of
any s tim ul ation, at a ny i nsta nt each t ra nsduct ion channel a t a st ereoci lium 's
ti p m a y be ei the r clos ed (A a nd F ) or open (E ). The grea ter proba bil ity i s
that t he c hannel is c lose d. W hen the hair bundle is de flecte d w ith a posi tive
sti m ulus, the spring or t ip l ink, exe rts a force on the tra p d oor and opens
the c hanne l (B , C and D ). The i nflux of K + ion s into the h a ir c ell c auses
it to depola rize . Pushing the hai r bundle in the oppos ite direction com pres ses
th e spri ng ( tip li nk), ens uri ng t hat th e c hann el rem ai ns cl os ed (G) . Th is
pr ev ent s th e in fl ux of K + i on s a nd ca us e s t he ce ll to hy pe rpo la ri ze .
D
endolymph
Potential difference between
Endolymph and cell interior
K+
Hi
Lo Na+
E n d ol ym p h
perilymph
apic al sur fac e
o f hair ce ll
Ek= 58 log
Kout
Kin
= 0mV
stere ocilia
kin ocilium
Hi K+
Lo Na+
basa l surf ace
of h air ce ll
tigh t ju nctions
sy naptic ribb on s
affe re nt ner v es
F ig. 1
Pe ri ly mp h
Hi Na+
Lo K+
Potential difference between
Perilymph and cell interior
Ek= 58 log
Kout
Kin
= ~-70mV
Rest
Inhibition
Excitation
K+
K+
-45 mV
Ca
++
K+
C a+ +
K+
K+
R e st : Sm al l am o unt of K + lea ks i nto ce ll from ra tt li ng c hann els of s t ere oci li a. Th e l ea king
K + de pola rize s cel l there by ope ning vol ta ge sens i tive C a + + cha nnel s res ult in g in s po nt aneous
re lea s e of tra nsm it te r a nd exc it at io n of affe rent ne rve fi bers (not s how n). T h e dep ol ari za ti on
ca use s K + to l eave cel l in ba sa l reg i on .
E xc ita tion : S te reoci lia are bent t here by opening channe ls wh ich re sul ts in l arg er e ntry of K +
int o cel l. The influx of K + ca use s a dd iti onal de polari zat ion th e reby op eni ng m ore vol tage gate d
C a + + cha nnels . The ad dit iona l C a + + ch a nnels th at a re now open c au s e a l arger i nflux of C a + +
i n th e b as e . T he i nfl u x of C a + + c aus e s m o re t ra ns m i t te r t o be re le as e d an d t hu s a gre at er
exc ita tion of t he affere nt fibe rs (not show n ).
I nh ib it ion : S tere oci li a a re bent in op posit e d irec ti on t here by cl os ing cha nnels . T hi s res ul ts
i n l es s i nflu x of K + a t ch a nnel s in t h e tips o f s t ere ocil ia . K + e ffl ux oc curs at base a nd i s not
rep leni sh ed b y infl ux at s te reoc ili a. C ons equ e ntl y, t he h air ce ll hy perpol ari zes t here by c losing
volt age sens it iv e C a + + ch a nnels at th e bas e res ult ing in a sm all er rel ease, o r even n o re lea se o f
tr ans m it te r. Di s cha rge o f a ffe rent fibe r is th ere fore re duc ed t o a rat e e ven be low th e res t ing
le vel.
Hi K+
low Na+
Hi Na+
low K+
Hi K+
low Na+
Hi K+
low Na+
Hi Na+
low K+
small leakage of
K+ into cell
K+ into cell
-45 mV
No K+ into cell
-70 mV
Next, we are going to
build a cochlea
Stapes
Basilar membrane
Sound is changed from
a pressure wave in the air
into mechanical movements
on the basilar membrane
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GIF decompressor
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round window
Traveling waves on basilar membrane
oval window
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GIF decompressor
are needed to see this picture.
round window
The structure of the
basilar membrane causes it
to perform a
frequency to place transformation
Basilar Membrane
has continuously changing dimensions along its length
Apex
responds maximally to low frequencies
flexible
wide and thin
Stiff
Narrow and thick
Base
responds maximally to high frequencies
Basilar membrane converts frequency to a place of maximal response
Frequency-to-Place Transformation in the Cochlea
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The motion on the
basilar membrane
causes shearing of the cilia on
hair cells and thereby causes
the hair cells to
depolarize and hyperpolarize in
response to sound
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GIF decompressor
are needed to see this picture.
