NEURO: Auditory and Vestibular Systems Flashcards
What are hair cells?
sensory receptors of both the auditory and vestibular systems in the inner ear
Describe the basic architecture of the hair cells.
Stereocilia
-arranged in hair bundles filled with actin and sit on top of hair cells
Auditory Nerve Fibre
-synapses with hair cells and projects to the brain
Overlying Extracellular Matrix
- tectorial membrane in the auditory system
- otoconial membrane in maculae of vestibular system responsible for linear motion
- cupula in cristae of vestibular system responsible for rotational movement
The function of the hair cell
converts motion of stereocilia into the release of neurotransmitter, which is then converted into electrical activity sent to the brain
Describe the stereocilia bundles.
The stereocilia are arranged in ‘bundles’ (eg. 30-300 stereocilia in each bundle in the ear).
Within the bundle stereocilia can be connected via a number of links:
LATERAL-LINK CONNECTORS: hold the bundle together to allow it to move as a unit (e.g. top connectors, shaft connectors and ankle links)
TIP LINKS: distorts the tip of the stereocilia mechanically. This distortion allows channels to open and close with cilia movement. The current flows in proportionately.
What happens when the stereocilia bundle moves?
when the bundle leans in towards the tallest stereocilia, tip links stretch and pull on top of the stereocilia, distorting the tip of the stereocilia; this allows channels to open, permitting the influx of positively charged ions into stereocilia, which is critical to the transduction process.
if the bundle leans towards the shortest stereocilia, tip links are compressed, reducing the amount of distortion at the tip of stereocilia, meaning ion channels close and there is no flow of current into the cell
Stereocilia transduction
1) Hair bundle pushed towards the tallest stereocilia, tip-links stretch tip of stereocilia and open ion channels
2) Outside of the stereocilia is a potassium (K+) rich fluid called Endolymph. When the tip-links stretch the tip of the stereocilia and distort it, they allow the influx of K+ influx, depolarising the hair cell
3) Voltage-gated Ca2+ channels open, and Ca2+ triggers neurotransmitter release at the synapse
4) Neurotransmitter binds to afferent neurones triggering an action potential
Hair cells as water motion detectors
Fish and amphibians have a lateral line system along both sides of their body
these are mechanoreceptors which provide information about movement through water or the direction/velocity of water flow along the body
What is the smallest functional unit of the lateral line?
neuromast
-consists of a hair cell and a cupula that connects the stereocilia bundles of the hair cells with the water surrounding the fish
Where are neuromasts located?
Some neuromasts in canals
Superficial neuromasts on surface
Neuromasts function similarly to the mammalian inner ear
A gelatinous cupula encases the hair cell bundle and moves in response to water motion
-movement of the cupula in different directions changes the firing rate of afferent neurone
Where are the auditory and vestibular systems located?
inner ear
What is the inner ear formed of?
- semicircular canals (vestibular system)
- cochlea (auditory system)
- afferent nerves (vestibulocochlear nerve or CN VIII)
Vestibulocochlear nerve (CN VIII)
Afferent nerve formed from Vestibular Nerve and Cochlear Nerve and transmits sound and balance information from inner ear to the brain
Vestibular system
three semicircular canals that provide the sense of balance and motion, located in the inner ear
has different receptors to sense linear and rotational motion
What are the different types of motion?
There are two types of motion, under which fall three kinds: LINEAR MOTION: - x-axis translation: front/back - y-axis translation: left/right - z-axis translation: up/down
ROTATION:
- roll: rotation around the x-axis
- pitch: rotation around the y-axis
- yaw: rotation around the z-axis
How does the ear sense rotation?
Hair cells in ampulla at the semicircular entrance with the
overlying extracellular matrix as the cupula:
-cupula detects endolymph flow through the semicircular canal
-as we rotate in one direction, endolymph moves in the opposite direction, pushing the cupula in the opposite direction to the rotation of the head and opening ion channels on hair cells, making afferent nerve fire
Role of each semicircular canal
The inner ear uses the semicircular canals to sense rotation.
Hair cells at different canal entrances register different directions:
>Posterior semicircular canal: responsible for roll
>Anterior semicircular canal: responsible for pitch
>Horizontal semicircular canal: responsible for yaw
Otolith organs
the mechanical structures in the vestibular system with hair cells that sense both linear acceleration/motion and gravity
> Utricular Macula: hair cells are arranged in a curve on a lateral plane so that as we move sideways (left/right) they pick up changes in direction
> Saccular Macula: hair cells are arranged in “up/down” and “forward/backwards” directions and can therefore detect both up/down motion and forward/backwards motion
How does the ear sense linear motion?
The otolith organs in the ear are sensitive to linear acceleration [gravity is also acceleration].
Hair cells in the utricular macula and saccular macula have an otolithic membrane as an overlying extracellular matrix which:
- detects motion of crystals (otoconia) which sit on top of this membrane
- under acceleration/movement, crystal layer is displaced, deflecting the stereocilia and opening ion channels and firing action potentials in afferent nerve
Describe the different grades of microtia (incomplete development of the outer ear).
GRADE I: a less than complete development of the external ear with identifiable structure 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 eardrum. Grade III microtia is the most common.
GRADE IV: absence of the total ear or anotia
Tympanic membrane role
membrane at the end of the ear canal which receives sound vibrations from the outer ear and transmits them through the auditory ossicles which connect the membrane to the inner ear
Describe the ossicles.
They are found in the middle ear.
They are the smallest bones in the human body. They connect the tympanic membrane to the oval window in the cochlea.
Three bones make them up:
· Malleus- connects to the tympanic membrane
· Incus- levers the malleus as it pushes at the top of the incus, turning small motion into bigger motion by acting as an amplifier
· Stapes- connected to the incus
The point, where the tympanic membrane connects to the auditory ossicles, is called the…
The chamber that separates the eardrum from the cochlea is usually filled with…
Manubrium of malleus
air
What happens after vibration is transmitted to auditory ossicles?
Incus amplifies vibration,
stapes pushes on the oval window of the cochlea, which in turn causes compression of fluid within the cochlea.
This compresses affects the basilar membrane, which is critical to sound transduction. It is more rigid at one end (closer to the stapes) than the other. At the rigid end, it responds to higher frequencies, and at the floppy end, it responds to lower frequencies.
What is glue ear - Otitis Media (OM)?
- middle ear chamber fills with fluid which impedes motion of ossicles and changes eardrum function
- eardrum pushes against the fluid, causing for loss of amplification
- reduces middle ear gain, raises hearing thresholds
- common in small children (<5 yrs)
Cochlea chambers (inner ear)
3 chambers:
· Scala vestibuli
· Scala media
· Scala tympani
Inner and outer hair cells of the organ of Corti
The inner hair cells do the actual hearing and send signals to the brain.
The outer hair cells act as amplifiers *(not involved in any hearing)
