Auditory and Vestibular Systems

Question 1

Provide the basic structure of hair cells

Answer 1

Hair bundles- vertically aligned hairs are rigid stereocilia that sit on top of the hair cell
At the end of the cell is the synapse that attaches to a nerve fibre that projects to the brain
Sitting around/on top the hair cell bundle is an extracellular matrix to protect the hair bundle
- In the auditory system it’s the tectorial membrane, in the maculae it’s the otoconial membrane and in cristae it’s the cupula

Question 2

What are stereocilia bundles and what links can be found within them?

Answer 2

Stereocilia are arranged in ‘bundles’ (e.g. 30-300 stereocilia in each bundle in the ear)
Within the bundle stereocilia can be connected via a number of links:
- Connectors: Lateral-link, top connectors, shaft connectors (connect the shaft of the stereocilia) and ankle links (at the bottom)
- Tip links: Found at the top of the cilia

Question 3

What are the stereocilia links’ functions?

Answer 3

Stereocilia bundles; lateral link connectors
Lateral-link connectors between the shafts of stereocilia hold
the bundle together to allow it to move as a unit

Stereocilia bundles; top links
Tension in the ‘Tip-links’ distorts the tip of the stereocilia mechanically
This distortion allows channels to open and close with cilia movement.
Current flows in proportionately.

Question 4

What happens when Tip-links open ion channels?

Answer 4

‘Tip-links’ open ion-channels.
Endolymph (extracellular fluid) high in K+.
Potassium ion (K+) influx due to opening of ion channels depolarises the cell.
Voltage gated Ca2+ channels open.
Ca2+ triggers neurotransmitter release at the synapse.
Post-synaptic potential in nerve fibre triggers an action potential.
When the tip-links move in the other direction they close the stereocilia ion channels
Displacement of the cilia causes a change in membrane potential

Question 5

What is the inner ear formed of?

Answer 5

The inner ear is formed of:
Semi-circular canals (vestibular system)
Cochlea (auditory system)
Vestibular and cochlear nerve merge to form the vestibulocochlear nerve

Question 6

What is the function of the semi-circular canals?

Answer 6

Semi-circular canals; sensing rotation
Rotation causes fluid motion in the semi-circular canals.
Hair cells at different canals entrances register different directions.
For example the posterior semi-circular canal captures the roll motion, so the fluid will flow around faster in that canal.

Question 7

How do hair cells aid in sensing rotation?

Answer 7

Hair cells for sensing rotation
Cilia are connected to the gelatinous cupula.
Under motion, fluid in the canals lags to due to inertia, pulling the cupula in the opposite direction to the rotation of the head.
Cilia are displaced, depolarising hair cells.

Question 8

What are the otoliths? How do they work?

Answer 8

For capturing linear motion you have the otoliths
In the otolith organs they are sensitive to linear acceleration.
Gravity is also acceleration.
Utricular macula- aligns the otolith organs so that they can capture left and right motion
The secular macula captures forwards/back movement and up/down movement

How the otolith organs work
Hair cells are topped by a rigid layer of otoconia crystals on top of the otolithic membrane
Under acceleration the crystal layer is displaced, deflecting the cilia and displacing the gelatinous layer with the hair cells

Question 9

What does the auditory system consist of?

Answer 9

The ear which allows and translates the changes in air pressure into motion of fluid in the cochlea which translates it to electrical activity.
This goes through the cochlear nucleus, olivary complex, lateral lemniscus, inferior colliculus, medial geniculate body and finally the auditory cortex.

Question 10

What is the pinna and its function?

Answer 10

The pinna (the outer ear)
Size and shape varies from person to person.
Gathers sound from the environment and funnels it to the eardrum.
Made entirely of cartilage and covered with skin.

The outer ear filters, influencing the frequency response.
Pinna features influence the entering sound differently.
The flange is amplifying between 3000 and 5000Hz
The concha amplifies about 5kHz
The meatus amplifies low frequencies

Question 11

What are the different grades of Microtia?

