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.
What is oscillopsia?
Bouncing vision - world moves as you move - caused by loss of receptor hair cells
What are some ways of measuring eye movement?
1. Silicone contact lens with search coil - this is gold standard, but expensive $700 per lens
2. Hamish Macdougall eye video goggles
What are some ways of measuring postural response?
The Equitest posture platform: Force sensors in the platform pick up the pressure the person applies to keep a stable posture. (Parachute harness used for safety reasons!)
How does postural stability of astronauts change after space flight?
Postural stability at its worst on day 1, then improves
How many receptor hair cells are there in each inner ear?
About 74,000 - 6 times number of cochlear receptors
Is the structure of the vestibular system unique to humans?
No, much the same in all animals - from frogs to fish to humans.
By what stage in fetal development are the receptor structures adult size?
By week 22 of pregnancy
Where are the receptors for angular rotation located?
In each semicircular canal at the ampulla.
Where are the otolithic receptors for linear acceleration?
The otolithic receptors are in the utricle and saccule.
What receptors fire when you shake your head horizontally?
Receptors in the horizontal semicircular canals
What happens when you nod your head vertically?
Receptors in the vertical canals
Where are the 7,500 hair receptors located?
They are embedded in the crista and project into the cupula - a gelatinous mass which seals off the semicircular duct at the crista and extends from the crest of crista to the roof of the ampulla
How are receptor cells activated?
Forces which bend the cilia towards the kinocilium excite the cell
Forces which bend the cilia away from the kinocilia inhibit the cell
So each receptor is "oriented" or "polarized". It prefers forces causing bending towards the kinocilium.
How are the receptor cells on each crista oriented?
All in exactly the same direction, so that every receptor on that crista is is excited by head rotations (angular forces) in one direction and inhibited by angular forces in the opposite direction.
How does the cupula affect movement of the cilia?
An angular rotation of the head causes fluid movement in the semicircular canal so that...
the cupula is displaced and
the receptor hair cells are bent
(If bent toward the kinocilium then all 7500 are excited; if away from the kinocilium then all 7500 are inhibited)
Where are the linear force detectors located?
In the otolithic maculae, the utricular macula and the saccular macula. Maculae are plates of cells
What do hair cells involved in linear force detection project into?
Into the otolithic membrane - a gelatinous mass with otolconia on top
How are receptor cells activated in the otoliths?
Linear movements cause the relatively dense otoconia to move with respect to the otolithic macula. As the otoconia move, so the otolithic membrane shifts, bending the cilia of the receptor cells. In direction of kinocilia - depolarisation; away from kinocilia - hyperpolarisation
Why do we need 7,500 vestibular receptor cells in each ear?
Different receptor cells provide different information. There are different parallel systems telling brain different things about the stimulus
What different information can vestibular receptor cells provide?
Some have transient responses (just at onset).
Some have sustained responses (during the entire stimulus)
What are the two types of otolithic neuron?
Irregular otolithic neurons - respond well to changes in force. Transient response.
Regular otolithic neuron - responds well to maintained force. Sustained response.
Where do vestibular afferent neurons project to in the brain?
To the vestibular nuclei in the brainstem
How is vestibular input distributed laterally in the brain?
Vestibular afferent input projects to the ipsilateral vestibular nuclei – meaning the input from left receptors goes to left vestibular nucleus (VN) and the input from right receptors goes to right VN
Where are the vestibular nuclei (VN) in the brain?
Groups of neurons symmetrically placed on each side of the midline of the brain stem.
Is afferent input from the vestibular system excitatory or inhibitory or both?
It is all excitatory. OMG sooo excited about my head moving OMG!!!
What are the spontaneous firing rates of vestibular neurons?
Many neurons have very high spontaneous rates – some fire at 100 spikes/sec continuously without any stimulation.
How much incoming activity to the ipsilateral vestibular nucleus will damage to the vestibular nerve remove?
