W9 - Neuro: Auditory & Vestibular Systems Flashcards
What is the basic architecture of a hair cell?
Overlying extracellular matrix, which has different names in different systems:
-Tectorial membrane (in auditory organs),
-otoconial membrane (in maculae)
-Cupula (in cristae)
Hair bundles on luminal surface sitting on top of hair cells. These hair cells have a synapse attached to it nerve fibre that projects to the brain. There are supporting cells there too on top of basal lamina.
What are Stereocilia bundles?
- 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. The things that bind them together are:
- Lateral-link Connectors: top connectors, shaft connectors and ankle links.
- Tip links: Found at the top of the cilia
What are lateral link connectors (stereocilia bundles)?
Lateral-link connectors between the shafts of stereocilia hold the bundle together to allow it to move as a unit. So when one moves, they all move.
What are tip links (stereocilia bundles)
- 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.
How do Tip-links connected to ion channels?
- ‘Tip-links’ open ion-channels.
- Endolymph high in K+.
- Potassium ion (K+) influx depolarises the cell by making it more positive.
- Voltage gated Ca2+ channels open.
- Ca2+ triggers neurotransmitter release at the synapse.
- Post-synaptic potential in nerve fibre triggers an action potential
This basically means when the hair moves in one direction, tip links pull, the K+ gates open and the accumulated K+ at the top can enter the cells. When it moves in the other direction, these gates close and the K+ are not able to enter.
Displacement of the cilia causes a change in membrane potential.
How are hair cells used as water motion detectors: fish and amphibians
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. Water can flow through these and interact with superfical neuromasts 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 inadulthood.
- More aquatic living species retain them.
How are hair cells in mammals used in the auditory and vestibular system?
Hair cells in mammals: The auditory and vestibular systems.
The inner ear: The auditory and vestibular systems
The inner ear is formed of:
* Semicircular canals (vestibular system)
* Cochlea (auditory system)
Vestibulocochlear or 8th Cranial Nerve (Vestibular and cochlear nerve)
Semicircular canals (Vestibular system)
Cochlea (Auditory system)
What is the vestibular system?
The vestibular system: balance and motion in
mammals.
It does this by translating movements, velocity and rotation into electrical activity to be able to explain.
Linear motion: up/down, left/right, backwards/forwards.
a) Positive x-axis translation: moving forward
b) Positive y-axis translation: moving left
c) Positive z-axis translation: moving up
Rotation: roll, pitch, yaw.
a) Roll: Rotation around x-axis: side ways roll
b) Pitch: Rotation around y axis: Forward roll
c) Yaw: Rotation around z axis :sitting on the spot
How do we sense rotation?
Semicircular canals: sensing rotation?
* Rotation causes fluid motion in the semicircular canals.
* Hair cells at different canals entrances register different directions.
- Posterior semi-circular canal, allows flow of fluid.
- Anterior semicircular canal, allows for determining the rotation of the head around a lateral axis such as when a person nods their head up and down
- Horizontal canal, the rotation of yaw is captured.
- Cilia are connected to the gelantinous 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 hair cells.
What are the otolith organs?
Orientation and motion in mammals
*In the otolith organs they are sensitive to linear acceleration.
* Gravity is also acceleration.
- Ultricular macula: left and right
- Circular macula: backward and forward and up and down.
How the otolith organs work
* Hair cells are topped by a rigid layer of otoconia crystals.
* Under acceleration the crystal layer is displaced, deflecting the cilia.
The crystals would move opposite to whichever direction we travel, which then gets detected.
What is the auditory system?
Sound: rapid variation of air pressure
* Longitudinal pressure waves in the atmosphere.
* Imagine a slinky spring being pushed and pulled along its length.
The ear translated changes in air pressure, which all sounds into motion of fluid within the cochlea. This translates to electrical activity and send it to cochlear nucleus. This goes to the olivary complex -> lateral leminiscus -> inferior colliculus -> medial geniculate body -> auditory cortex
What are the wavelengths and frequency?
There are longitudinal pressure waves in the atmosphere.
* 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
What are the various 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.
What are the sound pressure levels?
- the decibel scale:a “log” of ratio relative to 20uPa:
20log 10(amplitude/20)
20uPa 0dB SPL
200uPa 20dB SPL
2000 uPa 40dB SPL etc.
What is the pinna?
The outer ear.
* 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.
