Neuro 10 - Sound conduction and Transduction

Question 1

What are the components of the middle ear?

Answer 1

Eardrum, stapes, incus, malleus

Question 2

Outline the sound conduction process in the ear

Answer 2

1. Pinna (outer ear) - localises sound —> funnels it towards tympanic membrane down EAM
2. Changes in air pressure causes TM to vibrate —> vibration transferred from stapes/incus/malleus to Cochlea (fluid filled).
3. Cochlea has hair cells —> movement of fluid in cochlea causes response is sensory neurones –> pass along CN 8 (vestibulocochlear) —> brainstem nuclei —> thalamus —> Medial geniculate nucleus —> auditory cortex in temporal lobe

Question 3

Air in the middle ear is continuous with the nasal cavity. How is the pressure equalised?

Answer 3

By the eustachian tube

Question 4

What bones are the ossicles and where are they located

Answer 4

Malleus
Incus
Stapes

Located behind the tympanic membrane

Question 5

What happens to ear pressure when you fly

Answer 5

1. When flying, atm pressure falls - but ear pressure is still at ground level pressure
2. Ear pressure > atm pressure
3. This causes the air pocket behind the tympanic to expand —> causing the tympanic membrane to bulge

Question 6

What is the function of ossicles

Answer 6

Fluid in the inner ear resists movement - so it needs to be given a kick (e.g. underwater, can’t hear as liquid reflects sound from outside. Bones provide the kick to amplify sound pressure)

Pressure = Force/SA

Question 7

What is the oval window (OW)?

Answer 7

Oval window = intersection of middle ear w/ inner ear - directly contacted by stapes.

Pressure at OW > TM if:

1. Force OW>TM
2. SA of OW smaller than tm

By the time vibrations reach OW, 20X greater force than when they contacted the TM

Question 8

What are the components of the inner ear?

Answer 8

1. Cochlear - spiral shape
2. 3 fluid filled chambers (/scala):
   - Scala vestibuli (vestibular duct)
   - Scala media (cochlear duct)
   - Scala tympani
   (tympanic duct)
3. Reisners membrane - separates scala vestibuli and scala media
4. Basilar membrane - separates scala tympani and scala media. Basilar membrane also contains organ of corti - auditory receptor neurones

Question 9

What are the fluid types in the inner ear

Answer 9

1. Perilymph (CSF-like) = low K and high Na

2. Endolymph = high K and low Na

Question 10

Frequency is key to understanding speech. What translates changes in frequency into speech?

Answer 10

Auditory cortex

Question 11

Explain inner ear function

Answer 11

1. Sound/changes in air pressure transferred from stapes to basilar membrane (through endolymph movement).
2. Frequency determines how far the wave travels - higher frequencies travel further along the basilar membrane
3. Fluid enters via stapes –> flows down Scala vestibuli to helicotrema (where Scala vestibuli and scala tympani meet) –> Fluid enters the Scala tympani which causes a bulge in the round window

Question 12

Explain how the basilar membrane works

Answer 12

Basilar membrane contains hair cells - outer and inner hair cells. More outer (20K) than inner (3.5K). All hair cells contain stereocilia.

Inner hair cells - primary sensory cells - generate APs in auditory nerves - stimulated by fluid movements

Outer hair cells - shorten on depolarisation, lengthen on hyperpolarisation

Hair cells increase the amplitude and improves the quality of sound

Question 13

Where do hair cells form synapses

Answer 13

On neurones in the spiral ganglion - 8th CN (vestibulocochlear)

Question 14

Describe the path of the signal after the hair cell to auditory cortex

Answer 14

1. Afferents ganglion enter brainstem at level of CN 8 —-> innervates dorsal and ventral cochlear nucleus
2. Signals travel from cochlear nucleus —> superior olivary nucleus (both sides) —> lateral lemniscus —-> inferior colliculus —-> medial granitic nucleus —-> auditory nucleus

Question 15

Frequency selectivity is performed by what?

Answer 15

Performed by the basilar membrane - auditory nuclei are present throughout which code different frequencies

Question 16

At what frequencies is it difficult for the basilar membrane to detect?

Why?

What are the 2 mechanisms to handle this?

Answer 16

Below 200Hz or above 20000Hz

Because the basilar membrane doesn’t move much.

Tonotopy and phase locking

Question 17

What is the phase locking mechanism?

Answer 17

It only works at LOW frequency.

Timing the neural firing - matching the firing to frequency of the sound - (determine frequency by measuring time from peak to peak on sine wave, cycles per second)

Question 18

What is tonotopy

Answer 18

When the frequency becomes too high, firing becomes random. To accommodate for this, tonotopy is used for very high frequencies.

Question 19

What mechanisms do mild frequencies use?

Answer 19

Phase locking (low frequency) AND tonotopy (high frequency)

Question 20

What is the inter-aural time difference?

Answer 20

Difference in arrival time of sound between 2 ears

This difference is detected by neurones in the brainstem - allows sound localisation

Question 21

What are the 2 ways of conduction?

Answer 21

1. Air conduction via ear

2. Bone conduction directly via vibrations of cochlea (e.g. using construction drill)

Question 22

How can the bone conduction vs air conduction be used to determine the cause of hearing loss

Answer 22

If Bone conduction (shake head) better than air conduction (listening to speech), the problem lies before the cochlea (e.g. ossicles)