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Organ of Corti
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Basilar membrane
kTime™
a
basilar and
membrane
decompressor
o see this picture.
shearing of stereocillia
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Animation decompressor
are needed to see this picture.
Organ of Corti
basilar membrane
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Animation decompressor
are needed to see this picture.
Why are there two types of
hair cells?
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98% of the fibers that project into the central auditory system
are innervated by inner hair cells!!
98%
What are the outer hairs doing?
Answer: they act as amplifiers
of the mechanical motion of
the basilar membrane generated
by sound
Hi K++
----depolarization hyperpolarization
release of transmitter
Evoked mechanical responses of isolated cochlear outer hair cells.
Electromotility: OHC can change length in
response to voltage change
Direct evidence of an active mechanical
process in the organ of Corti
depolarized hyperpolarized
+
+
+
_
_
_
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are nee ded to see thi s picture.
Dancing hair cell
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YUV420 codec decompressor
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42
Outer hair cells are the only cells in the body that express
prestin. Even inner hair cells do NOT have prestin.
+
+
+
+
+
+
+
+
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Show movie of how Prestin works
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Positive feedback loop
Basilar
membrane
motion
Sound
stimuli
Hair bundle
deflection
Change in
length of
hair cells
OHC
Membrane
potential
change
IHC
Sensory signal
transmission
Normal response with cochlear
amplifier
Apex
base
Basilar membrane
response without cochlear
amplifier
Apex
base
How motion of basilar membrane
generates tuning curves in
auditory nerve fibers and thereby
imparts frequency selectivity
to auditory nerve fibers
50 dB SPL
base
apex
6 kHz
60
8 kHz
Intensity (dB SPL)
7 kHz
50
40
30
20
9 kHz
10
5
10 kHz
6
7
8
9
Frequency ( kHz)
10
11
30 dB SPL
base
Tuning Curve
The most basic feature
of an auditory neuron
apex
6 kHz
60
8 kHz
Intensity (dB SPL)
7 kHz
50
40
30
20
9 kHz
best frequency
10
5
10 kHz
6
7
8
9
Frequency ( kHz)
10
11
tuning curves
tuning
in animals
curves with
in normal
no outer
animals
hair cells or
in animals without prestin gene
Sound intensity
high
low
low
frequency
high
How is the tonotopic organization
that was first established on the
basilar membrane
preserved in in the
central auditory system?
Flow of Information Along the Central Auditory Pathway
Auditory
cortex
Medial
geniculate
Medial
geniculate
Inferior
Inferior
colliculus
colliculus
Cochlear
nucleus
Auditory
nerve
Cochlear
nucleus
Superior
olive
Superior
olive
Auditory
nerve
The Frequency Representation on the Cochlea is Preserved in Every Nucleus
of the Central Auditory System, and thus the Auditory System is Tonotopically
Organized
auditory cortex
Auditory
cortex
medial geniculate
Inferior colliculus
cochlear nucleus
Auditory
nerve
Cochlear
nucleus
Medial
geniculate
Medial
geniculate
medial geniculate
Inferior
Inferior
colliculus
colliculus
superior olive
Superior
olive
Superior
olive
Inferior colliculus
Cochlear
nucleus
cochlear nucleus
Auditory
nerve
The Frequency Representation on the Cochlea is Preserved in Every Nucleus
of the Central Auditory System, and thus the Auditory System is Tonotopically
Organized
Auditory
cortex
Medial
geniculate
Medial
geniculate
Inferior
Inferior
colliculus
colliculus
Cochlear
nucleus
Auditory
nerve
Cochlear
nucleus
Superior
olive
Superior
olive
Auditory
nerve
The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the
Central Auditory System, and thus the Auditory System is Tonotopically Organized