Transcript inner ear - CLAS Users

INNER
EAR
Two Halves:
 Vestibular--transduces motion and pull of gravity
 Cochlear--transduces sound energy
(Both use Hair Cells)
Subdivision into spaces containing endolymph
(blue), and spaces containing perilymph (red)
The
Endolymphatic
Sac
 Termination of
vestibular aquaduct
 Outside 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
A Cross Section Shows the 3 Scala
Within S. Media is the Organ of Corti
I = Inner Hair Cells
O = Outer Hair Cells
P = Pillar 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 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
Frequency Analysis - the
Traveling Wave
 Bekesy studied cochleae from cadavers,
developed the Traveling Wave theory
 1. Response always begins at the base
 2. Amplitude grows as it travels apically
 3. Reaches a peak at a point determined by
frequency of the sound
 4. Vibration then dies out rapidly
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:
 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 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
Synapse Basics
 Pre-Synaptic cell
contains vesicles
 Gap between cells is
Synaptic Cleft
 Post synaptic cell may
show darkened area
adjacent to membrane
Afferent & 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 cells
 Synapse 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 (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 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
Electrocochleography
The SP/AP Ratio