1. Structure of the Inner Ear and Its Mechanical Response
CHAPTER 7
Structure of the Inner Ear and Its Mechanical Response

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

11. Within S. Media is the Organ of Corti

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

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

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

18. 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

19. 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

20. The Traveling Wave
BASE
APEX

21. 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

22. 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

23. Tip Links

24. Resting (or Membrane) Potentials
Inner Hair Cell = mV
Outer Hair Cell = - 70 mV

25. Stereocilia bent toward tallest row
Potassium flows into cell
Calcium flows into cell
Voltage shifts to a less negative value
More neurotransmitter is released

26. Cochlear Mechanics Passive: Bekesy’s Traveling Wave Theory
Active: Outer Hair Cell Motion enhances stimulation of inner hair cells

27. 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.

28. 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