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Two Halves:Vestibular--transduces motion and pull of gravityCochlear--transduces sound energy
(Both use Hair Cells)
INNER EAR
Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)
The Endolymphatic
SacTermination of
vestibular aquaductOutside of temporal
bone; next to dura mater lining of the brain
Thought to maintain endolymphatic volume/pressure
Cochlea is Divided into 3 “Scala”
Scala Vestibuli Reissner’s Membrane
Scala Media Basilar Membrane
Scala Tympani
Helicotrema - the opening between 2 outer Scala
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)
Cross-Section of the Cochlea
Third Turn
Second Turn
First Turn
A Cross Section Shows the 3 Scala
Within S. Media is the Organ of Corti
I = Inner Hair Cells P = Pillar Cells
O = Outer Hair Cells D = Deiter’s Cells
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
Cochlear Functions
Transduction- Converting acoustical-mechanical 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-chemicalOuter Hair Cells have both forward and
reverse transduction--
Mechanical Electro-chemical
Mechanical Electro-chemical
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
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
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 shapesharp tip (shows best sensitivity at one freq)steep high frequency tailshallow low frequency tail
Tuning Curves
Passive Only
Active + Passive
More on Tuning & Tuning Curves:
Seen for basilar membrane, hair cells, nerve cells
Frequency of “tip” is called the CHARACTERISTIC FREQUENCY
OHC Length and CF
High Freqs Low Freqs
Tectorial Membrane
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
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
Tip Links
Resting (or Membrane) Potentials
Inner Hair Cell = - 45 mV Outer Hair Cell = - 70 mV
Stereocilia bent toward tallest row
Potassium flows into cellCalcium flows into cell
Voltage shifts to a less negative value
More neurotransmitter is released
Synapse Basics
Pre-Synaptic cell contains vesicles
Gap between cells is Synaptic Cleft
Post synaptic cell may show darkened area adjacent to membrane
AfferentAfferent & Efferent Neurons
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
IHC Innervation Pattern
OHC Innervation Pattern
Inner hair cellsSynapse at the base
with up to 20 afferent neurons
“Divergence”
Efferents synapse on afferent dendrites under IHCs
IHC activation alters firing rate
Afferent neurons have their cell bodies in the Spiral Ganglion (4)
An Action Potential (or Spike)
IHC activation alters firing rate
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
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
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 potentialLasts for duration of stimulus.
Compound Action Potential (CAP)
Summation of APs in large number of VIIIth nerve neurons
following onset (and offset) of stimulus
Ele
ctro
coch
leog
raph
y
The
SP
/AP
Rat
io