Auditory and Vestibular Flashcards
Hair cell
A cell generally composed of stereocilia, a cell body and a synapse (onto an afferent nerve fibre).
Stereocilia
Are rigid, non-motile, actin filled rods, or “hairs”.
Afferent nerve
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.
Endolymph
A potassium-rich extracellular fluid is 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.
Tip links
Found at the top of the cilia
Connectors
Lateral-link, top connectors, shaft connectors and ankle links.
Stereocilia bundles: TIP links
- 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.
Stereocilia bundles: Lateral link connectors
teral-link connectors between the shafts of stereocilia hold
the bundle together to allow it to move as a unit
Hair cells
- 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.
Inner ear is formed of:
- Semicircular canals (vestibular system)
* Cochlea (auditory system)
Semicircular canals: sensing rotation
- 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
Hair cells for sensing rotation
- 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.
Orientation and motation in mammals
- In the otolith organs they are sensitive to linear acceleration.
- Gravity is also acceleration.
How the otolith organs
- Hair cells are topped by a rigid layer of otoconia crystals.
- Under acceleration the crystal layer is displaced, deflecting the cilia.
Hair cells as water motion detectors: The lateral line system
- 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.
Auditory system
Cochlear nucleus Olivary complex Lateral lemniscus Inferior colliculus Medial geniculate body Auditory cortex
Sound: Rapid variation of air pressure
• Longitudinal pressure waves in the atmosphere.
Frequency and wavelength
λ = c/f
c = speed of sound (344m/s)
f = frequency
λ = wavelength
Sound level
- 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.
Sound pressure level
The decibel scale: a “log” of ratio relative to 20Pa: 20log 10(amplitude/20) 20Pa 0dB SPL 200Pa 20dB SPL 2000 Pa 40dB SPL
The pinna
- 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.
Filtering by the pinna
The outer ear filters, influencing the frequency response.
Pinna features influence the entering sound differently
Grade 1 microtia
A less than complete development of the external ear with identifiable structures and a small but present external ear canal
Grade 2 microtia
A partially developed ear (usually the top portion is underdeveloped) with a closed stenotic external ear canal producing a conductive hearing loss.
Grade 3 microtia
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 4 microtia
Absence of the total ear or anotia
The tympanic membrane
The ‘ear-drum’ vibrates in response to sound.
Middle ear bones (ossicles) are visible through the membrane.
The ossicles
Smallest bones in the human body.
Connects the tympanic membrane to the oval window of the cochlea.
Glue ear (otitus media/OM)
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.
The cochlea and basilar membrane
The cochlea: fluid filled spiral canal divided by a flexible membrane.
Basilar membrane filters sound according to frequency.
The organ of Corti
The organ of Corti sits on top of the basilar membrane, within the scala media.
Inner and outer hair cells are mounted on it.
The organ of Corti in action
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.
Outer hair cell: ‘rock and roll’ hair cell
Outer hair cells are motile.
Influx of positive ions makes the outer hair cells contract
The cochlear amplifier
• 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.
The ‘battery’ driving cochlear hair cell
- 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.
