Cochlear Biomechanics

Question 1

What antibiotic caused hearing loss?

Answer 1

Kanamycin

Question 2

How did Kanamycin cause hearing loss?

Answer 2

It selectively killed off OHC, but left the IHC alone. When the OHC are killed off, we don’t see any reaction from the BM until more intensity is added. At 40dB, we begin to see displacement of the BM

Question 3

Why is BM displacement not linear?

Answer 3

BM displacement as a function of intensity is not a straight line. With OHC, the displacement is linear until 40dB. After this it stays in a horizontal line until at 80dB, we see displacement linearly again. This is because the IHC start getting action at 40dB

Question 4

What is the function of OHC?

Answer 4

To increase fluid disturbance and add energy into the cochlea. The IHC can respond to soft sounds if the OHC disturb the fluid enough.

Question 5

What is the max that an OHC can expand?

Answer 5

40dB

Question 6

What does a tuning curve represent?

Answer 6

Level of the sound against the frequency of the sound to find the cell’s favorite frequency. You don’t have to give it as much energy to get it to start firing.

Question 7

What does a tuning curve look like with OHC loss?

Answer 7

You need to give it significantly more energy to react. The curve is much more broad. You can’t hear low intensity sound and can’t tell frequencies apart.

Question 8

What are OHC tuned for?

Answer 8

Low thresholds and also responsible for sharp tuning/fine frequency differences.

Question 9

What are OAE?

Answer 9

Sound coming out of the ear?

Question 10

How does OAE happen?

Answer 10

When you put sound into a cochlea, the prestin motility add more energy in and more fluid disturbance it can go back in the opposite direction and pushes into the oval window in the other direction. Ossicles vibrate in reverse, which causes a sound.

Question 11

Why are OAE useful?

Answer 11

Clinically useful because if you damage the OHC in a particular region of the cochlea, there is no Prestin motility and no reflected sound wave, therefore no OAE. Can measure integrity of the OHC. If some OHC are intact (some are killed off), then the OAE will be reduced.

Question 12

How many afferent fibers are in each ear?

Answer 12

30,000 in each ear
27,000 are connected to IHC
3,000 are connected to OHC

Question 13

How many hair cells are in a cochlea?

Answer 13

15,000 total for each ear
12,000 are OHC
3,000 are IHC

Question 14

What are the two types of afferent fibers and how are they connected?

Answer 14

1.) Type 1: 90% of the fibers and are connected to IHC; highly myelinated
2.) Type 2: 10% of the fibers and are connected to OHC; un-myelinated

Question 15

Why is myelination important?

Answer 15

It increases the speed of information traveling across the neuron. The electrical signal can jump across the Nodes of Ranvier

Question 16

How long is the action potential?

Answer 16

1 millisecond

Question 17

Why does spontaneous firing happen?

Answer 17

There is always some calcium coming into the permeable cell wall of the hair cell. As a result, there is always some neurotransmitter moving into the synapse, so dendrites are always picking it up

Question 18

Why do we care about spontaneous firing rates?

Answer 18

They matter because of the fact that every auditory nerve fiber will increase its firing rate as the intensity of the sound/level of sound increases. It will not infinitely increase.

Question 19

What is the dynamic range of a cell?

Answer 19

Range of intensity of which the fiber will increase its intensity rate as the sound increases. Only about 50dB

Question 20

What is a high spontaneous firing rate?

Answer 20

It takes almost no sound at all for the cells to fire (low threshold); 0-50dB

Question 21

What is a low spontaneous firing rate?

Answer 21

It takes a lot of sound for the sounds to fire (high threshold)l 70-120dB

Question 22

What is physiological recruitment?

Answer 22

As you increase the level of sound, fibers with nearby favorite frequencies will also respond to that sound. Provides an additional cue for the brain to use and code the large range of intensities that we can respond to.

Question 23

What are the 2 mechanisms for coding intensity?

Answer 23

1.) Fibers have low, med, or high SFR will determine the 50dB ranges they will respond to but they don’t all respond to the same 50dB range.
2.) Physiological recruitment: fibers that don’t love that frequency, but if you make it loud enough it will respond to it. Broadening of the envelope of BM displacement as the level of sound increases.

Question 24

What are 2 things that efferent fibers are doing in the cochlea?

Answer 24

1.) Protection from loud noises
2.) Improving signal to noise ratio