Cochlear Physiology

Question 1

What does the cochlea do when it first receives a signal/sound?

Answer 1

Determine frequency, intensity, and temporal cues of signal in order to transmit it correctly to the higher auditory pathway.

Question 2

What is the pathway of the acoustic signal?

Answer 2

Sound is found in the air, with compressions and rarefactions.
Compressions and rarefactions travel through the outer ear, which cause the TM to move.
Acoustic signal is altered to a mechanical signal by the impedance matching system of the middle ear.
The movement is translated to the oval window.
This movement is directly related to the frequency, intensity, and timing of the original signal.

Question 3

What does compression cause the oval window to do? Rarefaction?

Answer 3

Compression causes the stapes footplate to move inward. Rarefaction causes it to move outward.

Question 4

What is the Traveling Wave by George von Bekesy?

Answer 4

The movement of the cochlear partition, with the key player being the basilar membrane, in relation to the input arriving at the oval window.

Question 5

What does the vibration of the stapes result in?

Answer 5

Displacement of non compressible cochlear fluids along with a compensatory displacement or bulge of the round window.

Question 6

What does the pressure wave that originates at the oval window create?

Answer 6

Temporary pressure differences between the scala vestibuli and scala tympani- which cause displacement of the basilar membrane.

Question 7

What do compression waves do to the basilar membrane? Rarefaction waves?

Answer 7

Compression drives the BM down, and rarefaction drives it up.

Question 8

Where does the BM have greater displacement? Why?

Answer 8

Greater displacement longitudinally than radially, because of it’s attachments and characteristics.

Question 9

In which direction does the wave travel?

Answer 9

Always from the base to the apex.

Question 10

What is the cochlear operation of frequency analysis dependent on?

Answer 10

Graded width, graded stiffness, and graded mass.

Question 11

Explain the graded width of the BM

Answer 11

The width of the BM increases from base to apex- greater at apex than at base

Question 12

Explain the stiffness of the BM

Answer 12

The stiffness of the BM decreases from base to apex- apex is flaccid, base is stiff

Question 13

Explain the graded mass of the BM

Answer 13

Increases in mass from base to apex- apex is more massive than the base.

Question 14

As stiffness of a vibrating object increases…

Answer 14

Resonant frequency increases.

Question 15

As mass of a vibrating object increases…

Answer 15

Resonant frequency decreases.

Question 16

How does the traveling wave represent the frequency and intensity of the stimulus from the oval window?

Answer 16

Frequency- location of maximum displacement of the BM

Intensity- amount of deflection of the BM

Question 17

What does tonotopic arrangement mean?

Answer 17

Frequency analysis

Question 18

Where do different frequencies reach their maximum deflections along the BM?

Answer 18

High frequencies cause maximal displacement in the basal region of cochlea, low frequencies cause maximal displacement of the BM in the apical region of the cochlea. Low frequency will simulate both base and apex, but the max displacement will be in the apex.

Question 19

What does the amplitude of the traveling wave look like? What does it result in?

Answer 19

Builds in amplitude as it moves away from base of cochlea, then decays quickly after reaching it’s maximum. Results in “envelope” of activity that is asymmetric in shape.

Question 20

Explain the excitation of the OHC’s.

Answer 20

Cilia are embedded in the tectorial membrane- shearing action is produced between the two when the wave moves along the BM. Shearing action is greatest at the point of maximum displacement of the basilar membrane.

Question 21

Explain shearing force in excitation of OHCs

Answer 21

Shearing force is greater than vertical force would provide on its own due to pivot points for tectorial and basilar membranes being hinged at different points.

Question 22

What is the cochlear amplifier action/electromotility of OHC?

Answer 22

The OHC contracts when BM moves upward and expands when BM moves downward. Results in greater deflection of cilia at the point of maximum displacement.

Question 23

Explain depolarization/excitation of IHCs

Answer 23

Cross links hold the stereocilia together, causing bundle to move in unison when shearing force is applied. Tip links course from shaft of one stereocilia to the tip of shorter stereocilia in the next row. As stereocilia bundle is deflected in direction of tallest row, tension increases in the tip link, which pulls open the mechanically gated ion channels located near the tip of the cilia.

Question 24

What does a rarefaction wave result in in IHC excitation?

Answer 24

Will displace the BM upward and result in deflection of stereocilia bundle toward the tallest stereocilia/away from modiolus- which will cause transduction channels to open and K+ to flow into the cell, initiating depolarization. This will lead to CA 2+ channels opening which aids in repolarizing cell and releasing neurotransmitters from base of hair cells.

Question 25

What neurotransmitter is released by the IHC? What does it do?

Answer 25

Glutamate- it diffuses across synaptic cleft, activates receptors on postsynaptic membrane of afferent nerve fibers and depolarizes fibers to generate an action potential.

Question 26

What happens in hyperpolarization/inhibition of hair cells?

Answer 26

A compression wave, displacing the BM downward, will stretch the tectorial membrane and cilia are pulled toward the modiolus. Tip links become slack, disallowing any ion flow into the cell. Increases number of transduction channels that are closed and results in hyperpolarization of the hair cell.

Question 27

What does hyperpolarization result in?

Answer 27

Decreases neurotransmitters from hair cell and leads to a decreased frequency of firing of the auditory nerve fibers.

Question 28

What is the first system to perform analysis of an incoming signal/sound?

Answer 28

Cochlea