Hearing

Question 1

How does sound travel?

Answer 1

Via longitudinal waves in the air

Question 2

What are longitudinal waves?

Answer 2

Changes in air pressure- has areas of compression (peaks) and rarefraction (troughs)

Question 3

What is the eardrum also known as?

Answer 3

Tympanic membrane

Question 4

Where do sound waves travel in humans?

Answer 4

Ear canal- sound waves vibrate off of ear drum

Question 5

How sensitive is our hearing and what frequencies can we hear?

Answer 5

Very sensitive- can detect signals that vibrate ear drum by a few picometres- amplifies signals by more than 100 fold

• can hear frequencies of 20Hz - 20000Hz

Question 6

What are the 3 small ear bones in the inner ear and where are they located?

Answer 6

Malleus
Incus
Stapes

• create sound waves with varying frequencies in the perilymph of cochlea

Question 7

What is the perilymph?

Answer 7

Type of extracellular fluid with components (e.g. sodium, potassium) similar to normal extracellular fluid

Question 8

What does the active process enhance and what is this?

Answer 8

Frequency selectivity = capacity to discriminate between similar tones

Question 9

What is the organ of corti and where is it found?

Answer 9

= strip of sensory epithelium running 33mm along the spiral cochlea

Question 10

What is the key component of the organ of corti?

Answer 10

Hair cells = sensory receptors of inner ear

Question 11

How many hair cells does the organ of corti contain?

Answer 11

16000

Question 12

How are the hair cells in the organ of corti arranged?

Answer 12

one row of inner hair cells (IHCs) + 3 rows of outer hair cells (OHCs)

Question 13

Describe the structure of the organ of corti

Answer 13

Made up of tectorial + basilar membrane
Hair cells lie within these 2 membranes with the IHCs connected to auditory nerve

Question 14

What is the tectorial membrane and what is it attached to?

Answer 14

= extracellular matrix that covers the apical surface (highest point) of the organ of corti
- attached to stereocilia of OHCs

Question 15

Why does the sound wave need to be amplified?

Answer 15

sound waves travel at different speeds in air vs perilymph

Question 16

Define: amplification

Answer 16

The process of increasing the volume of sound

Question 17

What chamber is the perilymph located in?

Answer 17

Scala vestibuli

Question 18

How do sound waves travel through the perilymph?

Answer 18

1. Stapes transmit sound waves through to the scala vestibuli
2. Sound wave activates hair cells within organ of corti depending on the frequency

Question 19

What are the 2 different fluids in the organ of corti and where are they found?

Answer 19

Perilymph- scala vestibuli + scala tympani
Endolymph- scala media

Question 20

What is the main difference between the 2 fluids in organ of corti?

Answer 20

Endolymph has higher potassium content

Question 21

What is the basilar membrane and what happens when it vibrates?

Answer 21

= extracellular matrix that covers the basal surface (lowest point) of the organ of corti

Vibration =
1. Hair cells deflected = stereocilia move in a back and forth direction
2. = opens mechanically-gated ion channels = hair cell depolarisation releasing action potentials = converting sound wave into electrochemical signal

Question 22

What are the 3 chambars in the organ of corti?

Answer 22

Scala vestibuli
Scala media = cochlear duct
Scala tympani

Question 23

What is ear hair cell morphology optimised for?

Answer 23

Mechanosensation

Question 24

How are the hair cells optimised for mechanosensation?

Answer 24

Stereocilia within hair bundle are organised in rows of decreasing height
- bundle deflection only in direction of longest stereocilia leads to increased probability of mechanoreceptors opening

Question 25

What are mechanoreceptors?

Answer 25

= sensory receptors that respond to mechanical stimuli (touch, sound)

Question 26

Where is the kinocilium located?

Answer 26

Produced 1st during developmeny and moves to outer edge of top of hair cell as ear matures and then the stereocilia form

Question 27

Do stereocilia have a cytoskeleton and why?

Answer 27

They have a very structured actin cytoskeleton as they have to remain upright

Question 28

How are stereocilia connected?

Answer 28

via tip links that connect top of one stereocilia to another

Question 29

What are roolets and what is their function?

Answer 29

= actin filaments in stereocilia- anchor stereocilia into specialised actin network = cuticular plate

Question 30

What happens if certain proteins that make up tip links are removed?

Answer 30

Mechanosensitivity of stereocilia is decreased which could lead to deafness

Question 31

How can stereocilia be seen in a lab?

Answer 31

1. Hair bundles stained with phalloidin 2. Can be viewed via light or electron microscope depending on resolution

Question 32

Name and describe the model that explains how mechanotransduction works

Answer 32

THE TETHER MODEL: 1. Tip link connected to mechanosensitive channel 2. Tension causes channel to open = influx of calcium ions 3. Channels are also opened in other stereocilia 4. Adaption phase = motor protein slios down which relieves tension in tip link = mechanosensitive channel closes quickly 5. Upper tip link climbs up stereocilium to re-establish tension

Question 33

Name 2 proteins that form tip links

Answer 33

Protocadherin 15 Cadherin 23

Question 34

Name 2 places where mechanosensitive channels are located in the ear

Answer 34

Stereocilia apical membrane

Question 35

What 2 TMCs are candidates for essential functioning of mechanoreceptor channels and whats the evidence for this?

Answer 35

TMC1 + TMC2: - Both are selectively expressed in developing hair cells at the onset of mechanosensitivity - Mutations of TMC1 = deafness - Absense of both = hair cells of mice lack mechanosensory responses to forward deflection of hair bundles - Transient exogenous expression of either restores mechanosensitivity in hair cells in mice who have had them knocked out

Question 36

How is the organ of corti adapted for function?

Answer 36

= gradual changes in structure + morphology from base to apex e.g. height of stereocilia + width and thickness of basilar membrane - stiff and taute at base- picks up high frequencies - More flaccid at apex- can move freely so picks up lower frequencies

Question 37

What happens when the cochlea recieves a pure tone acoustic stimulus?

Answer 37

1. Pressure differences between its liqiud filled compartments cause basilar membrane to oscilate at frequency of stimulation 2. Waves propagate from base of cochlea towards apex 3. As wave progresses, it grows in amplitude but drecreases in wavelength and the quickly vanishes when it reaches relevant frequency position on basilar membrane

Question 38

Describe the active process

Answer 38

OHCs locally activated and enhance basilar membrane vibration 1. Depolarisation causes OHCs to shorten as well as enhances vibrations via contractions and hyperpolarisation causes them to elongate 2. IHCs detect the vibrations and activate afferent neurons e.g. auditory nerve

Question 39

Where are piezo2 channels found in the ear?

Answer 39

apical membrane