Auditory and vestibular systems Flashcards
What are hair cells?
sensory receptors of both the auditory and vestibular systems in the inner ear
Describe the Structure of a hair cell.
Stereocilia
-arranged in hair bundles filled with actin and sit on top of hair cells
Auditory Nerve Fibre
-synapses with hair cell and projects to brain
Overlying Extracellular Matrix
- tectorial membrane in auditory system
- otoconial membrane in maculae of vestibular system responsible for linear motion
- cupula in cristae of vestibular system responsible for rotational movement
What is the Function of the hair cell?
converts motion of sterocilia into release of neurotransmitter, which is then converted into electrical activity sent to the brain
How many stereocilia are there in each bundle?
30-300 stereocilia in each bundle in the ear
How are stereocilia connected to each other?
· Lateral-link connectors: top connectors, shaft connectors and ankle links
· Tip links: found at the top of the cilia and are critical to the transduction process
What happens when stereocilia bundle moves?
when bundle leans in towards 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 bundle leans towards shortest stereocilia, tip links are compressed, reducing amount of distortion at tip of stereocilia, meaning ion channels close and there is no flow of current into the cell
Describe the process of 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
Describe the function of 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 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 system located?
inner ear
What is the inner ear composed of?
Semicircular canals
-form the vestibular system
Cochlea
-forms the auditory system
What is the 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
What is the 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
How is rotation sensed?
Hair cells in ampulla at semicircular entrance with overlying extracellular matrix as the cupula:
- cupula detects endolymph flow through semicircular canal
- as we rotate in one direction, endolymph moves in opposite direction, pushing the cupula in the opposite direction to the rotation of the head and opens ion channels on hair cell, making afferent nerve fire
What is the Role of each semicircular canal?
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
What are 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/backward” directions and can therefore detect both up/down motion and forward/backwards motion
Describe the process of Sensing Linear Motion.
Hair cells in utricular macula and saccular macula have an otolithic membrane as an overlying exrtracellular 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
What is the Auditory System?
parts of the ear needed for hearing and the pathway responsible for transferring sound information to the brain
goes from ear to the cochlea, and then to a number of different brain regions
What is sound?
variation in pressure of air molecules that travels as longitudinal pressure waves in the atmosphere
Define Wave compression.
Wave compression
Define Wave rarefaction.
rate at which compression and rarefaction of a wave occur
What is the Relationship between frequency and wavelength?
inversely proportional y=c/f c=speed of sound f=frequency y=wavelength
The more areas of compression there are…
the shorter the wavelength and the higher the frequency
What is sound converted into?
neuronal activity in the brain
What is the function of The outer ear (pinna)?
collects sound from air and funnels it to the eardrum
different pinna features influence the frequency response
What pinna features amplify high frequencies?
flange and concha
What pinna features amplify low frequencies?
meatus
What is Microtia?
abnormally small pinna with 4 different grades, anywhere from a slight deformation of the pinna which will lead to differences in the amount of amplification that the individual gets, to a complete absence of the pinna and an absence of the eardrum and ear canal.
*grade III microtia more common
What is the function of the The middle ear?
transmits and further amplifies sounds from the outer ear (pinna) to the inner ear
What is the eardrum called?
tympanic membrane
What is the function of the Tympanic membrane (eardrum)?
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
What are the Auditory ossicles?
· 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
Point where the tympanic membrane connects to the auditory ossicles is called the…
Manubrium of malleus
What happens after vibration is transmitted to auditory ossicles?
Incus amplifies vibration,
stapes pushes on oval window of cochlea, which in turn causes motion of fluid within the cochlea
Chamber that separates the eardrum from the cochlea is usually filled with…
air
What is Otitus Media (OM) - Glue Ear?
- 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)
What are the Cochlea chambers (inner ear)?
3 chambers:
· Scala vestibuli
· Scala media
· Scala tympani
What is the function of the Basilar membrane?
separates scala media and scala tympani
Describe the separates scala media and scala tympani.
narrow and stiff at the end closest to the oval window
wide and compliant and the opposite end (apex)
What is the Role of basilar membrane?
filters sound according to frequency
- resonates at high frequency close to oval window
- resonates at low frequency at the apex
What sits on top of the basilar membrane?
organ of corti
-sits within scala media
What is the Organ of corti?
sensory organ containing inner and outer hair cells necessary for hearing
What is the extracellular matrix of hair cells in the auditory system?
tectorial membrane
-moves up and down
What happens when basilar membrane moves/vibrates?
there is motion of organ of corti, pushing hair cells towards the tectorial membrane, displacing the stereocilia, opening ion channels and causing firing in afferent nerve
Describe the function of the Inner and outer hair cells of organ of corti.
- Inner hair cells are involved in transduction and send a signal to the brain
- Outer hair cells are involved in amplification (not involved in any hearing)
What is the Role of inner hair cells?
transduction and send a signal to the brain
Role of outer hair cells
amplification
*(not involved in any hearing)
Describe the Outer hair cell mobility.
- Prestin within outer hair cell allows it to be mobile
- influx of positive ions makes outer hair cells contract
- decreases in voltage makes the outer hair cell expand
What effect does the Cochlear amplifier have?
As the basilar membrane moves upwards, it pushes against the tectorial membrane, which makes the outer hair cell bundle lean. As it leans, it opens up the channels on top of the hair cells, allowing an influx of positive ions, increasing voltage inside the cell and the outer hair cell contracts.
As it contracts, it pulls the basilar membrane up even further, causing more pressure on the tectorial membrane because the two membranes are now closer together. This causes the inner hair cell bundle to lean, opening ion channels and resulting in an influx of positive ions into the inner hair cell, meaning the auditory nerve therefore produces more action potentials. In this way, we amplify whatever signal is coming in.
Where does the endolymph sit in the auditory system?
in the scala media of the cochlea
What does endolymph do?
creates a positive voltage within scala media (+80mV), and the difference (140mV) between this and the voltage inside the hair cell (-60mV) is so great that the changes in the voltage of the hair cell are much more rapid than in normal neurones, causing more amplification
if the endolymph were not so K+ rich, then inner hair cell output (of cochlea nerve) would be halved, making sound perceptually quieter
endolymph also makes outer hair cells more motile, maintaining their function and therefore maintaining amplification
