NEURO: Auditory And Vestibular Systems

Question 1

Hair cells in the ear convert motion into electrical activity of the brain (mechanotransduction)

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 which is then turned to electrical activity and sent to the brain.

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.

Order of structure:

• Overlying extracellular matrix.
• Hair bundle
• Lumenal Surface
• Hair cell itself
• Synpase
• Supporting cell
• Basal lamina
• Nerve fibre

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

Outline the tip link transduction mechanism (action potential initiation).

Answer 5

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

1. Motion of the hair cell causes the ‘tip-links’ open ion-channels.
2. Endolymph high in K+.
3. Potassium ion (K+) influx depolarises the cell.
4. Voltage gated Ca2+ channels open (VGCCs).
5. Ca2+ triggers neurotransmitter release at the synapse
6. Postsynaptic potential in nerve fibre triggers 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 closing of the ion channels leading to hyperpolarisation.

Question 6

What is the lateral line system?

Answer 6

Hair cells can be used as water motion detectors by fish and amphibians.

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 is the vestibular system responsible for?

Answer 9

The vestibular system is responsible for balance and motion in mammals.

Question 10

What are the 6 different types of motion?

Answer 10

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 (head moving from shoulder to shoulder in the lateral plane - like watching a metronome)
• pitch: rotation around the y-axis (forwards and backwards - imagine nodding yes)
• yaw: rotation around the z-axis (side to side - nodding no)

Question 11

How does the ear sense rotation?

Answer 11

The inner ear uses the semicircular canals to sense rotation.

Rotation causes fluid motion (endolymph) in the semicircular canals. Hair cells at different canals entrances register different directions. At the entrances of the semi circular canals are ampulla. There are sensory receptors here (hair cells with stereocilia on top), and extracellular matrix is a cupula (detecting endolymph).

• 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 displaced, depolarising the hair cells. Fluid moves in the opposite direction, pushing the cupula so the channels in the hair cells open making the afferent nerve fire and thus detect rotational movement.

Question 12

How does the ear sense linear motion?

Answer 12

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.

The utricular macula is arranged of hair cells in a curving lateral plane so changes in direction can be picked up in a sideward motion.

The saccular macula can detect up/down motions (hair cells arranged up and down) and forwards and backwards (hair cells arranged that way.)

Question 13

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

Answer 13

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 14

What is sound?

Answer 14

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 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 occur determine the distance between two peaks in a wave (wavelength) and the rate at which the pressure cycles between the compression and rarefaction (frequency).

Frequency and wavelength are inversely related:

λ = c/f

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

Compression = region in a longitudinal wave where the particles are closest together.

Rarefaction = region in a longitudinal wave where the particles are furthest apart.

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. We hear from 20 to 200,000,000µPa.

Question 17

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

Answer 17

Sound pressure level (SPL) is the pressure level of a sound, measured in decibels (dB).

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 18

What is the pinna?

outer ear

Answer 18

It makes up the outer ear.
The size and shape varies from person to person. It gathers sound from the environment and funnels it into the external auditory canal (a short tube that ends at the eardrum) and to the eardrum (tympanic membrane). It is 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 influence the frequency response.

The different pinna features influence the entering sound differently.
For example:
- the flange adds a very small high frequency amplification
- the meatus amplifies low frequencies
- the concha amplifies high frequencies

Question 20

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

Answer 20

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 known as anotia.

Question 21

Describe the tympanic membrane.

middle ear

Answer 21

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 22

Describe the ossicles.

Answer 22

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.

3 bones make up the ossicles:

• the malleus (connects to the ear drum)
• the incus (acts as a lever –> turns a small motion into a larger motion (amplifier))
• the stapes (connects to incus and oval window of cochlea)

Question 23

Describe the transduction mechanism through the middle ear

Answer 23

Vibrations through the eardrum are transmitted through the ossicles and amplified at the point of the incus, leading to a pushing motion onto the oval window of the cochlea.

Question 24

What is glue ear (otitis media)?

Answer 24

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. There is a loss of amplification, and thus a reduction in hearing ability.

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

Question 25

Describe the workings of the inner ear.

Answer 25

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

The stapes pushes on the oval window causing compression of the cochlear fluid causing basilar membrane movement. This compression affects the basilar membrane, which is critical to sound transduction.

The travelling wave of endolymph moves through cochlear fluid causing basilar membrane movement.

Question 26

How does basilar membrane filter sound?

Answer 26

Basilar membrane filters sound
according to frequency.

The basilar membrane is more rigid at the stapes end compared to the other floppy apex end where its wider. At the rigid end, it responds to higher frequencies, and at the floppy end, it responds to lower frequencies.

Question 27

Describe the cochlear and its 3 chambers.

Answer 27

The cochlea: fluid filled spiral canal divided by a flexible membrane.

3 chambers of the cochlea:

• Scala Vestibuli (upper)
• Scala Media
• Scala tympani

The scala vestibuli and scala media are separated by a membrane. The scala media and scala tympani are separated via basilar membrane.

Question 28

Describe the wave travelling through the inner ear.

Answer 28

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 29

Describe the organ of corti.

Answer 29

The organ of corti 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 (leans sideways), 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 30

What is the difference between inner and outer hair cells?

Answer 30

The inner hair cells do the actual hearing and send signals to the brain (transduction). - contain afferent synapse

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

Question 31

Describe how the outer hair cells interact with the tectorial membrane to contract the inner hair cells - leading to cochlear amplification.

Answer 31

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 (the basilar membrane moves upwards), it pushes on the tectorial membrane, which makes the hairs lean. This opens the channels, allowing +ve ion influx and increasing the voltage of the cell.
This causes the cell to contract, pulling the basilar membrane up further towards the tectorial membrane; now, the inner hair cell is in contact with the tectorial membrane, and it’s hairs are leaning - causing the influx of +ve ion influx into the hair cell. This makes that hair contract as well.

This means the auditory nerve fires more action potentials and thus amplification of the signal occurs. - this is cochlear amplification.

Question 32

Why is cochlear amplification of sound so important?

Answer 32

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

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

Question 33

What is the ‘battery’ driving the cochlear hair cells?

Answer 33

The high potassium concentration of the endolymph in the scala media create a 2x amplification,, and elevating the voltage to +80 mV. - this is the ‘battery’ driving the cochlear hair cells.

The hair cell like all neurones holds its intracellular voltage at the negative value at -60mV so when its channels open it can depolarise and fire an action potential.

The difference (140mV) between the different inside (scala media) and outside of the cell (hair cell) allows for rapid changes in voltage than in normal neurons.

Question 34

What is the importance of the endolymph in sound amplification?

Answer 34

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 (outer hair cells) would be much smaller, again, making sounds much quieter.

Question 35

Glossary

Answer 35

Hair cell – A cell generally composed of stereocilia, a cell body and a synapse (onto a afferent nerve fibre).
Stereocilia – Are rigid, non-motile, actin filled rods, or “hairs”.
Afferent nerve – The nerve cell stimulated, via synapse, by hair cells. This is the 8th cranial nerve in the case of auditory and vestibular hair cells.
Endolymph – A potassium rich extracellular fluid critical to the function of hair cells.
Basilar membrane – 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.