Auditory and Vestibular Systems Flashcards
Hair cell
A cell generally composed of stereocilia, a cell body and synapse (onto a different nerve fibre).
Stereocilia
Rigid, non-motile, actin filled rods, or “hairs”
Which cranial nerve is the afferent nerve for the auditory and vestibular hair cells?
8th Cranial nerve
Endolymph
A potassium rich extracellular fluid critical to the function of hair cells.
Basilar membrane
The membrane which houses auditory hair cells.
Function of the basilar membrane
Selectively vibrates to different frequencies at different points along its length, this underlies perceptual frequency selectivity
Hair bundle
Filled with actin, stiff rods and do not move around like hair. The stereocillia are bundled together into these hair bundles and sit on top of the hair cell.
Function of the hair cell
Synapses onto an auditory/sensory nerve fibre and projects to the brain. The hair cell converts motion of the stereocilia into release of neurotransmitters which is converted into electrical activity that is sent to the brain.
Basic architecture of a hair cell
Nerve fibres underneath touching the basal lamina.
The basal lamina is next to the supporting cell. The hair cell contains the lumenal surface where the hair bundles sit. On top of the hair bundles is the overlying extracellular matrix
What is the overlying extracellular matrix in auditory organs?
Tectorial membrane
Otoconial membrane
Overlying extracellular membrane in the maculae responsible for linear motion
Cupula
In the cristae in the vestibular system responsible for rotational movement
How many stereocilia are in each hair bundle?
30-300 stereocilia
How are stereocilia connected?
- Lateral-link connecters: top connectors, shaft connectors and ankle links
- Tip links: found at the tope of the cilia
How do Lateral link connectors work?
Used between the shafts of the stereocilia to allow them to move as a unit in the same direction at the same time
How do Tip links work?
Tension in the tip-links distorts the tip of the stereocilia mechanically.
- If the stereocilia moves in one direction, it puts pressure on the tip-links and when they stretch, they pull on the top of the stereocilia. This deforms the stereocilia opening the ion channels.
- If pulls it in the other direction, reducing the amount of deformation and the ion channels close.
Summary of hair cell movement
- “Tip-links” open the ion channels
- Potassium ion from the endolymph cause the depolarisation of the cell.
- The voltage decrease opens other channels.
- Voltage-gated Ca2+ channels open on the cell body. This means the cell releases neurotransmitter.
- Ca2+ triggers neurotransmitter release at the synapse.
- Post-synaptic potential in nerve fibre triggers an action potential
What is the lateral line system?
In most fish and amphibians - they have the lateral line system along both sides of their body. This is a series of mechanoreceptors that provides information about movement through water or the direction and velocity of water flow. These mechanoreceptors (neuromasts) are in canals called lateral line canals. The water flows into the channels and along the canals. It pushes against the cupula. If it goes in the direction of the tallest stereocilia, then it opens ion channels causing depolarisation and the nerve fibres fire. If it goes in the other direction, it closes channels. The neuromasts function similarly to the mammalian inner ear
Where are the auditory and vesitbular system located?
In the inner ear
What does the inner ear system consist of?
Semicircular canals that form the vestibular system
Cochlea (named due to shell shape) that form the auditory system
Afferent nerves (from both systems) - the vestibulocochlear (8th) cranial nerve that forms two separate nerves; the vestibular and cochlear nerve.
What are the 6 different types of motion?
LINEAR:
- Up/down - positive Z-axis translation
- Left/Right - positive Y-axis translation
- Backwards/Forwards - positive X-axis translation
ROTATIONAL:
- Roll - rotation around x-axis
- Pitch - rotation around y-axis
- Yaw - rotation around z-axis
How does the vestibular system sense rotation?
Using semi-circular canals. Rotation causes movement in these canals.
The hair cells register different directions.
Roll: posterior semi-circular canal
Pitch: Anterior semi-circular canal
Yaw: Horizontal semi-circular canal
In which direction does the fluid move during rotation of the head?
Fluid moves in the opposite direction to the motion of the head
How do the semi-circular canals sense rotation?
There is ampulla that is an opening containing sensory receptors. The stereocilia detect the flow of endolymph through the semi-lunar canals. As rotated, the fluid moves in the opposite direction, this pushes the cupula and opens the channels to fire the afferent nerve fibre.
What is the cilia in the hair cells sensing rotation connected to?
Cilia are connected to the gelatinous cupula
What is inertia?
This is when under motion, the fluid in the canals lag pulling the cupula in the opposite direction to the rotation of the head.
What are cilia?
Displaced, depolarising hair cells
Which organs are sensitive to linear acceleration?
Otolith organs
What are the hair cells in the otolith organs called?
Macula that pick up linear motion
What are the two types of macula and their functions?
- Utricular macula: hair cells are arranged in an arch on a lateral plane so pick up changes in direction
- Saccular macula: detect up/down motion as the hair cells are arranged in an up/down directions and forwards and backwards as they are in opposing direction as well.
How do the otolith organs work?
- Detect the motion of crystals rather than the motion of fluid.
- Topped by a layer of otoconia crystals; heavy and dense compared to the gelatinous membrane.
- Under acceleration, the crystal layer is displaced deflecting the cilia.