1. Dimensions of the cochlear partitions
Cochlea: Narrower toward the apex
Basilar membrane: Narrower at the base and wider toward the apical end
Apical end: Flaccid and under no tension
Base end: Stiff and under a small amount of tension

2. Cochlear microstructure
Organ of Corti
Lies on the basilar membrane in the scala media
Contains many different types of specialized cells

3. Organ of Corti

4. Pillar cells or rods of Corti
Provide structure and support
Inner and outer pillar cells
Form a tunnel called tunnel of Corti
Contains fluid called cortilymph

5. Hair cells
On the outer side of the outer pillar/rod cells: Outer hair cells (OHC)
(Away from modiolus)
On the inner side of the inner pillar/rod cells: Inner hair cells (IHC)
(Toward the modiolus)

6. Supporting cells OHC supported by cells of Deiter and Hensen
IHC supported by border cells of the inner sulcus

7. Reticular lamina
Formed partly by the phalangeal processes of the Deiter’s cells.

8. OHC
Around in number
Test-tube shaped
Three-five rows

9. IHC
Around 3400 in number
Flask shaped
Single row

10. Stereocilia Hairlike projections Project from the top of IHC and OHC
Graded in length
Have cross-bridges called tip-links

11. Stereocilia of OHC and IHC
6-7 rows per OHC
Each row has a W shaped arrangement
IHC:
2-4 rows per IHC
Each row in a shallow U-shaped arrangement

12. Tectorial membrane
Gelatinous transparent membrane projecting from spiral lamina
Attaches loosely on outer edge to the Deiter’s and Hensen’s cells.
Longest stereocilia of OHC embedded in inferior surface of tectorial membrane
Stereocilia of IHC not embedded in tectorial membrane

14. Physiology of the cochlea
Mechanical response of cochlea in response to sound
Two major functions:
Analysis of sound into components:
Frequency/Spectral analysis
Transduction of sound:
Converting mechanical energy into electrochemical/neural energy 14

15. Mechanical response of cochlea to sound
Basilar membrane moves in response to stapes vibration
This vibration takes the form of a ‘traveling wave’ (von Bekesy)
Wave motion starts at the base and moves toward the apex 15

16. Instantaneous basilar membrane pattern and envelope of traveling wave16

17. Characteristics of traveling wave
Points of maximum displacement: Peak
Peak depends on the frequency of incoming sound
Each part of basilar membrane ‘tuned’ to a particular frequency called critical/center/characteristic frequency (CF) 17

18. Low frequencies: Localized toward the apex High frequencies: Localized toward the base18

19. Frequency/Spectral analysis
Cochlea acts as a series of bandpass filters
Incoming sound broken down into individual sinusoidal components
Basilar membrane vibrates in response to each of these components 19

20. Basilar membrane mechanics
Greater the stimulus level, greater the amount of basilar membrane displacement
Temporal pattern of basilar membrane vibration follows that of incoming sound 20

21. Input-output functions
At mid-intensity levels, basilar membrane vibration increases with intensity in a non-linear or compressive fashion
At very low (< 30 dB SPL) and very high (> 90 db SPL), linear vibration
This is frequency dependent: Only for the CF
Above and below the CF: Linear input-output functions 21

22. Steps in transduction
Mechanical vibrations translated into neural responses in the auditory nerve
Stapes vibration sets inner ear fluid into vibration
Basilar membrane vibrates
Shearing motion of tectorial membrane
Stereocilia of haircells bend 22 22

23. Bending of stereocilia 23 23

24. Transduction, Cont’d.
Increase in number of open ion channels at tip links
Influx of positive pottassium and calcium ions into the cell body
Resting membrane potential of endolymph: + 80 mv
Intracellular resting potential:
OHC: -70mv
IHC: - 45mv
Change in membrane potential
Release of neurotransmitter
Neuron at the base of the haircell fires 24 24

25. Transduction, Cont’d.
Increase in number of open ion channels at tip links
Influx of positive pottassium and calcium ions into the cell body
Resting membrane potential of endolymph: + 80 mv
Intracellular resting potential:
OHC: -70mv
IHC: - 45mv
Change in membrane potential
Release of neurotransmitter
Neuron at the base of the haircell fires 25 25

26. Differences between OHC and IHC
IHC: True sensory part of the cochlea
Only forward transduction (Mechanical to neural)
OHC: Both sensory and motor functions
Forward and backward transduction (Mechanical to neural, and neural to mechanical) 26 26

27. Physiology of OHC and IHC
OHC display ‘motility’ (change in length) in response to stimulation. IHC do not.
Because of muscular contractions within the cell body and efferent connections to the OHC bodies.
Thought to be basis of ‘cochlear amplifier’ and ‘active cochlea’ 27 27

28. What is the cochlear amplifier?
‘Feedback loop’ within the Organ of Corti, because of OHC motility.
Forward transduction: Basilar membrane moves, stereocilia are deflected, tranduction current flows into the OHC. This transduction current causes a change in the receptor potential (depolarization).
Reverse transduction: Depolarization triggers the motility of the OHC. This change in length exerts force on the basilar membrane, deflecting the stereocilia and so on.
OHC feed energy back into the basilar membrane!! 28 28

