NEURO: Auditory and Vestibular Systems

Question 1

Describe the basic architecture of the hair cells.

Answer 1

On the lumenal surface, we have a hair bundle (which is actually filled with actin filaments, so they’re more like stiff rods). These stereocilia are bundled together and sit on top of a hair cell.

The hair cell synapses onto an auditory nerve fibre and projects to the brain. The hair cell itself converts motion of the stereocilia into release of neurotransmitter.

On top of the stereocilia we have an overlying extracellular matrix, which is essentially a gelatinous substance.
It’s called the tectorial membrane in the auditory organs, the otoconial membrane in the maculae, and the cupula in the cristae.

Question 2

Describe the stereocilia bundles.

Answer 2

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 3

What is the function of the lateral link connectors?

Answer 3

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

Question 4

What is the function of the tip links?

Answer 4

Tension in the tip links distorts the tip of the stereocilia mechanically. This distortion allows channels to open and close with cilia movement. The current flows in proportionately.

Question 5

Expand on how an action potential is initiated in a hair cell.

Answer 5

Outside of the stereocilia, we have the endolymph, which is a potassium ion rich fluid.

With movement, the tip links open up the ion channels, allowing K+ in.
This potassium ion influx depolarises the cell, which causes the voltage-gated calcium channels (VGCCs) to open.

The Ca2+ triggers neurotransmitter release at the synapse. The post-synaptic potential in the nerve fibre triggers an action potential.

When the hair bundle is pushed in the direction of the tallest stereocilia, we get depolarisation of the hair cell. If it is pushed in the opposite direction, we get hyperpolarisation.

Question 6

What is the lateral line system?

Answer 6

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 7

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

Answer 7

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 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. The more aquatic-living species retain them.

Question 8

What is the inner ear formed of?

Answer 8

• semicircular canals (vestibular system)
• cochlea (auditory system)
• afferent nerves (vestibulocochlear nerve or CN VIII)

Question 9

What are the different types of motion?

Answer 9

There are two types of motion, under which fall three kinds:
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 10

How does the ear sense rotation?

Answer 10

The inner ear uses the semicircular canals to sense rotation.

The posterior semicircular canal is used to sense roll.
The anterior semicircular canal is used to sense pitch.
The horizontal semicircular canal is used to sense yaw.

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 11

How does the ear sense linear motion?

Answer 11

The otolith organs in the ear are sensitive to linear acceleration [gravity is also acceleration].

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 12

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

Answer 12

It goes from the inner ear to the:

• cochlear nucleus, then to the
• olivary complex, then to the
• lateral lamniscus, then to the
• inferior colliculus, then to the
• medial geniculate body, then to the
• auditory cortex

Different parts are responsible for different functions. For example, the olivary complex is responsible for computing location, and the inferior colliculus to the auditory cortex separate out auditory objects.

Question 13

What is sound?

Answer 13

Sound is the rapid variation of air pressure.

It is longitudinal pressure waves in the atmosphere (imagine a slinky spring being pushed and pulled along its length).

Question 14

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

Answer 14

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:

λ = c/f

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

Question 15

Describe a human’s response to sound pressure level.

Answer 15

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 16

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

Answer 16

The decibel scale is the ‘log’ of ratio relative to 20μPa: 20log10(amplitude/20).

so: 
20μPa           0dB SPL
200μPa        20dB SPL
2000μPa      40db SPL
etc.

Question 17

What is the pinna?

Answer 17

It makes up the outer ear.
The size and shape varies from person to person. It gathers sound from the environment and funnels it to the eardrum. It is made entirely out of cartilage and covered with skin.

Question 18

Describe the filtering by the pinna.

Answer 18

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.
For example:
- the flange adds a very small high frequency
- the meatus amplifies low frequencies
- the concha amplifies high frequencies

Question 19

Describe the different grades of microtia (incomplete development of the outer ear).

Answer 19

GRADE I: a less than complete development of the external ear with identifiable structure 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.

GRADE IV: absence of the total ear or anotia

Question 20

Describe the tympanic membrane.

Answer 20

It is found in the middle ear.

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

Question 21

Describe the ossicles.

Answer 21

They are found in the middle ear.
They are the smallest bones in the human body. They connect the tympanic membrane to the oval window in the cochlea.

Three bones make them up:

• the malleus (connects to the ear drum)
• the incus (acts as a level)
• the stapes

Question 22

What is glue ear?

Answer 22

It is a condition where the middle ear (which is normally filled with air) fills with fluid which impedes the motion of the ossicles.
It reduces middle ear gain, raising the hearing thresholds. You lose amplification, so you get a reduction in your ability to hear.

It is very common in small children (<5 years old) and can lead to developmental problems.

Question 23

Describe the workings of the inner ear.

Answer 23

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

This compresses 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 24

Describe the wave travelling through the inner ear.

Answer 24

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 25

Describe the organ of corti.

Answer 25

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 26

What is the difference between inner and outer hair cells?

Answer 26

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 27

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

Answer 27

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 28

Why is the amplification of sound so important?

Answer 28

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 29

What is the importance of the endolymph in sound amplification?

Answer 29

The high potassium concentration of the endolymph in the scala media create a 2x amplification,, and elevating the voltage to +80 mV.

If it were not potassium-rich, then the inner hair cell out put (of the cochlear nerve) would be halved, making sound perpetually quieter. Also, the cochlea amplification would be much smaller, again, making sounds much quieter.