Lecture 21 + 22- Hearing

Question 1

Use the example of a tuning fork to explain what sound is…

Answer 1

• The fork vibrates forward create a zone of compression (high pressure) and backwards to create a zone of refraction (low pressure)
• This high to low pressure oscillations on repeat are what sound waves are

Question 2

What are two important features of sound waves? What is the unit for both?

Answer 2

• Frequency= number of cycles per second, same as pitch (unit= hertz)
• Amplitude= size of sound wave from peak to trough, same as loudness (unit= decibels)

Question 3

What range of soundwave frequencies can humans detect?

Answer 3

Between 20- 20,000 kilohertz, range declines with age

Question 4

What is our threshold of hearing determined by? What happens for a deaf person?

Answer 4

• What is audible
• Looks like a U on a graph as is caused by the interaction of loudness/ amplitude and pitch/ frequency i.e. if extreme frequency need to increase volume in order to hear
• For deaf people the shape of the graph is the same it is just shifted up (have a higher threshold for hearing

Question 5

What frequency range does our optimum hearing (i.e can hear at lowest volume) occur and why?

Answer 5

• 700-1200 hertz

- Here because this is where our voices sit (have adapted to hear humam voice the best)

Question 6

True or false is damage to hearing permanent?

Answer 6

True

Question 7

At what dB is the threshold of hearing? At what decibel level do we experience discomfort and at what level do we experience immediate damage?

Answer 7

• 0dB= threshold of hearing
• 120dB= discomfort
• 130-140dB= immediate damage.

Question 8

What are the three broad sections of the ear?

Answer 8

• Outer ear
• Middle ear
• Inner ear

Question 9

What makes up the outer ear/ functions of each part?

Answer 9

• Pinna (auricle)= Flaps of skin and cartilage which sound waves reflect off. This directs sound to the..
• External auditory canal
• Tympanic membrane= ear drum which vibrates when sound waves hit it

Question 10

What makes up the middle ear and functions of each part?

Answer 10

• 3 small bones the malleus, incus and staples= these bones take the vibrations from the tympanic membrane and transfer them across the middle ear to the inner ear where the staples connects with the oval window
• Auditory (eustachian tube)= role is to equalise pressure across the tympanic membrane. Does this by bringing in extra air via the pharynx (is connected to it).

Question 11

Broadly what are parts of the inner ear + their roles?

Answer 11

• Semicircular canal= loops invovled in balance
• Cochlea= snail shell like spiral invovled in sensation of hearing
• Vestibulocochlear nerve= once excited by hearing fires action potentials.

Question 12

What is collectively the name for the 3 bones in the middle ear?

Answer 12

The ossicles

Question 13

What are the two branches of the vestibulocochlear nerve?

Answer 13

• Vestibular branch

- Cochlear branch

Question 14

What are the three chambers are there in the cochlear and their relative positions in a cross section? What are the filled with and how does this contrast with previous structures we have talked about?

Answer 14

• Upper= scala vestibuli
• Middle= Scala media
• Lower= Scala tympani

Filled with fluid which contrasts to the middle and outer ear which are air filled

Question 15

What fluid does both the scala vestibular and scala tympani resemble in terms of concentration? What is this called?

Answer 15

• Perilymph

- Resembles makeup of extracellular fluid: high sodium, low potassium

Question 16

What fluid fills the scala media and what does it resemble in terms of concentrations?

Answer 16

• Endolymph

- Resembles intracellular fluid= low sodium, high potassium

Question 17

What structure sits in the middle cochlear?

Answer 17

The Organ of Corti

Question 18

What is the organ of corti made up of?

Answer 18

• Basilar membrane= layer of cells which organ of corti sits on
• Hair cells (projections of cilia from cells)
• Tectorial membrane sits on top of hair cells

Question 19

How does sound transduction occur up until the point where hair cells in organ of corti move?

Answer 19

1) sound waves cause tympanic membrane to vibrate
2) middle ear bones move.
3) The staple acts as pump on the oval window (boundary to inner ear) causing it to move. This results in a pressure increase in the scala vestibuli (top chamber of cochlear) and the ressiner’s membrane bows out transferring pressure to the scala media (middle chamber)
4) This causes the membrane associated with the scala media the basilar membrane to move and because this is the membrane at the base of the organ of corti this causes the hair cells to move against the tectorial membrane and be deflected.

Question 20

In sound transduction what happens after deflection of the stereocilia?

Answer 20

• The deflection of stereocilia causes the opening of mechanically-gated ion channels (open by force) which lets potassium in depolarizing the hair cell
• This depolarization spreads in a wave down the cilia and results in voltage-gated calcium channels opening. Calcium comes in and results in vesicle fusion and neurotransmitter release into the afferent nerve fibres of the cochlear nerve

Question 21

How come the cilia are all different lengths but still deflect as one?

Answer 21

-They are connected by thin bands of protein (tip links) which mean when one deflects it pulls the others along and causes their mechanically gated channels to open as well.

Question 22

Describe the central pathways…

Answer 22

• Auditory receptors in cochlea
• Brain stem neurons (cochlear nucleus)
• Medial Geniculate Nucleus
• Auditory cortex (each side receives info from both ears)

Question 23

How is pitch/ frequency determined?

Answer 23

Pitch (frequency) – discrimination is determined by activity in hair cells at specific
points on basilar membrane i.e the basilar membrane is tuned to respond to different frequencies along it’s length.

Base= narrow and stiff excited by high frequency sounds.
Move further away= floppy and wide responding to low frequency sounds.

Question 24

How is intensity (loudness) encoded?

Answer 24

Is encoded in the number of impulses per second in auditory nerve fibres. Bigger sound= more deflection= more neurotransmitter release.

Question 25

How is duration encoded?

Answer 25

The sound is signaled by duration of the afferent discharge caused by the stimulus

Question 26

How is direction encoded?

Answer 26

The sound source is indicated by time difference in activation of receptors in each ear, and by intensity differences in each ear

Question 27

What is deafness? What are the two types/ causes?

Answer 27

-Raised threshold to sound stimuli

2 causes:
-Impaired sound transmission through outer or middle ear (conduction deafness)
:Blockage or infection
-Damage to receptors or neural pathways (sensorineural deafness): - exposure to loud noises, tumour, meningitis etc

Question 28

What happens during infection and thus causes conduction deafness?

Answer 28

– Infection causes a build-up of fluid in middle ear (usually just air)
– Prevents correct sound transmission (sound waves find it harder to travel)

Question 29

Exposure to loud sounds causes….

Answer 29

-Damage to hair cells in the organ of corti (can’t be reversed= permanent!)

Question 30

What is an acoustic neuroma (vestibular schwannoma)?

Answer 30

– Benign brain tumour (abnormal growth of Schwann cells on the vestibular nerve causing pressure)
– Hearing loss in one ear
– Tinnitus (ringing or feeling like can hear own bodily functions)
– Detected early cause symptoms so obvious and usually okay

Question 31

What is a Glomus Tympanicum?

Answer 31

– Usually grows behind the ear drum
– Hearing loss and tinnitus
– Surgery can restore hearing

Question 32

What is Ear Canal cancer?

Answer 32

• Very rare with 5-year survival rate of just 35% (super aggressive)
• Hearing loss
• Surgery removes the auditory canal, ear drum and ossicles resulting in permanent hearing loss