Neuro: Auditory and Vestibular Systems

Question 1

What are some example uses of hair cells?

Answer 1

• As water motion detectors
• As gravity and balance detectors
• As sound detectors

Question 2

Describe the basic architecture of the hair cells.

Answer 2

• Individual hairs are rigid stereocilia and they make up hair bundles
• Hair bundles are attached to a hair cell which sits beneath the luminal surface.
• At the end of the hair cell there is a synapse.
• Synapse attaches to nerve fibres that project to the brain.
• Sitting above/around the hair cell bundle to protect it is a gelatinous extracellular matrix.
• In the auditory system the extracellular matrix is called the tectorial membrane. In the maculae it is the otoconial membrane. In the cristae it is the Cupula. All have slight variations but overall they do very similar things.

Question 3

Describe the stereocilia bundles.

Answer 3

• The stereocilia are arranged in ‘bundles’ (eg. 30-300 stereocilia in each bundle in the ear).

Within the bundle stereocilia can be connected via a number of links:

LATERAL-LINK CONNECTORS: top connectors, shaft connectors and ankle links

TIP LINKS: found at the top of the cilia

Question 4

What is the function of the lateral link connectors?

Answer 4

Holds stereocilia together so bundles operate in unison.

Question 5

What is the function of the tip links?

Answer 5

When there is stretch, there is distortion of the top of stereocilia opening channels, allowing channels to open and close with cilia movement. Current flows in proportionately.

Question 6

What happens when there is tension in the tip links?

Answer 6

• Pull open K+ channels
• Endolymph (fluid sitting outside of the cell) high in K+
• Potassium ion (K+) influx depolarises the cell (cell becomes more positive)
• Voltage gated Ca2+ channels open (usually activate at around -30mV)
• Ca2+ enters the cell and triggers neurotransmitter release into the synaptic cleft from the vesicles by fusing at the synapse.
• The neurotransmitter spreads into the synaptic cleft and attaches to receptors on the postsynaptic cell.
• Post-synaptic potential in nerve fibre triggers an action potential (firing increased).

When the tip-links move in the other direction, the potassium ion channels close, reducing the flow of K+ ions into the cell (hyperpolarisation).

Question 7

What is the lateral line system?

Answer 7

• Most fish and amphibians have a lateral line system along both sides of their body. The lateral line system is simply a series of mechanoreceptors.
• The mechanoreceptors provide information about movement through water or the direction and velocity of water flow. This is important for schooling, for example.

Question 8

How do hair cells function as water motion detectors in fish and amphibians?

Answer 8

• Superficial mechanoreceptors, or neuromasts, are found on the surface; some neuromasts are found in the lateral line canals.
• The neuromasts function similarly to the mammalian inner ear. A gelatinous cupula surrounds sensory hair bundles. There is also a hair cell beneath this and a nerve fibre. The cupula encases the hair cell which stops water coming into direct contact with it. When the cupula moves in one direction, the stereocilia lean in the same direction causing an increase in firing rate. So in this way there is transduction of water motion

Most amphibians are born (i.e. tadpoles) with lateral lines. Some (e.g. salamander) lose them in adulthood. The more aquatic-living species retain them.

Question 9

What is the inner ear formed of?

Answer 9

• semi-circular canals (vestibular system)
• cochlea (auditory system)
• These two merge together to form the vestibulocochlear nerve (CN VIII)

Question 10

What are the different types of motion?

Answer 10

There are two types of motion

LINEAR MOTION:

• x-axis translation: front/back
• y-axis translation: left/right
• z-axis translation: up/down

ROTATION:

• roll: rotation around the x-axis
• pitch: rotation around the y-axis
• yaw: rotation around the z-axis

Question 11

How does the ear sense rotation?

Answer 11

The inner ear uses the semicircular canals to sense rotation.

• Roll is sensed by the posterior semicircular canal
• Pitch is sensed by the anterior semicircular canal
• Yaw is sensed by the horizontal semicircular canal

The cilia are connected to the gelatinous cupula. Under motion, the fluid in the canals lags due to inertia, pulling the cupula in the opposite direction to the rotation of the head. The cilia are then displaces, depolarising the hair cells.

Question 12

How does the ear sense linear motion?

Answer 12

For capturing linear motion e.g. gravity and acceleration there are the otolith organs.

• Left and right is sensed by the Utricular macula
• Forward and back is sensed by Saccular macula (angled)
• Up and Down is also sensed by Saccular macula (angled)

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

Question 13

How does information get from the ear to the auditory cortex?

Answer 13

• There is the inner ear which translates the changes in air pressure (sound) into motion of fluid within the cochlear
• This then translates this into electrical activity which sends it to the cochlear nucleus
• This then goes to the oligarchs complex
• Lateral lemniscus
• Inferior Colliculus
• Medial Geniculate Body (thalamus)
• And finally to the Auditory Cortex

Question 14

What is sound?

Answer 14

• Sound is the rapid variation of air pressure.

- Longitudinal pressure waves in the atmosphere (imagine a slinky spring being pushed and pulled along its length).

Question 15

Define what wavelength and frequency are, and describe the relationship between the two

Answer 15

The rate at which the compression and rarefaction of a wave to occur determine the distance between the two peaks in a wave (known as the wavelength).

