The Inner Ear and Hair Cells (CH 12, pg. 283-300)
I.
Sound – changes in pressure
A) Stimulus Parameters – Frequency (pitch) & Amplitude (loudness)
1. Humans – frequency range 20 Hz to 20kHz, pressure in dB
B) Tympanic Membrane –> Ossicles (amplify sound pressure) -> Cochlea
Fig. 12-3
C) The Cochlea Uncoiled
1. Ossicles transmit pressure to cochlea through Oval Window
2. Basilar Membrane –
a) tapered, narrower (and stiffer) at oval window (base)
b) wider, less stiff at apex
c) each point on basilar membrane tuned to a particular frequency
Amplitude of vibration
Fig. 12-5
3
Text
http://www.neurophys.wisc.edu/animations/
http://www.youtube.com/watch?v=yADT4gvzwE4
Endolymph (high K+), +80 mV
Perilymph (0 mV)
Fig. 12-4
Fig. 12-6
3) Vibration of basilar membrane
causes bending of hair cells
II. Hair Cells
A. Kinocilium , Stereocilia – contain actin filaments
B. Tip Links – stretching opens cation selective channels –
1. Channels located near the tips of the cilia
C. Steps in the process of mechanotransduction
1. K+ enters at distal ends of cilia
(endolymph)
2. Depolarizes Cell
3. Voltage sensitive Ca+2 channels open
4. Calcium entry causes exocytosis of
vesicles containing synaptic transmitter
5. Transmitter release --> depolarization &
action potential in postsynaptic cell
(vestibular or auditory afferent)
D. Evidence – record extracellular currents from saccular hair cells
1. EM of hair cells & locations
of recording electrode
Hudspeth, J. Neurosci. 2: 1-10,
1982
2. Some important controls
Recording electrode
In Contact
Adjacent to it
Stimulus probe
Toward &
away from
kinocilium
Perpendicular
to it
E. Frequency Tuning (Selectivity)
1. Frequency response of two hair cells
in turtle cochlea
2. A pulse of sound produces resonant
oscillations
3. Injecting current (depolarizing cell)
produces oscillations at same frequency
Fettiplace, TINS, 10: 421-425, 1987
Mechanism of Frequency tuning (CH 13, Box B)
Endolymph (high K+)
a) K+ enters at tip links
b) Depolarization
c) Voltage-gated Ca2+
opens
d) Opens Ca2+ - gated K+
channel
e) K+ leaves 
repolarization
f) Ca2+ channel closes
g) K+ channel closes
h) Depolarization from tip
link channels
Perilymph (low K+)
F. Transient signaling – Adaptation (Purves, Ch 14, Box B)
1. Maintain sensitivity to transient stimuli
Hair cell bent by a constant amount
Probability of K+ Channel Being Open
Fig. 13-Box B
Basilar Membrane Filtering
City Traffic
Conversation
Fan at the back of this room
Mosquito
(Sellick, Patuzzi, and Johnstone, 1983)
Prestin in action
Outer Hair Cells are “Pre-Amps”