29. Evidence for active versus passive cochlea
History: von Bekesy measured broad traveling waves in dead cochleae. However, finely-tuned tuning curves measured for auditory neurons.
Fine tuning of basilar membrane is lost when OHC die.
Compressive non-linearity (as seen in input-output curve) lost when OHC are damaged.
Most important evidence for active cochlea: Oto-acoustic emissions. 29 29

30. Otoacoustic emissions (OAE)
Responses that originate within the inner ear
May be spontaneous or evoked
Generated due to
The feedback system in the cochlea
Active processes within the cochlea
Non-linearities within the cochlea 30 30

31. Tuning in the basilar membrane
Vibration of basilar membrane can be described by a ‘tuning curve’
Amplitude required for the basilar membrane to vibrate at a certain constant displacement, as a function of the frequency of the input sound 31 31

32. Neural connections in the cochlea
Afferent and efferent fibers of the VIIIth cranial nerve (auditory nerve)
Afferent: From organ of Corti to brain
Efferent: From brain to organ of Corti
Peripheral processes of auditory nerve neurons enter the cochlea through small openings on the edge of the osseous spiral lamina
These openings are called ‘habenulae perforata’
These fibers are then gathered in the modiolus 32

33. Organization of the auditory nerve bundle in the modiolus
Fibers from the apex in the middle
Fibers from the base on the outside
This nerve bundle then goes to the cochlear nucleus in the brain 33

34. Afferent fibers Around 30,000 neurons in man
Only 5-15% of these innervate the OHC
These are called Type II or outer spiral fibers
One neuron connects to one OHC (one-to-many)
Rest innervate the IHC
These are called Type I or radial fibers
Many neurons connect to one IHC (many-to-one) 34

35. Originate from the olivocochlear bundle in the auditory brainstem
Efferent fibers
Originate from the olivocochlear bundle in the auditory brainstem
Efferent fibers synapse on the afferent nerves innervating the IHC
Efferent fibers synapse directly on the OHC 35

36. Discharge pattern of a neuron
Neural spike has an initial large potential shift
Following this, refractory or rest period of around 1 msec 36

37. Other terms
Spontaneous discharge rate: Neuron’s discharge rate without a stimulus
Threshold: Minimum stimulus level needed to cause an increase in the discharge rate above the spontaneous discharge rate 37

38. Spontaneous rate and thresholds
Neurons with high spontaneous rate: Low threshold
Neurons with low spontaneous rate: High threshold 38

39. Rate-Level function Also called input-output or intensity function
Increase the level of the acoustic stimulus and measuring changes in the discharge rate of a single neuron

40. Response area Also called isolevel or isointensity curve Iso: “Same”
Plotting how a neuron fires in response to sounds of different frequencies at a fixed intensity 40

41. Tuning curve Define a certain threshold for a neuron
Plot the level of the tone required for this neuron to discharge at this threshold, as a function of the frequency of the tone 41

42. Encoding of frequency Two theories: Place theory
Tonotopic organization
Temporal theory
Based on the periodic nature of nerve firing

43. Place theory
Different neurons in the nerve respond to different frequencies
Frequency of input determined by which neuron(s) in the auditory nerve fires at the greatest rate

44. Temporal theory
For lower frequencies (< 5000 Hz), discharge rates of neurons are proportional to the period of the input stimulus.
So for lower frequencies, rate of discharge of the neuron also provides information about the frequency of the stimulus.

45. Encoding of intensity
Increase in discharge rate with increase in stimulus intensity
However, for most neurons, increase in discharge rate only occurs for a limited range of input intensity
Possible that discharge rate of many neurons may be combined to account for the 140 dB dynamic range of the ear

46. Psychoacoustics Branch of psychophysics
Relation between the physical aspects of sound and the perception of sound
Psychoacoustic correlate of intensity:
Psychoacoustic correlate of frequency: 46

47. Threshold Threshold of detection Absolute threshold
Just detectable level of sound
Threshold of discrimination
Difference limen (DL) or Just noticeable difference (JND)
Just detectable difference between two sounds 47

48. 1000 Hz tone
4000 Hz tone 48

49. Minimum audible curve Plot of threshold as a function of frequency
Minimum audible pressure (MAP)
Measured using headphones
Minimum audible field (MAF)
Measured in free field using loudspeakers 49

50. 1000 Hz
10 Hz
2000 Hz
20 Hz
4000 Hz
100 Hz
8000 Hz
250 Hz
10000 Hz
500 Hz 50

51. Dynamic range of hearing
Range between threshold of hearing and threshold of pain
About 140 dB in normal hearing listeners 51

52. Temporal integration
Signal must contain some critical amount of energy to be detected.
If time is short, then needs more power to be detected (power = energy/time)
For sounds below a certain duration (300ms), the threshold of detection decreases with duration. 52

53. Ear summates or integrates energy over time.53