Heaps. About 1,000,000 spikes/sec
Why is the horizontal canal system often called a "push-pull" system?
During a horizontal head rotation to the left, for instance, receptors in the left horizontal canal are excited while receptors in the right horizontal canal are inhibited. That bilateral symmetry acts centrally to enhance the response.
Why does UVD cause vertigo, nystagmus postural instability?
Because spontaneous firing that signals stability is not being received from one side. So given that one side is firing, and only a bit, this is interpreted as a long duration rotational stimulus.
How are the two vestibular systems connected?
Each connect ipsilaterally to vestibular nucleus, but the two vestibular nuclei are joined by commissural fibres which are functionally inhibitory. Each vestibular nucleus acts to silence the other.
What is Muller's doctrine?
The brain only gets the information that a cell has fired, not what caused it to fire.
The crossing inhibition of the two sides of the VN has what effect on the signal coming from each side?
Weak side is inhibited more and strong side is strenghtened – this increases the neural imbalance and increases sensitivity (but also means that when there is a dysfunction it's magnified).
How can the VOR be quantified?
One measure of VOR is VOR GAIN - the ratio of how large the eye movement response is in relation to head movement stimulus.
What are typical VOR gain values in full light/total darkness?
Typical values are close to 1.0 in full light, dropping to 0.7 when testing is done in total darkness.
How can VOR be inhibited (when you're reading a book and a bus turns a corner)?
Cerebellar inhibition projects down to the vestibular nuclei cells and silences them – just stops the transmission of information
How does vection cause feeling of movement?
There are long indirect pathways from the visual cortex processing movement (middle superior temporal area) down to the vestibular nuclei. These VN neurons receive direct vestibular input but also respond to moving visual fields.
What did Henn demonstrate about how nerve cells of the monkey vestibular nucleus respond to stimulus?
Neurons in monkey vestibular nuclei can be activated in exactly the same way by either...
1. Rotation of the animal in darkness OR
2. By moving visual input (optokinetic input).
So these cells fire indistinguishably for
1. the animal being rotated in darkness
2. or the animal being stationary and in light.
Why is sine wave testing of vestibular input in eye movement control inadequate?
Because other sensory inputs may be controlling eye movement response - feeling of neck moving, visual cues, prediction of L-R pattern etc.
What way was discovered by Curthoys et al. of testing eye movement response in isolation?
Brief, abrupt, unpredictable head rotation (about 15 degrees) while subject asked to look straight ahead - to see if the eyes compensate in the first 50ms epoch. If the person has to make a saccade at the end of the head rotation then they have an inadequate VOR.
How does a normal subject respond to impulsive head rotation?
Eye velocity matches head velocity almost exactly.
Does dynamic VOR recover after loss of one vestibular system?
No, dynamic symptoms DO NOT show the rapid recovery which occurs for most static symptoms after unilateral loss.
Two reasons why unilateral vestibular disease is so disabling...
1) Patients who have a distorted vestibular input due to disease will receive inaccurate sensory information about their head position and movement because the detectors which would normally sense this movement are not functioning properly, but in addition
2) because of this erroneous sensory input, other responses which normally would be generated by the sensory input from those diseased receptors will also be in error. So the eye movements which usually compensate for head movement will not work appropriately so the visual stimulus will be smeared AND the person will be receiving inaccurate information about their head movement.
What happens to gravity-sensing otoliths after two weeks of microgravity?
Test of toadfish showed 7-fold increase in sensitivity
How can the brain "turn up" sensitivity of primary afferent otolithic neurons?
In the vestibular system, just as in the cochlea there are vestibular efferents: nerve fibres which project from the brainstem out to contact the receptor hair cells or primary afferents. Those fibres allow the brain to control the sensory input it receives.
Does space flight alter the structure of vestibular receptors?
Apparently, rats sent into space have anatomical changes to otolith hair receptor cells by day 2 of flight and highly significant ones by day 14