Auditory and Vestibular

Question 1

Hair cell

Answer 1

A cell generally composed of stereocilia, a cell body and a synapse (onto an afferent nerve fibre).

Question 2

Stereocilia

Answer 2

Are rigid, non-motile, actin filled rods, or “hairs”.

Question 3

Afferent nerve

Answer 3

The nerve cell stimulated, via a synapse, by hair cells. This is the 8th cranial nerve in the case of auditory and vestibular hair cells.

Question 4

Endolymph

Answer 4

A potassium-rich extracellular fluid is critical to the function of hair cells.

Question 5

Basilar membrane

Answer 5

The membrane which houses auditory hair cells. The basilar membrane selectively vibrates to different frequencies at different points along its length, this underlies perceptual frequency selectivity.

Question 6

Tip links

Answer 6

Found at the top of the cilia

Question 7

Connectors

Answer 7

Lateral-link, top connectors, shaft connectors and ankle links.

Question 8

Stereocilia bundles: TIP links

Answer 8

• 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 9

Stereocilia bundles: Lateral link connectors

Answer 9

teral-link connectors between the shafts of stereocilia hold
the bundle together to allow it to move as a unit

Question 10

Hair cells

Answer 10

• Tip-links’ open ion-channels.
• Endolymph high in K+.
• Potassium ion (K+) influx 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.

Question 11

Inner ear is formed of:

Answer 11

• Semicircular canals (vestibular system)

* Cochlea (auditory system)

Question 12

Semicircular canals: sensing rotation

Answer 12

• Rotation causes fluid motion in the semicircular canals.
• Hair cells at different canals entrances register different directions

Roll: rotation around X-axis - Posterior semicircular canal
Pitch : Anterior semicircular canal
Yaw: Rotation around Z-axis

Question 13

Hair cells for sensing rotation

Answer 13

• Cilia are connected to the gelatinous cupula.
• Under the 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 14

Orientation and motation in mammals

Answer 14

• In the otolith organs they are sensitive to linear acceleration.
• Gravity is also acceleration.

Question 15

How the otolith organs

Answer 15

• Hair cells are topped by a rigid layer of otoconia crystals.
• Under acceleration the crystal layer is displaced, deflecting the cilia.

Question 16

Hair cells as water motion detectors: The lateral line system

Answer 16

• Most fish and amphibians have a lateral line system along both sides of their body.
• Mechanoreceptors provides information about movement through water or the direction and velocity of water flow.
• Important for schooling.
• Some mechanoreceptors, or neuromasts are in canals.
• Superficial neuromasts are on the surface.
• Neuromasts function similarly to the mammalian inner ear.
• A gelatinous cupula encases the hair cell bundle and moves in response to water motion.
• Most amphibians are born (i.e. tadpoles) with lateral lines.
• Some (e.g. salamander) lose them in adulthood.
• More aquatic living species retain them.

Question 17

Auditory system

Answer 17

Cochlear nucleus 
Olivary complex 
Lateral lemniscus
Inferior colliculus 
Medial geniculate body 
Auditory cortex

Question 18

Sound: Rapid variation of air pressure

Answer 18

• Longitudinal pressure waves in the atmosphere.

Question 19

Frequency and wavelength

Answer 19

λ = c/f
c = speed of sound (344m/s)
f = frequency
λ = wavelength

Question 20

Sound level

Answer 20

• The difference in amplitudes between the quietest sounds we can hear is massive
• Normal air pressure: 100k Pascals.
• We can hear a .000000001% change in pressure.

Question 21

Sound pressure level

Answer 21

The decibel scale:
a “log” of ratio relative to 20Pa:
 20log 10(amplitude/20)
20Pa 	0dB SPL
200Pa 	20dB SPL
2000 Pa	40dB SPL

Question 22

The pinna

Answer 22

• 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.

Question 23

Filtering by the pinna

Answer 23

The outer ear filters, influencing the frequency response.

Pinna features influence the entering sound differently

Question 24

Grade 1 microtia

Answer 24

A less than complete development of the external ear with identifiable structures and a small but present external ear canal

Question 25

Grade 2 microtia

Answer 25

A partially developed ear (usually the top portion is underdeveloped) with a closed stenotic external ear canal producing a conductive hearing loss.

Question 26

Grade 3 microtia

Answer 26

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

Question 27

Grade 4 microtia

Answer 27

Absence of the total ear or anotia

Question 28

The tympanic membrane

Answer 28

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

Question 29

The ossicles

Answer 29

Smallest bones in the human body. | Connects the tympanic membrane to the oval window of the cochlea.

Question 30

Glue ear (otitus media/OM)

Answer 30

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.

Question 31

The cochlea and basilar membrane

Answer 31

The cochlea: fluid filled spiral canal divided by a flexible membrane. Basilar membrane filters sound according to frequency.

Question 32

The organ of Corti

Answer 32

The organ of Corti sits on top of the basilar membrane, within the scala media. Inner and outer hair cells are mounted on it.

Question 33

The organ of Corti in action

Answer 33

The 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 34

Outer hair cell: ‘rock and roll’ hair cell

Answer 34

Outer hair cells are motile. | Influx of positive ions makes the outer hair cells contract

Question 35

The cochlear amplifier

Answer 35

• Prestin in short conformation state • Outer hair cell contracts • This pulls the basilar membrane toward the tectorial membrane. 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 36

The ‘battery’ driving cochlear hair cell

Answer 36

* 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.