2. 1 Inner Ear Physiology

3. 2

4. 3 Transduction Tympanic membrane Acoustical/mechanical Oval window Mechanical/hydraulic Basilar & tectorial membrane Hydraulic/mechanical Hair Cells (stereocilia) Mechanical/electric Hair Cells (base) Electrical/chemical Auditory Neuron Chemical/electrical

5. 4 Block diagram of the cochlea

6. 5 Frontal view of unrolled cochlea

7. 6 Effect of stapes insertion in cochlea

8. 7 Effect of outward motion of stapes.

9. 8 Effect of outward movement of stapes

10. 9

11. 10 Theories of Hearing Place Coding Theories Resonance theory (Helmholtz, 1885) Traveling wave theory (Georg von Bekesy 1928-1958- Nobel Prize 1961) Many other theories including standing wave theory, pressure pattern theory, frequency analytic theory, etc. See Zemlin for details. Temporal Coding Theories Telephone theory (Rutherford, 1886) Volley theory (Wever, 1949)

12. 11 Place Theory: Impedance characteristics of the basilar membrane Mass reactance Stiffness reactance Resistance Apical end is 5x wider and 100x more flaccid than basal end.

13. 12 Resonance Theory The BM acts as a series of filters or tuned rods (analogy of piano strings). It performs a frequency analysis (Fourier Analysis) of the incoming signal.

14. 13 Traveling Wave Theory Most popular TW theory developed by Bekesy (1928-1958). Used cochlear models and direct observation of cochlea.

15. 14 Traveling wave

16. 15 Traveling wave for complex tones Basilar Membrane Traveling Wave 8 kHz + 2 kHz 16 kHz + 2 kHz + 1 kHz 16 kHz + 8 kHz + 4 kHz + 2 kHz

17. 16 Traveling wave and non- linearities

18. 17 Traveling wave and upward spread of masking Excitation patterns and can be used to represent traveling wave. Notice that low frequencies can mask high frequencies (e.g., 1000 masking 2000 Hz) better than the high frequencies mask lows (e.g., 1000 masking 500 Hz).

19. 18 Telephone Theory Problem is that the absolute refractory period is about 1 msec. That is, a hair cell can fire only once per millisecond (1000 times per second or 1000 Hz). Therefore, it would not be possible to transmit sounds to the CNS greater than 1000 Hz. Another problem is that damage to certain parts of the basilar membrane (basal end) helps confirm a place theory.

20. 19 Volley Theory Modification of Telephone theory. Receptor cells fire in groups: first one, then another, then a third Resetting: the first group of cells to fire are resetting while the second and third groups fire and so on

21. 20 Combining Place and Volley Theories FrequenciesPredominate Theory Comments < 300, 400 HzVolleyThe traveling wave is too broad to allow for specificity. Mid FrequenciesBoth theories apply > 4000, 5000 HzTraveling Wave Temporal coding not supported in high frequencies

22. 21 Electrical potential of the cochlea

23. 22

24. 23 Role of Hair Cells on Hearing Thresholds Normal behavioral thresholds are due to amplification of motion by OHCs Slight to moderate hearing loss is due to a breakdown of OHCs, and not IHCs Severe hearing loss is due to the breakdown of OHCs and IHCs

25. Nerve Cell Anatomy Axon & Myelin Soma Synapses Dendrites Terminal Boutons Neurotransmitter

26. 25 Auditory Neurons are Bipolar

27. 26 Block diagram of auditory neuron

28. Sensitivity (Detection) vs. Specificity (Identification) Specificity: Many-to-one arrangement Sensitivity: One-to-many arrangement IHC OHC Nerve Fiber Nerve Fiber Afferent Innervation of Inner and Outer Hair Cells

29. 28 Innervation

30. 29 Summary