Middle Ear Functions

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Middle Ear Functions

• Impedance Matching -- amplification of sounds to
  overcome difference in impedance between the air
  of EAC and the fluid of the inner ear.
• Filtering -- resonant frequency is approximately
  1000 Hz, functions as bandpass filter.
• Acoustic Reflex -- Contraction of Stapedius
  muscle in response to loud sounds

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INNER EAR

• Two Halves
• Vestibular--transduces motion and pull of gravity
• Cochlear--transduces sound energy
• (Both use Hair Cells)

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Subdivision into spaces containing endolymph
(blue), and spaces containing perilymph (red)
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Cochlea is Divided into 3 Scala

• Scala Vestibuli
• Reissners 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 (85 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 Deiters Cells

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The Stereocilia on IHCs and OHCs

• OHCs (at top)
• V or W shaped ranks
• IHC (at bottom)
• straight line ranks

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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-chemical
• Outer 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 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