2.2 Cochlear Physiology l Flashcards
What are the three main theories for how the cochlea analyzes sound?
Place Theory, Frequency Theory, Place Folley Theory.
Which of the three main theories is currently accepted?
Modern Place theory proposed by Bekesy upon his discovery of the travelling wave.
For the place/resonance theory, who had proposed it and what was that study based on?
Helmholtz proposed the place/resonance theory based on his study of resonators
The place theory was first proposed (1884) by Helmholtz based on his study on resonators. He thought that the cochlea= a group of _____________ resonators of different natural frequencies.
INDEPENDANT resonators (at the level of the rods of corti)
What is another way to call the place/resonance theory?
Travelling wave theory
What direction does the travelling wave move?
From base to apex
What happens to the amplitude of the wave as it passes the BM?
The amplitude first increases and then once it is over the peak, it drops down quickly
Where is the peak location?
The peak location is determined by the frequency of the sound signal
What is meant by the travelling wave being asymmetric? (2)
- Steeper on the low frequency side (apex)
- Broader wave for low frequency signals
What is the relationship of the travelling frequency to the signal frequency? (2)
- Travelling frequency does not equal signal frequency
- Travelling speed < sound speed
What is the relationship of the travelling wave of a low frequency tone to the basilar membrane?
The lower frequency waves have a wider envelope which covers a larger distance on the basilar membrane
What are three major discoveries from this Bekesy’s research?
Travelling feature of the vibration (that there is a travelling wave); peak location change with frequency; asymmetric envelope
What is the cause of the wave slowing down?
Increase of mass of the basilar membrane thus takes longer to vibrate at the apex
What is another way to say this “slowing down” concept of waves at the BM?
Phase lag or Time delay
What kind of wave are the periodic signals measured in for the time and phase domains? (2)
- Sine waves in the time domain
- Vector rotation in the phase angle
Would the frequency of the most left curve be the lowest or highest compared to the other 4 curves? What is the range of frequencies tested in this graph (does it go across all frequencies?)
Largest = 300hz which is the highest frequency
The range covered in 50-300hz, and doesn’t cover all frequencies
Describe the pendulum analogy as it relates to the vibration.
A force is applied to all the pendulum at the same time. Objects with smaller mass respond faster.
Summarize the three main size changes that occurs from the base to the apex regarding the BM OC and HC: (3)
- The width of the basilar membrane increases (mass increases).
- The thickness of the basilar membrane decreases (stiffness decreases).
- Organ of Corti size increases and hair cell height increases (vibrates slower).
Why does the BM respond more slowly to the travelling wave at the apex?
Apical BM has larger mass and smaller stiffness
How would you describe the width and duration of an impulse signal?
The spectrum is wide and the duration is very short.
Which pairs would result decrease in stiffness and increase in mass?
From base to apex:
1. Increase in BM width
2. Slight decrease in thickness
3. Increase in OC size and height of HCs
1 and 2 result decrease in stiffness
1 and 3 result increase in mass
What are the three limitations of Bekesy’s study of the travelling wave?
Dead cochleae
Poor sensitivity of equipment
Used a high intensity of sound
What is the lack of consistency of the auditory nerve and the basilar membrane?
ANF responses are sharply tuned but not BM vibration.
Describe how the active component of the cochlea is different from the passive?
The active component depends on OHCs, which improves frequency selectivity and sensitivity of the cochlea. The active component requires healthy cochleae whereas the passive component does not.
Explain the meaning of the fact that receptor potentials are graded.
The magnitude of their potentials depend upon the intensity of the stimulus.
What does the receptor potential in the inner hair cell vs the outer hair cell lead to?
The receptor potential in the inner hair cell leads to neurotransmitter release.
The outer hair cells lead to OHC motility. (length change
What are Macromechanics? (as it relates to the ear)?
Macromechanics refers to how the basilar membrane vibration causes the Organ of Corti to vibrate. (Traveling wave and Radial Vibration)
What are Micromechanics? (as it relates to the ear)
Micromechanics refers to the vibration of different parts in the OC and the interaction among those parts.
What are the two factors that cause the BM vibrates up and down?
The different dimensions (size) between the SV and ST
The different flexibility between the oval window and the round window. The BM bends up and down to balance the pressure.
Which direction does the vibration amplitude change? What is another word to describe this force?
Radial direction, which means that it spreads out from a central point. Another word to describe this is shearing.
Describe what is happening in A and B. What is the name for this model?
This is the shearing model. The organ of corti and tectorial membrane rotates as a result of radial vibration. (TM and OC have different pivot points)
What leads to the shearing effect?
Rotation causes the shearing effect. The stereocilia of the OHCs are connected to the tectorial membrane, so when they rotate there is a shearing between the TM and the reticular lamina.
What are the borders around the endolymph?
Tight junction along Ressiner’s membrane, the stria vascularis and along the reticular lamina
What is the travelling wave theory?
- Sound pressure applied to oval window
- Pressure is transmitted as a travelling wave along BM
- Peak displacements for high frequencies are toward the base and high frequencies are toward the apex
