Download Outer Hair Cells

Cochlear Functions
• Transduction- Converting acousticalmechanical energy into electro-chemical
energy.
• Frequency Analysis-Breaking sound up
into its component frequencies
Transduction• Inner Hair Cells are the true sensory
transducers, converting motion of
stereocilia into neurotransmitter release.
Mechanical Electro-chemical
• Outer Hair Cells have both forward and
reverse transduction-Mechanical  Electro-chemical
Mechanical Electro-chemical
Hair Cell Activation
• Involves Ion Flow into cell
• Through channels in the stereocilia
• Bending stereocilia causes # of open
channels to change.
• Toward Modiolus = Fewer channels open
• Away from Modiolus = More open
Ion Channels are opened by “TIP
LINKS”
• Tip Links connect tip of shorter stereocilia
to the side of a stereocilium in the next
taller row
• Bending toward taller rows pulls tip links
• Bending toward shorter rows relaxes tip
links
Tip Links
Resting (or Membrane) Potentials
• Inner Hair Cell = - 45 mV
• Outer Hair Cell = - 70 mV
Stereocilia bent toward tallest row
• Potassium flows into cell
• Calcium flows into cell
• Voltage shifts to a less
negative value
• More neurotransmitter is
released
Sensory Physiology
• The basic unit of the nervous system
is the Neuron or nerve cell
• Neurons undergo voltage changes– SPONTANEOUSLY
– IN RESPONSE TO STIMULATION
The Neuron
Neural Activity
• Post-synaptic Potentials-- Local, Variable
changes in voltage near synapse
• Action Potentials-- Conducted through axon, “all
or none,” “spike”
• For image of AP’s traveling down an axon:
http://bio.winona.msus.edu/berg/ANIMTNS/actpot.htm
An Action Potential (or Spike)
Action Potentials
• Are generated spontaneously
– At a slow rate by some neurons
– At a faster rate by some neurons
• And occur more frequently with
STIMULATION
• Spike rate increases through a range of
about 30 dB
Spike Rate (APs/sec)
Spike Rate Increases Thru a 30
dB Range
90
80
70
60
50
40
30
20
10
0
Spike Rate
0
5 10 15 20 25 30 35 40 45 50 55 60
Stimulus Level (dB SPL)
Cochlear Mechanics
• Passive: Bekesy’s Traveling Wave Theory
• Active: Outer Hair Cell Motion enhances
stimulation of inner hair cells
The Traveling Wave
BASE
APEX
Bekesy’s Theory describes
Passive Mechanics
• Based on work in “dead” cochleae
• Highly damped -- not sharply tuned
• Active Undamping occurs in live and
healthy cochleae
• Like pumping on a swing--adds amplitude
The Active Component Adds to
Bekesy’s Traveling Wave
The Active Component
• Improves Sensitivity for soft sounds
• Improves frequency resolution
Frequency Tuning Curves Show
these Effects
= plots of response threshold as a function of
frequency
They have a characteristic shape
• sharp tip (shows best sensitivity at one freq)
• steep high frequency tail
• shallow low frequency tail
Tuning Curves
Passive Only
Active + Passive
More on Tuning & Tuning
Curves:
• Frequency of “tip” is called the
CHARACTERISTIC FREQUENCY
• Can be seen for:
 basilar membrane,
hair cells,
 nerve cells
Tonotopic Organization
• Mapping of Characteristic Frequency by
place in the auditory nervous system
• In the Auditory portion of cranial nerve VIII
the map is spiral-shaped
with highest CF neurons on the outside,
lowest CF neurons at the center.
Head-Related Effects
• Head-Baffle--the mere presence of your
head alters the sound field.
• Head Shadow - loss of energy at far ear for
frequencies above approx 2000 Hz
Signs of Peripheral Activation
• Otoacoustic Emissions (OAEs)
• Cochlear Potentials
Otoacoustic Emissions
• Low-level sounds produced by the cochlea
and recordable in the external ear canal.
•
•
•
•
Spontaneous
Click-evoked
Distortion Product
Stimulus Frequency
Recording OAEs
Spectrum of Sound in Ear Canal
OAEs: Measures of Cochlear Health
Cochlear Potentials:
• Resting Potentials: voltages which exist
without external stimulation
e.g., Endolymphatic Potential,
Cell Membrane Potential
• Stimulus-Related Potentials: voltages
occurring in response to sounds
We’ll talk about 3 of these from the cochlea
Cochlear Microphonic
• Least valuable from a clinical standpoint.
• Is an alternating current (AC) response that
mirrors the waveform of low to moderately
intense sound stimuli
• Appears to arise from outer hair cells in the
basal-most turn of the cochlea
Summating Potential (SP)
• Is a direct current or DC potential
• Lasts for duration of stimulus.
Compound Action Potential
(CAP)
• Summation of APs in large number of
VIIIth nerve neurons
• following onset (and offset) of stimulus