Hearing Physiology

Question 1

What is sound?

Answer 1

Is the vibration of a medium and spreads out as a wave of pressure
Pressure waves dissipate as you get further away

Question 2

How can we describe sounds?

Answer 2

• Frequency (Hz) (pitch)

- Intensity (loudness) (dB)

Question 3

What range of frequencies do humans have?

Answer 3

20-20000 when young but this decreases when we get older

Question 4

What is the pressure ratio between the quietest detectable sound and an instantly damaging sound?

Answer 4

1:100 trillion

Question 5

How can loudness be measures?

Answer 5

-We plot the quietest sound that can be heard at each frequency on an audiogram (the threshold) dB SPL
But this gives hard to interpret curves
-This is corrected by referencing actual sound pressure level against hearing thresholds in young adults with normal hearing. dbHL

Question 6

Is the human ear more senstive to certain frequencies?

Answer 6

-Yes

At different frequencies we need different ammounts of energy to hear sounds

Question 7

Why do we have 2 ears?

Answer 7

• Allows localisation of sound in horizontal plane

- Sound carries first and louder to side it is closest to

Question 8

What 2 main structures make up the outer ear?

Answer 8

• Pinna

- Ear canal

Question 9

Describe role of pinna?

Answer 9

• Amplifies and filters incoming sounds
• Directional dependant filtering at certain frequencies
• Its shape is important in localisation

Question 10

What do the outer ear structures combine to do?

Answer 10

• Increase sound pressure level by up to 20dB.

- Frequency filtering useful for sound localisation in vertical and front-back planes.

Question 11

What 2 main structures make up the middle ear?

Answer 11

• Tympanic membrane

- Ossicles

Question 12

How do ossicles of middle ear form embryologically?

Answer 12

• 1st branchial arch = malleus and incus

- 2nd arch = stapes

Question 13

What are the 2 protective muscles in the middle ear and what do they do?

Answer 13

• Stapedius reflex; protects cochlea from damage from large sounds by contracting
• Tensor tympani; protects against sounds also (really it doesnt but it does for exam)

Question 14

What does the 3 bone system we have allow for?

Answer 14

Allows for more efficient transmitting of high frequency sound.
Gives sensitivity to high frequency sounds.

Question 15

What does the eustachian tube connect?

Answer 15

-Connects nasopharynx to the middle ear and allows air to enter and leave the middle ear

Question 16

Describe role of eustachian tube?

Answer 16

• Allows air to enter and leave middle ear
• Closed at rest, opens during swallowing and valsalva.
• Keeps air pressure in middle ear space the same as ambient atmospheric air pressure.
• Allow stympanic membrane to vibrate

Question 17

What are the main structures in the inner ear?

Answer 17

Cochlea

Vestibule

Question 18

What are the main functions of the cochlea and vestibular apparatus?

Answer 18

• Cochlea; contains sensory epithelium for hearing, the organ of corti.
• Vestibular apparatus; contains sensory structures for balance and head movements

Question 19

What is organ of corti?

Answer 19

Sensory epithelium containing auditory hair cells

Question 20

Describe role of stria vascularis?

Answer 20

Regulates ionic and metabolic functions of scala media.

Question 21

What is the basilar membrane?

Answer 21

The thing that vibrates and different parts vibrate better at different frequencies.
-It is stiff and light at one end and lexible and heavy at the other.

Question 22

Where are stereocilia found in the hair cells?

Answer 22

Form bundle at apical pole of the hair cell.

Question 23

What is the tallest cilium and its significance?

Answer 23

• Kinocilium
• When it moves away there is depolarisation
• When it moves towards there is hyperpolarisation

Question 24

What do inner hair cells do?

Answer 24

Is a receptor that picks up vibrations of basilar membrane, it converts mechanical to electrical energy.

Question 25

What do outer hair cells do?

Answer 25

Are motile and when stimulated by basilar membrane they change shape and stiffness.
They feed energy back into basilar membrane (reverse transduction)
Acts as amplifier increasing amount of vibration on basilar membrane (cochlear amplifier)
Also improves frequency selectivity

Question 26

Describe the central processing of sound?

Answer 26

-1st order neurons in the spinal ganglion of the cochlea
-Travel in CN8 through acoustic meatus to cerebello-pontine angle.
-Synapse on cochlear nuclei (junction of medulla and pons)
-From there via Olive and trapezoid body to inferior colliculus (midbrain).
-to medial geniculate body (thalamus)
and superior temporal gyrus (cortex)

Question 27

Describe what can cause conductive hearing loss and solutions?

Answer 27

• Causes: -ear canal; wax, foreign body
• tympanic membrane; perforation
• ossicles; congenital fusion, damage from infection
• middle ear space; fluid instead of air

-Inner ear works fine if you can get sound to it by another route

Question 28

What may cochlear implants be used for?

Answer 28

-Can put one one cochlear nucleus in the brainstem if there is no auditory nerve but not great results.

Question 29

How can we screen for hearing in babies?

Answer 29

OHc’s move due to otoacoustic emissions which can be seen in a quick screen for babies.

Question 30

How are hair cells vulnerable and why is this bad?

Answer 30

• Ageing, sensineural hearing loss
• Loss of IHC’s = no signal to brain
• Loss of OHC’s = basilar vibration is insufficiently amplified loss of sound discrimination

Question 31

What is good and less useful about hearing aids?

Answer 31

They are just like a mic so will correct conductive hearing loss.
-Doesnt completely alleviate disability of sensineural hearing loss: loss of discrimination, central processing effects, loss of dynamic range