Auditory system- Lecture 13 and 14

Question 1

What is hearing needed for?

Answer 1

An early warning system with 360 degree coverage and works at a distance. Communication and speech

Question 2

What is sound?

Answer 2

A longitudinal pressure wave in which air molecules are close together and then pulled apart

Question 3

What is frequency?

Answer 3

1/t (sec)- the number of oscillations which occur in one second.

Question 4

What range of frequencies can a young person hear?

Answer 4

from 20Hz to 21kHz

Question 5

To hear lower frequency sounds we need …… volume

Answer 5

Higher

Question 6

At what frequency do we require the lowest volume to hear it?

Answer 6

2000Hz- most sensitive to this frequency

Question 7

What volume is the threshold for risk of hearing loss?

Answer 7

90 dB

Question 8

What are the two components of the outer ear?

Answer 8

Pinna and auditory canal

Question 9

What is the function of the outer ear?

Answer 9

To direct sound to the tympanic membrane. Responsible for the external perception of sound

Question 10

How long is the ear canal?

Answer 10

28mm tube

Question 11

By how much does the ear canal amplify speech?

Answer 11

from 10 dB to 15 dB

Question 12

Between what range does the ear canal vibrate?

Answer 12

1.5kHz to 7kHz

Question 13

Describe why the middle ear is necessary in terms of impedance mismatching

Answer 13

Sound waves need to be transferred into the cochlear which is fluid filled and denser than air. There is poor energy transfer from air to water as most is just reflected. The middle ear acts as an impedance transformer to improve this transfer

Question 14

How does the middle ear act as an impedance transformer?

Answer 14

The area of the foot of the stapes is smaller than the tympanic membrane so the force exerted is spread over a smaller area which increases the pressure (pressure=force/area) so this increase in pressure helps the transfer into the cochlear

Question 15

How much does pressure increase between the tympanic membrane and stapes?

Answer 15

25x

Question 16

What is the cochlear?

Answer 16

The organ of hearing- a frequency analyser

Question 17

What are the two ways the cochlear acts as a frequency analyser?

Answer 17

Mechanical response of the cochlear and mechano-electrical transduction by hair cells

Question 18

Which cranial nerve is the auditory nerve?

Answer 18

8 VIII

Question 19

How long is the cochlear when uncoiled?

Answer 19

35mm

Question 20

How many turns of a spiral does the cochlear have?

Answer 20

2.5

Question 21

What are the three compartments of the cochlear?

Answer 21

Scala vestibuli, Scala media and Scala tympani

Question 22

What fluid is inside the Scala vestibuli and Scala tympani?

Answer 22

Perilymph (High Na+)

Question 23

What fluid is inside the Scala media?

Answer 23

Endolymph (high K+)

Question 24

What membrane separates the Scala Vestibuli and Scala media?

Answer 24

Reissner’s membrane

Question 25

What membrane separates the Scala media from the Scala tympani?

Answer 25

The basilar membrane

Question 26

What cells line the Scala media and secrete K+ to produce endolymph?

Answer 26

Stria vascularis

Question 27

What is the cellular component of the cochlear responsible for transduction of sound?

Answer 27

Organ of Corti

Question 28

On what membrane does the organ of Corti sit?

Answer 28

The basilar membrane

Question 29

What cells make up the organ organ of corti?

Answer 29

Inner and outer hair cells and the tectorial membrane

Question 30

Which hair cells have stereocilia imbedded in the tectorial membrane?

Answer 30

Outer hair cells

Question 31

What fluid baths the hair cells inside the Organ of corti?

Answer 31

Perilymph

Question 32

Where does the stapes footplate connect to the cochlear?

Answer 32

The oval window

Question 33

What direction does the basilar membrane go in?

Answer 33

From base to apex

Question 34

Describe the travelling wave of the basilar membrane

Answer 34

Displacement begins at base due to movement of the stapes in and out. BM oscillates from base to apex

Question 35

What determines the location of greatest displacement on the basilar membrane?

Answer 35

Frequency

Question 36

Where do low frequency sounds displace the BM most?

Answer 36

The apex which is wider and more flexible

Question 37

Where do high frequency sounds displace the BM most?

Answer 37

At the base- thinner and more rigid.

Question 38

What is laser interferometry?

Answer 38

Shining a laser at different parts of the basilar membrane to measure velocity and displacement. When measuring firing from hair cell fibres at this area of highest displacement, curve is similar to tuning curve

Question 39

How many rows of inner hair cells are there?

Answer 39

1

Question 40

How many rows of outer hair cells are there?

Answer 40

3

Question 41

Describe the tectorial membrane

Answer 41

A gelatinous structure made of proteoglycans

Question 42

What does movement of the basilar membrane cause?

