Download Physiological Acoustics

CHAPTER 7
Structure of the Inner Ear and Its
Mechanical Response
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)
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 (85 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
The Stereocilia on IHCs and OHCs
• OHCs (at top)
• V or W shaped ranks
• IHC (at bottom)
• straight line ranks
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
The Traveling Wave
BASE
APEX
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
Cochlear Mechanics
• Passive: Bekesy’s Traveling Wave Theory
• Active: Outer Hair Cell Motion enhances
stimulation of inner hair 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