Answer 11

Grade I: A less than complete development of the external ear with identifiable structures and a small but present external ear canal
Grade II: A partially developed ear (usually the top portion is underdeveloped) with a closed stenotic external ear canal producing a conductive hearing loss.
Grade III: Absence of the external ear with a small peanut-like vestige structure and an absence of the external ear canal and ear drum. Grade III microtia is the most common form of microtia (see photo).
Grade IV: Absence of the total ear or anotia.

Question 12

What is in the middle ear?

Answer 12

The tympanic membrane (the middle ear)
The ‘ear-drum’ vibrates in response to sound.
Middle ear bones (ossicles) are visible through the membrane.

Question 13

What are the three ossicles? What do they do?

Answer 13

The ossicles:
- The malleus
- The incus
- The stapes
Smallest bones in the human body.
Positioned to create amplification by the incus levering and that pushes the stapes inwards and outwards which contacts the oval window (the route through which sound enters the inner ear)
Connects the tympanic membrane to the oval window of the cochlea.

Question 14

What is glue ear/ otitis media?

Answer 14

Middle ear fills with fluid which impedes motion of the ossicles.
Reduces middle ear gain, raises hearing thresholds.
Very common in small children (<5 yrs) - can lead to development problems, in terms of language development

Question 15

What are the three chambers that compose the cochlea?

Answer 15

The cochlea is composed of three chambers:
- Scala vestibuli
- Scala media
- Scala tympani
The most important arguably is the Scala media cause it contains the basilar membrane and the components for hearing
When the stapes moves on the oval window, it causes compression and rarefaction of the lfuid in these chambers
That creates a travelling wave along the basilar membrane

Question 16

What are the cochlea and basilar membrane? What is the function of the basilar membrane?

Answer 16

The cochlea: fluid filled spiral canal divided by a flexible membrane.
Basilar membrane filters sound according to frequency.
At one end, close to the oval window, the basilar membrane is narrow and stiff
This vibrates more to high frequency
At the apex, the other end, it is wide and compliant so moves slower so prefers lower frequencies

Question 17

Whats is the organ of corti?

Answer 17

Within the Scala media sits the organ or corti
The organ of corti sits on top of the basilar membrane, within the Scala media.
Inner and outer hair cells are mounted on it.
In the organ of corti you have these outer hair cells and inner hair cells and on top of the organ you have the tectorial membrane (as the extracellular membrane

Question 18

What does the organ of corti do?

Answer 18

Motion of the organ of corti on the basilar membrane causes displacement of the stereocilia.
Outer hair cells contact the tectorial membrane.
Inner hair cells do not.

Question 19

What do the outer and inner hair hair cell do?

Answer 19

The inner hair cells have an afferent synapse so the flow of electrical activity goes from the hair cell to the brain
The outer hair cells have an efferent synapse so they are receiving information from the brain
Outer hair; ‘rock and roll’ hair cell
Outer hair cells are motile.
Influx of positive ions makes the outer hair cells contract
Prestin is the protein which allows the change in conformation of the hair cell

Question 20

How does the movement of the hair cells act as the cochlear amplifier?

Answer 20

As they shift in one direction, there is an influx of positively charged ions into the outer hair cell
Prestin in short conformation state
That causes the outer hair cell to contract
This pulls the basilar membrane toward the tectorial
membrane.
This means the inner hair cells move so it is amplifying the sound
Amplifies by as much as 50dB!!
Quiet sounds are amplified.
Loud sounds are not amplified – helps us deal with 120dB of dynamic range.
Tuning is sharper than the passive vibration of the basilar membrane

Question 21

What is the battery driving the cochlear hair cells?

Answer 21

The high potassium concentration of the endolymph of the Scala media creates a 2x amplification.
If it were not potassium rich then inner hair cell output (of the cochlea nerve) would be halved, making sound perceptually quieter.
And the cochlea amplification would be much smaller, again making sounds perceptually quieter.