The rate at which the pressure cycles between the compression and rarefaction is called the frequency.

Frequency and wavelength are inversely related - shorter wavelengths at higher frequencies:

λ = c/f

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

Question 16

Describe a human’s response to sound pressure level.

Answer 16

The difference in amplitudes between the quietest sounds we can hear is massive.

Normal air pressure is 100k pascals. We can hear 0.000000001% changes in pressure.

Question 17

How do we convert our scale of hearing to simple levels?

Answer 17

To cope with the very large range of hearing we use a logarithmic scale.

20 pico amps is equal to 0 decibels

20uPa = 0dB SPL
200uPa = 20dB SPL
2000uPa = 40dB SPL

Question 18

What is the pinna?

Answer 18

• Makes up the outer ear.
• Size and shape varies from person to person.
• Gathers sound from the environment and funnels it to the eardrum
• Made entirely out of cartilage and covered with skin.

Question 19

Describe the filtering by the pinna.

Answer 19

Parts of the outer ear amplify sound as it passes into the inner ear. The outer ear filters, influencing the frequency response.

The different pinna features influence the entering sound differently.

• Flange amplifies high frequency (3000-5000Hz)
• Meatus amplifies low frequencies
• Concha amplifies very high frequencies (above 5000Hz)

Question 20

What is microtia and describe the different grades.

Answer 20

• Incomplete development of the outer ear

Grade I - a less than complete development of the external ear (pinna) 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 - prevents external air reaching the air drum.

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. Most common form of
microtia

Grade IV - absence of the total ear (anotia).

Question 21

Describe the tympanic membrane.

Answer 21

• Found in the middle ear.
• The ‘ear-drum’ vibrates in responses to sound.
• The middle ear bones (ossicles) are visible through the membrane.

Question 22

Describe the ossicles.

Answer 22

• Smallest bones in the human body
• Connects the tympanic membrane to the oval window of the cochlea. Bones positioned to create amplification.

Three ossicles:
- Malleus (which is attached to the tympanic
membrane)
- Incus
- Stapes (srirrup shaped)

When there is motion on the membrane, this causes motion on the malleus, this causes the incus to lever which pushes the stapes inwards and outwards. This provides an amplified magnitude. This then contacts the oval window (the route through which sound enters the inner ear).

• Cavity containing the ossicles is usually filled with air.

Question 23

What is glue ear (otitus media)?

Answer 23

• Middle ear fills with fluid which impedes motion of the ossicles (causes resistance)
• Reduces middle ear gain, raises hearing thresholds (ear drum isn’t moving much)
• Very common in small children (<5 yrs) - can lead to language development problems.

Question 24

Describe the workings of the inner ear.

Answer 24

The ossicles translate motion of the ear drum into motion at the oval window. This causes compression of fluid within the cochlea.

This compression affects the basilar membrane, which is critical to sound transduction. It is more rigid at one end (closer to the stapes) than the other. At the rigid end, it responds to higher frequencies, and at the floppy end, it responds to lower frequencies.

Question 25

Describe the wave travelling through the inner ear.

Answer 25

The wave will rise gradually, peak, then decay rapidly.

The peak location of a wave depends on the stimulus frequency (if higher, closer to stapes, if lower, further away).

Question 26

Describe the organ of corti.

Answer 26

It sits on tops of the basilar membrane within the scala media. There are inner and outer hair cells mounted on it. The motion of the organ of corti on the basilar membrane causes displacement of the stereocilia, which opens up the ion channels, causing action potential firing.

Sitting on top of the hair’s extracellular matrix is the tectorial membrane, which moves up and down like a lever. The outer cells contact the tectorial membrane, while the inner hair cells do not.

Question 27

What is the difference between inner and outer hair cells?

Answer 27

The inner hair cells do the actual hearing and send signals to the brain.

The outer hair cells act as amplifiers, but don’t do any hearing.

Question 28

Describe how the outer hair cells interact with the tectorial membrane to contract the inner hair cells.

Answer 28

• There is a protein around the membrane of the outer hair cells called prestin, which allows it to be motile. It contracts with an increase in voltage, and expands with a decrease in voltage.
• As it expands, it pushes on the tectorial membrane, which makes the hairs lean. This opens the channels, allowing ion influx and increasing the voltage of the cell.
• This causes the cell to contract, pulling the basilar membrane up towards to tectorial membrane; now, the inner hair cell is in contact with the tectorial membrane, and it’s hairs are leaning. This makes that hair contract as well.

Question 29

Why is the amplification of sound so important?

Answer 29

The outer hair cells amplify sound by as much as 50 dB. Thus, quiet sounds are amplified largely, and the loud sounds are not amplified. This helps us deal with 120 dB of dynamic range.

Tuning is sharper than the passive vibration of the basilar membrane.

Question 30

What is the importance of the endolymph in sound amplification?

Answer 30

• The hair cell is maintained at -60mV whereas the scala media is kept at an usually high 80mV
• The high potassium concentration of the endolymph in the scala media create a 2x amplification
• If it were not potassium-rich, then the inner hair cell output (of the cochlear nerve) would be halved, making sound perpetually quieter.
• Cochlea amplification would be much smaller, making sounds much quieter.