1. 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

2. INNER EAR
Two Halves:
Vestibular--transduces motion and pull of gravity
Cochlear--transduces sound energy
(Both use Hair Cells)

3. Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)

4. Cochlea is Divided into 3 “Scala”
Scala Vestibuli
Reissner’s Membrane
Scala Media
Basilar Membrane
Scala Tympani
Helicotrema - the opening between 2 outer Scala

5. 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)

6. Cross-Section of the Cochlea
Third Turn
Second Turn
First Turn

8. A Cross Section Shows the 3 Scala

10. Within S. Media is the Organ of Corti

11. I = Inner Hair Cells P = Pillar Cells
O = Outer Hair Cells D = Deiter’s Cells

12. The Stereocilia on IHCs and OHCs
OHCs (at top)
V or W shaped ranks
IHC (at bottom)
straight line ranks

13. Cochlear Functions
Transduction- Converting acoustical-mechanical energy into electro-chemical energy.
Frequency Analysis-Breaking sound up into its component frequencies

14. 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

15. 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