Flashcards in 32-4 Vestibular system Deck (75)
What are the two main functions of the vestibular system of the inner ear?
It functions as a motor sensor to tell you:
where you are
whether you are moving or still
ensure retinal images is stable
How are angular head movements detected?
By receptors in the semicircular canals
How are linear head movements detected?
By receptors in the otoliths
Why must semicircular canals and otoliths work together?
Because most head movements generate both angular and linear accelerations
What systems are involved in balance?
Balance is achieved by integrating input from:
Vestibular input (inner-ear receptors)
Somatosensory input (receptors in muscles, joints, skin surface)
What are the three outputs of the balance system?
1. Eye movements to compensate for head movements and keep the image on retina stable
2. Sensation - you perceive movement, or head orientation, or stable equilibrium
3. Posture - your posture changes to ensure stability
What two areas of the brain affect transmission of vestibular systems?
The cerebellum - can silence transmission
The reticular formation - alertness has dramatic effects
How many semicircular canals are there in each labyrinth?
3 semicircular canals in each labyrinth
How many otoliths are there in each labyrinth?
2 otoliths in each labyrinth
What happens if you don't have a stable image on the retina?
You must have a stable image on the retina for clear vision. If the image moves across the retina:
1. your visual acuity is degraded – you can't detect details
2. it seems like the visual scene is bouncing ("oscillopsia")
3. it is unpleasant!
What are angular head movements?
Rotations around an axis (turning your head or nodding)
What are linear head movements?
In a straight line (like a car accelerating from a light)
What's the difference between active and passive movements?
Passive movements are when you slip or go over a bump in a car - trigger sensations and compensatory postural reflexes
Active movements are intentional, self-generated movements - you don't want compensatory reflexes
What is the difference in vestibular sensor activation in active and passive movements?
There is no difference in vestibular sensor activation - the brain must cancel something out
What is Meniere's disease?
Repeated episodes of violent vertigo and vomiting caused by disturbances usually in ONE ear.
Why does disactivating one labyrinth stop whole vestibular system from working?
Because labyrinths talk to each other - they are a paired system. They inhibit each other to keep each other under control. There is neural interaction at the brainstem between the inputs from each ear.
How do vestibular neurons fire when head is still?
At a high rate - 100 spikes/s in many neurons - but IMPORTANTLY, with equal neural input from both ears
Given that the two ears are paired, what happens during horizontal head rotation to the left?
That angular rotation EXCITES vestibular receptors in semicircular canals on the left side and simultaneously INHIBITS, or silences, receptors on the right side.
If one vestibular system is absent, why does this cause vertigo?
Because on side is active (even when head is stationary) and the other is silenced - this is identical state to when head is turning – activation on one side, inhibition of other. So it feels like head is turning + nystagmus + postural changes
What are the three symptoms of unilateral vestibular loss?
- The patient feels like they
are turning – a sensation called VERTIGO
- They have inappropriate compensatory postural responses
- The compensatory eye movement responses (NYSTAGMUS)
What is sensory conflict?
When sensory systems signal different things
What are some examples of visual-vestibular conflict?
1. At IMAX cinema, vestibular system says you're still; vision says you're moving. Vision – optokinetic information – wins. VECTION. Or inside an optokinetic drum.
2. In the case of unilateral vestibular dysfunction, it feels like you're moving, but vision says you're stable. Vestibular system wins. But people with UVD can learn to use visual cues to compensate - vestibular compensation.
What is the constant force of gravity on earth detected by the otoliths?
Why is going into space like sensory deprivation for the vestibular system?
Because it no longer feels the constant 1g pressure.
What happens when astronauts who have adapted to space gravity return to earth?
They have to be lifted out of capsule - balance completely thrown off after sensory deprivation
How long does a space flight have to be to change response of otoliths?
As little as two weeks
What happens when you turn to look at something in the horizontal plane?
Eye moves to object first, then head rotates as eyes back-track to compensate for head movements. If vestibular systems removed (Bizzi monkey experiments), eyes carried off target as head moves.
What is the response called whereby eyes move to compensate for head movement?
Vestibulo-ocular response (VOR)
How does VOR work mechanically?
Vestibular sensory information activates neurons in fast neural pathways to the eye muscles, which rotate the eyes in their socket to exactly compensate for head rotation.