Answer 42

The tectorial membrane to shear over the stereocilia of hair cells.

Question 43

In what direction do stereocilia bend to depolarise the hair cell

Answer 43

Towards the tallest

Question 44

In what direction do stereocilia bend to hyperpolarise the cell?

Answer 44

To the shortest

Question 45

Describe the excitation phase

Answer 45

When sound wave is in rarefaction phase, the stapes moves outwards and displaces the membrane upwards, shears tectorial membrane over the tallest stereocilia, depolarisation

Question 46

Which type of hair cell is rounder?

Answer 46

Inner

Question 47

How long are stereocilia in length?

Answer 47

5 microns

Question 48

What is between the rows of outer hair cells?

Answer 48

The reticular lamina

Question 49

What is the function of the reticular lamina?

Answer 49

to separate the endolymph and perilymph

Question 50

What side of hair cells are bathed in endolymph?

Answer 50

Top

Question 51

What is a tip-link

Answer 51

Connects stereocilia. Made of cadherin and protocadherin which are associated with MET channels TMC1 and TMC2.

Question 52

What ions flow through MET channels in neutral position?

Answer 52

Some K+

Question 53

What ion is at a high concentration inside the inner hair cell?

Answer 53

K+

Question 54

What is the voltage gradient from outside in the endolymph to inside the inner hair cell?

Answer 54

130mV (90mV outside and -40mV inside)

Question 55

What makes K+ move into the IHC when MET channels open?

Answer 55

The voltage gradient (not the concentration gradient as there isn't one)

Question 56

Describe the process of depolarisation of the IHC.

Answer 56

When the tectorial membrane shears over the stereocilia from shortest to tallest, the tip links are pulled tight which opens MET channels. K+ enters the stereocilia and IHC down the voltage gradient. This depolarises the IHC causing Ca2+ entry through voltage gates calcium channels. This causes the release of glutamate, the excitatory neurotransmitter, onto the spiral ganglion neurite

Question 57

What is adaptation?

Answer 57

The reduced response of the MET channels when stimulus is sustained

Question 58

What is the rapid adaptation mechanism of IHC?

Answer 58

Ca2+ entry into the stereocilia interacts with the MET channels which reduces the amount of K+ they let through- resets the sensitivity to K+

Question 59

How many possible slow adaption mechanisms are there?

Answer 59

2

Question 60

Describe the tension regulation slow adaptation

Answer 60

As stereocilia are deflected for a long time, the Ca2+ entry causes the myosin to slip down the actin filaments in the tip link- this reduces tension and closes MET channels

Question 61

Which type of hair cell secretes glutamate?

Answer 61

Inner hair cell

Question 62

What type of hair cell are most auditory nerve fibres connected to?

Answer 62

Inner hair cell

Question 63

Which hair cells are involved in biological amplification?

Answer 63

Outer hair cells

Question 64

Why do we need amplification?

Answer 64

For increased sensitivity and tuning

Question 65

What did Kemp realise in 1970s?

Answer 65

That when we direct sound into the cochlear, there is a weaker sound reflected out, so there must be an active process of amplification occuring.

Question 66

What is the electromotility of outer hair cells?

Answer 66

They shorten when depolarised and lengthen when hyperpolarised

Question 67

What protein do outer hair cells produce that give them motility?

Answer 67

Prestin

Question 68

What ion is required inside the outer hair cell to give motility?

Answer 68

Chloride

Question 69

Describe the mechanism of prestin and chloride

Answer 69

When the OHC is depolarised, the inside of the cell is more positive so any Cl- ions are drawn to the centre of the cell away from the membrane. When the cell is hyperpolarised, the Cl- ions move towards the membrane where the prestin molecules are imbedded with a Cl- binding site in it so when Cl- binds it changes the shape of prestin to be longer.

Question 70

Describe prestin

Answer 70

A modified anion transporter- doesn't move Cl- out, just moves it further into the membrane depending on membrane potential.

Question 71

Why is the prestin movement so quick?

Answer 71

It responds to fast fluctuations in charge across the membrane

Question 72

What do OHC do to the basilar membrane?

Answer 72

Displace it more so amplify the signal by increasing IHC sensitivity to sound

Question 73

What does damage to outer hair cells do?

Answer 73

Hearing loss, especially to soft sounds because we have no amplification

Question 74

Describe the size of OHCs at the base

Answer 74

Smaller SA

Question 75

How does the smaller SA mean that these OHCs can respond better to high frequency?

Answer 75

Smaller SA lowers capacitance which, along with resistance, are factors of TC. A smaller TC is needed in order for a membrane to be able to quickly flip the voltage across a membrane.