Auditory and Vestibular systems Flashcards
1
Q
Basic architecture of hair cells
A
Top to bottom: - overlying extracellular matrix: • tectorial membrane (in auditory organs) • otoconial membrane (in maculae) • cupula (in cristae) - hair bundle - lumenal surface - hair cell - synapse - supporting cells - basal lamina - nerve fibres
2
Q
Stereocilia bundles
A
- 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 and ankle links.
• Tip links: Found at the top of the cilia - Lateral-link connectors between the shafts of stereocilia hold
the bundle together to allow it to move as a unit - 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
3
Q
How do hair cells work?
A
- ‘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
- Displacement of the cilia causes a change in membrane potential
4
Q
Hair cells as water motion detectors (fish and amphibians)
A
- 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.
- Superfical neuromasts are on the surface.
- Neuromasts function similarly to 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.
5
Q
The inner ear
A
- The inner ear is formed of:
• Semicircular canals (vestibular system)
• Cochlea (auditory system) - there is also the vestibulocochlear or 8th cranial nerve
6
Q
Orientation and motion in mammals
A
Linear motion: - up/down (positive x-axis translation) - left/right (positive y-axis translation) - backwards/forwards (positive z-axis translation) Rotation: - roll (rotation around x-axis) - pitch (rotation around y-axis) - yaw (rotation around z-axis)
7
Q
Semicircular canals: sensing rotation
A
- Rotation causes fluid motion in the semicircular canals.
- Hair cells at different canals entrances register different directions.
- Parts of the semicircular canals:
• anterior semicircular canal
• cupula of anterior semicircular canal
• osseous canal
• perilymph
• endolymph
• horizontal semicircular canal
• ampullae
• posterior semi-circular canal
• cupulae of horizontal and posterior semicircular canals
8
Q
Hair cells for sensing rotation
A
- 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 haircells.
9
Q
Orientation and motion in mammals
A
- In the otolith organs they are sensitive to linear acceleration.
- Gravity is also acceleration.
10
Q
How do the otolith organs work?
A
- Hair cells are topped by a rigid layer of otoconia crystals.
- Under acceleration the crystal layer is displaced, deflecting the cilia.
11
Q
Parts of the auditory system
A
- auditory cortex
- medial geniculate body
- inferior colliculus
- lateral lemniscus
- olivary complex
- cochlear nucleus (the ear: outer, middle, inner)
12
Q
Sound: rapid variation of air pressure
A
- Longitudinal pressure waves in the atmosphere.
- Imagine a slinky spring being pushed and pulled along its length.
13
Q
Wavelength and frequency
A
- The rate at which the compression and rarefaction of a wave occur determine the distance between two peaks in the wave (wavelength) and the rate at which the pressure cycles between compression and rarefaction (frequency).
- Frequency and wavelength are inversely related.
• λ = c/f
• c = speed of sound (344m/s)
• f = frequency
• λ = wavelength
14
Q
Sound level
A
- The difference in amplitudes between the quietest sounds we can hear is massive
- Normal air pressure: 100k Pascals.
- We can hear a 0.000000001% change in pressure.
- the decibel scale:a “log” of ratio relative to 20 uPa:
‘20log 10(amplitude/20)’
• 20 uPa = 0dB SPL
• 200 uPa = 20dB SPL
• 2000 uPa = 40dB SPL etc.
15
Q
The pinna (outer ear)
A
- 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.
16
Q
Microtia (outer ear)
A
- 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
- Grade IV: Absence of the total ear or anotia.
17
Q
Tympanic membrane (middle ear)
A
- The ‘ear-drum’ vibrates in response to sound.
- Middle ear bones (ossicles) are visible through the membrane.
18
Q
The Ossicles (middle ear)
A
- Smallest bones in the human body.
- Connects the tympanic membrane to the oval window of the cochlea.
19
Q
Glue ear (otitis media (OM)) (middle ear)
A
- 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.
20
Q
Cochlea and basilar membrane (inner ear)
A
- The cochlea: fluid filled spiral canal divided by a flexible membrane.
- Basilar membrane filters sound according to frequency.
21
Q
Travelling wave (inner ear)
A
- Wave rises gradually, peaks, then decays rapidly.
- Peak location depends on stimulus frequency.
22
Q
Organ of corti (inner ear)
A
- The organ of corti sits on top of the basliar membrane, within the scala media.
- Inner and outer hair cells are mounted on it
- 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.
23
Q
Outer hair cell
A
- ‘rock and roll’ hair
- Outer hair cells are motile.
- Influx of positive ions makes the outer hair cells contract
24
Q
Cochlear amplifier
A
- made up of tectorial membrane, inner hair cell, outer hair cell and basilar membrane
- influx of +ve ions on outer hair cell
- Prestin in short conformation state
- Outer hair cell contracts
- This pulls the basilar membrane toward the tectorial membrane.
- larger influx of positive ions on inner hair cell
- 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.
25
Q
‘Battery’ driving cochlear hair cells
A
- 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.
