Vestibular function Flashcards
What is the vestibular system?
a sensory system essential in the control of posture and balance
Where is the vestibular system found?
In the inner ear, it is a series of fluid-filled membranous tubes, (labryinths), which are embedded in the temporal bone
Function of semi-circular canals?
Allow us to orientate ourselves in time and space
What does the vestibular apparatus consit of?
3 semi-circular canals (all at right angles to each other - 3D)
utricle
saccule
the 3 canals connect to the utricle and the utricle connects to the saccule
What are the swellings called at the bases of each semi-circular canal? What do they contain?
They’re called ampulla and contain sensory hair cells.
What do the utricle and saccule contain
they also contain hair cells
What is the collective name given to the utricle and saccule
otolith organs
What does the utricle sense?
back/front tilt and horizontal acceleration (car)
What does the saccule sense?
vertical acceleration (lift)
What do the semi-circular canals sense?
rotational acceleration (roundabout/spinning)
What do hair cells act as
sensory receptors
What are the hair cells embedded in?
A flexible gelatinous structure called the cupula that stretches across the entire width of the ampulla
What distorts the cupula?
movement of the endolymph fluid within the canals
What do the cilia of the hair cells in the cupula synapse direclty with?
the sensory neurons of the vestibular nerve (CN VIII)
How do the sensory organs detect rotational acceleration?
If the skull is rotated left or right from rest, the endolymph at first does not move (inertia) The semi-circular canals move when the skull moves.
The endolymph takes time to catch up. However the ampulla moves instantly because it is embedded in the skull.
If you rotate at constant velocity, the endolymph catches up and rotates at the same speed, removing the shearing forces, but this takes several seconds.
At what point do you become less aware that you are moving?
when the endolymph and semi-circular canals are moving at the same speed
In what direction does the endolymph bend the cupula?
in the opposite direction to movement
what are the smaller hair cells called?
stereocilia
what is the single, longer hair cell called?
kinocilium
What happens if you stop rotating suddenly?
Sudden stop will cause endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness.
when do you get an increased discharge of APs in the vestibular nerve?
When the cilia are distorted in the direction of the kinocilium (towards the longest hair)
when do you get a decreased discharge of APs in the vestibular nerve?
when the cilia are distorted away from the kinocilium - this leads to hyperpolarization and therefore a decrease in APs
where does integration of the sensory information relating to movement in time and space take place?
cerebellum
- vestibular nerve input
- change in tension of the muscles of posture to maintain balance
what is the collective name given to the sensory apparatus of the utricle and saccule (Otolith organs)?
maculae
how are the macula in the utricle orientated?
on the horizontal plane
how are the macula in the saccule orientated?
on the vertical plane
true or false: The maculae, like the ampulla, have a set of cilia (one kinocilium and a series of stereocilium) ?
True
What are embedded into the gelatinous otolith membrane?
CaCO3 crystals called otoliths
Tilting your head backwards moves the otolith crystals in which direction?
towards the kinocilium which causes depolarisation and increased discharge of APs. Opposite for forwards tilt.
What detects tilting of your head?
the macula in the utricle
what is affected more by gravity: otoliths or endolymph? and why?
otoliths as they have a density greater than endolymph
Moving of the head has what affect on otoliths?
it moves the otoliths and the otolith membrane in which they are embedded. This distorts the jelly, and moves the cilia.
vertically orientated macula in the saccule respond to what kinds of forces?
vertical forces e.g elevation in a lift
List all the inputs to the brain that keep it informed of the body’s position in space and any direction of movement
the three ampulla of the semi-circular canals and the two maculae of the otolith organs
via the vestibular nerve
where do most sensory afferents of the vestibular nerves end?
in the medulla
where do projections from the vestibular nuclei on one side project to?
ipsilaterally (same side)
bilaterally
contra-laterally to descending motor pathways
(and also the extraocular nuclei (extraocular muscles) - control eye movement).
where do the vestibular nuclei receive input from?
proprioceptors signalling limb and body position, also from neck and eye muscles
define the term for perception of movement and body position
kinaesthesia
Name 3 vestibular system reflexes
tonic labyrinthine reflexes
dynamic righting reflexes
vestibulo-ocular reflexes
what is the Tonic labyrinthine reflex
Keeps the axis of the head in a constant relationship with the rest of the body. Uses info from maculae and neck proprioceptors to keep head up etc.
what is the dynamic righting reflex
rapid postural adjustments that are made to stop you falling when you trip.
Long reflexes, involving extension of all limbs. Most profound in cats - ie they always land on their feet.
What is the vestibulo-ocular reflex
strong association is evident between the vestibular apparatus, the visual apparatus and postural control.
Within the medullary vestibular nuclei, afferents from the vestibular system synapse with afferent fibres travelling to the extraocular nuclei and thus influence eye movement
Static reflex of the vestibulo-ocular reflex
When you tilt your head, the eyes intort/extort to compensate, so that over a certain range, the image stays the right way up.
Dynamic vestibular nystagmus
Think about being on a roundabout - your head passively moves round – your eyes tend to fix on something until you can no longer see it. Your eyes then flick round to look straight ahead again.
By convention, direction of nystagmus is the direction of the rapid flick back R rotation = R flickback = R nystagmus
What is used to test vestibular function?
nystagmus
Describe the Post-rotatory nystagmus test
Subjects are rotated in a Barany chair. If rotate to the left then during acceleration get a left nystagmus.
At the end of rotation, for about 20 seconds, during deceleration get a right nystagmus.
Due to endolymph catching up and now pushing the cupula in the opposite direction.
only done in research labs
Describe the Caloric stimulation test for horizontal semi-circular canals
When the outer ear is washed with either cold or warm fluid, the temperature difference from core 37 degrees, gets through the thin bone and sets up convection currents which affect the endolymph.
Warm fluid (44 degrees) causes nystagmus towards affected side, cold (30 degrees) causes nystagmus away from affected side
Warm into R ear = R Nystagmus
Cold into R ear = L nystagmus
What does COWS stand for?
Cold Opposite, Warm Same (caloric stimulation test)
Why don’t you get nystagmus when you swim in cold water for example?
water would go into both ears
what does powerful, maintained stimulation of the vestibular system give rise to?
kinetosis (motion sickness)
most likely to occur if visual and vestibular system inputs to the cerebellum are in conflict e.g. if the vestibular system indicates rotation but the visual system does not
what symptoms do you get when the cerebellum generates a ‘sickness signal’
nausea, vomiting, BP and dizziness, sweating and pallor
What is Labyrinthitis?
Acute interference with normal vestibular function as a result of infection = all ANS symptoms + vertigo
What is vertigo?
is the perception (hallucination) of movement in the absence of movement.
There may also be nystagmus. Gross impairment of posture and balance, very disabling.
What is Ménière’s disease?
Vertigo, nausea, nystagmus and tinnitus = subjective noise.
Associated with over production of endolymph causing increased pressure.
The cause is unknown.
what leads to nystagmus at rest?
lesions of the brain stem
which drug can attack the inner ear?
streptomycin
