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.
What are the components of the auditory system?
Cochlear Nucleus Olivary Complex Lateral Lemniscus Inferior Colliculus Medial Geniculate Body Auditory Cortex
What is sound?
A rapid variation of air pressure like a slinky being pushed on one end, pushing air forms areas of compression and rarefaction (spread).
When does sound frequency increase?
The more areas of compression there are over a particular distance, the higher the frequency perceived.
What is a sound wavelength?
The rate at which the compression and rarefaction of a wave occur determine the distance between the two peaks.
What is a sound frequency?
The rate at which the pressure cycles between compression and rarefaction.
Equation for sound
Wavelength = speed of sound (344m/s) x frequency
What is the limit of human hearing?
20 micro pascals - humans are sensitive to changes in air pressure
Normal air pressure
100k Pascals
How many nanometres of movement can the tallest stereocilia of 4 micrometres detect?
0.3 nanometres
What scale is sound converted to?
Non-linear scale called the sound pressure level - a log scale where the amplitude in micro Pascals is divided by the limit of hearing
What is the decibel scale (the sound pressure level)?
A log of ratio relative to 20 micro Pascals
- 20 microPa = 0db SPL (decibels sound pressure level)
- 200 microPa = 20db SPL
The outer ear
- The pinna
- Shape and size varies
- Gathers sound from the environment and funnels it to the eardrum
- Made of entirely cartilage and covered with skin
Which parts of the pinna amplify the sound coming into the ear?
- The flange
- The meatus
- The concha
The flanga
Adds a very high frequency, very small, amplification
The meatus
Amplifies low frequencies
The concha
Amplifies high frequencies
Microtia
A developmental problem with the pinna
4 grades of microtia
- less than complete development of the external ear with identifiable structures and a small but present external ear canal
- Partially developed ear (usually the top portion is underdeveloped) with a closed stenotic external ear canal producing a conductive hearing loss
- Absence of the external ear with a small peanut-like vestige structure and absence of external ear canal and ear drum 4. Absence of the total ear or anotia
Function of the middle ear
Critical to the transduction process and also helps amplification
The tympanic membrane
- “The ear drum” vibrates in response to sound
- Middle ear bones, ossicles, are visible through the membrane
What is the manubrium of malleus?
- The manubrium of malleus in the middle ear drum is the point at which the ear drum connects to a bone.
What are the 3 smallest bones in the body?
The ossicles
- Malleus
- Incus
- Stapes
Malleus
Connects to the ear drum
Incus
Acts as a lever - an amplifier
Stapes
Creates a pushing motion on the oval window of the cochlea
Describe how the ossicles and the ear drum work together
There is vibration or movement at the ear dum. This is transmitted through the malleus, amplified at the point of incus and creates a pushing motion using the stapes on the oval window of the cochlea. This turns motion of the ear drum to motion of fluid within the cochlea.
What is glue ear?
When the chamber that separates the ear drum from the cochlea is filled with very viscous or gluey fluid rather than air.
- The ear drum can no longer move relatively fluid, it is now pushing against the fluid meaning that amplification is lost, leading to a reduction in the ability to hear.
What happens when glue ear occurs in small children?
Can lead to developmental as it is used to a lesser input
Consequence of glue ear
Middle ear fills with fluid which impedes motion of the ossicles. This reduces middle ear gain, raising thresholds.
In which animals is the tympanic membrane external?
Lizards and legs of crickets
Structure of the cochlea
- Oval window opening 3 Chambers: - Scala vestibuli - Membrane - Scala media - Basilar membrane for sound transduction - Scala tympani
Describe which areas of the inner ear respond preferentially to high/low frequencies
- The basilar membrane sits between the scala media and the scala tympani and is more ridge at one end and wide at the other.
- Close to the oval window, the basilar membrane resonates at high frequencies
- Wider and more compliant end, moves more slowly so responds to low frequencies
What is on top of the basilar membrane?
The organ of corti
Structure of the organ of corti
Contains inner hair cells
Outer hair cells
On top of the hair cells is the tectorial membrane that moves up and down like a lever.
Describe the action of the organ of corti
The basilar membrane moves up and down which pushes the outer hair cells towards the tectorial membrane. The tectorial membrane moves and pushes the hair cell sideways, this causes the hair cells to move sideways and open the ion channels that sit on top causing the firing of the afferent nerve.
Function of the inner hair cells
They do not touch the tectorial membrane. Perform the transduction and send a signal to the brain
Function of the outer hair cells
Act as an amplifier and do not do any hearing
How can the outer hair cell be tested for response to voltage?
The outer hair cell contains a substance called prestin which allows the hair cell to be motile. This means when there is an increase in voltage, prestin contracts. It is on top of the membrane, therefore causing the entire cell to contract.
How does the cochlear act as an amplifier?
- The basilar membrane moves upwards and pushes agains the tectorial membrane which causes the outer hair bundle to learn.
- As it leans, it opens the channels on the hair cell, causing an influx of positive ions, this increases the voltage inside the cell. The cell contracts. As it contracts, it pulls the basilar membrane further up.
- More pressure on the tectorial membrane causing an influx of positive ions into the inner hair cell. The auditory nerve will fire more so this means the signal is amplified.
Which sounds are amplified by the cochlear?
The quiet sounds are amplified but loud sounds are not amplified.
Why are sounds amplified?
To help with the tuning of the auditory nerve fibres. Tuning is sharper than the passive vibration of the basilar membrane.
What happens if the outer hair cells are removed?
The reduction in threshold is lost and loss 50db on hearing.
How much does the cochlear amplify?
Up to 50db
Where does the endolymph reside in the auditory system?
Sits in the scala media, elevating the voltage within the scala media to a positive voltage of +80mV.
How does the membrane potential increase in the hair cell?
When the ion channels are opened, the positive K+ ions enter and the membrane potential is elevated. Normally, the hair cell holds the intracellular voltage to a negative value.
What is the difference between the voltage on the inside of the cell and the outside of the cell?
About 140 mV - much larger than normal neurons.
Why does the difference between the voltage of the inside and outside of the cell have an effect?
The changes in voltage of the hair cell is much more rapid than in normal neurons.
How does the endolymph affect the volume of sound?
If the endolymph was less potassium rich, then the inner hair cell ouput would be halved, making the sound perceptually quieter. It would also be less motile so the cochlea would loss 50db.
