Two Halves: §Vestibular--transduces motion and pull of gravity §Cochlear--transduces sound energy...
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Transcript of Two Halves: §Vestibular--transduces motion and pull of gravity §Cochlear--transduces sound energy...
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Two Halves:Vestibular--transduces motion and pull of gravityCochlear--transduces sound energy
(Both use Hair Cells)
INNER EAR
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Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)
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The Endolymphatic
SacTermination of
vestibular aquaductOutside of temporal
bone; next to dura mater lining of the brain
Thought to maintain endolymphatic volume/pressure
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Cochlea is Divided into 3 “Scala”
Scala Vestibuli Reissner’s Membrane
Scala Media Basilar Membrane
Scala Tympani
Helicotrema - the opening between 2 outer Scala
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Fluids filling the Inner Ear
Perilymph- in S. Vestibuli and S. Tympani High Sodium / Low Potassium concentrations Low Voltage (0 to +5 mV)
Endolymph- in S. Media High Potassium / Low Sodium concentrations High Positive Voltage (80 mV)
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Cross-Section of the Cochlea
Third Turn
Second Turn
First Turn
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A Cross Section Shows the 3 Scala
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Within S. Media is the Organ of Corti
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I = Inner Hair Cells P = Pillar Cells
O = Outer Hair Cells D = Deiter’s Cells
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IHCs, OHCs And Their Stereocilia
OHCs (at top) 3, 4 or 5 rows Approx 12,000 cells 10 to 90 microns V- or W-shaped ranks of stereocilia
50 to 150 stereocilia per cell IHC (at bottom)
1 or 2 rows Approx 3,500 cells 35 microns straight line ranks of stereocilia 50 to 70 stereocilia per cell
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Cochlear Functions
Transduction- Converting acoustical-mechanical energy into electro-chemical energy.
Frequency Analysis-Breaking sound up into its component frequencies
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Transduction-
Inner Hair Cells are the true sensory transducers, converting motion of stereocilia into neurotransmitter release.
Mechanical Electro-chemicalOuter Hair Cells have both forward and
reverse transduction--
Mechanical Electro-chemical
Mechanical Electro-chemical
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Frequency Analysis - the Traveling Wave
Bekesy studied cochleae from cadavers, developed the Traveling Wave theory
1. Response always begins at the base2. Amplitude grows as it travels apically3. Reaches a peak at a point determined by
frequency of the sound4. Vibration then dies out rapidly
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Bekesy’s Theory describes Passive Mechanics
Based on work in “dead” cochleaeHighly damped -- not sharply tuned
Active Undamping occurs in live and healthy cochleae
Like pumping on a swing--adds amplitude
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The Active Component Adds to Bekesy’s Traveling Wave
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The Active Component
Improves Sensitivity for soft sounds
Improves frequency resolution
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Frequency Tuning Curves Show these Effects
= plots of response threshold as a function of frequency
They have a characteristic shapesharp tip (shows best sensitivity at one freq)steep high frequency tailshallow low frequency tail
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Tuning Curves
Passive Only
Active + Passive
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More on Tuning & Tuning Curves:
Seen for basilar membrane, hair cells, nerve cells
Frequency of “tip” is called the CHARACTERISTIC FREQUENCY
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OHC Length and CF
High Freqs Low Freqs
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Tectorial Membrane
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Hair Cell Activation
Involves Ion Flow into cellThrough channels in the stereocilia
Bending stereocilia causes # of open channels to change.
Toward Modiolus = Fewer channels openAway from Modiolus = More open
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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 linksBending toward shorter rows relaxes tip
links
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Tip Links
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Resting (or Membrane) Potentials
Inner Hair Cell = - 45 mV Outer Hair Cell = - 70 mV
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Stereocilia bent toward tallest row
Potassium flows into cellCalcium flows into cell
Voltage shifts to a less negative value
More neurotransmitter is released
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Synapse Basics
Pre-Synaptic cell contains vesicles
Gap between cells is Synaptic Cleft
Post synaptic cell may show darkened area adjacent to membrane
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AfferentAfferent & Efferent Neurons
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4 Types of Cochlear Neurons
INNER HAIR CELLS
> Multiple (10 to 20) Afferent synapses
> (Efferents synapse on afferent dendrites)OUTER HAIR CELLS:
> Large Efferent synapses engulf base of cell
> Small (& not very active) Afferent synapses
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IHC Innervation Pattern
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OHC Innervation Pattern
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Inner hair cellsSynapse at the base
with up to 20 afferent neurons
“Divergence”
Efferents synapse on afferent dendrites under IHCs
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IHC activation alters firing rate
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Afferent neurons have their cell bodies in the Spiral Ganglion (4)
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An Action Potential (or Spike)
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IHC activation alters firing rate
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Spike Rate Increases Thru a 30 dB Range
0
10
20
30
40
50
60
70
80
90
0 5 10 15 20 25 30 35 40 45 50 55 60
Stimulus Level (dB SPL)
Spik
e R
ate
(AP
s/se
c)
Spike Rate
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Cochlear Potentials:
Resting Potentials: voltages which exist without external stimulation
e.g., Endolymphatic Potential,
Cell Membrane PotentialStimulus-Related Potentials: voltages
occurring in response to sounds
We’ll talk about 3 of these from the cochlea
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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
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Summating Potential (SP)
Is a direct current or DC potentialLasts for duration of stimulus.
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Compound Action Potential (CAP)
Summation of APs in large number of VIIIth nerve neurons
following onset (and offset) of stimulus
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Ele
ctro
coch
leog
raph
y
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The
SP
/AP
Rat
io