Vestibular System-Graf Flashcards
What is the vestibular system good for?
- compensatory movements (eyes, head, body)
- postural control (upright stance)
- spatial orientation
- self-motion detection and perception
- cardiovascular, pulmonary adjustment
Most of the time, we are unaware of the existence of our sense of balance.
We perceive our sense of balance only, when something goes wrong!
Examples: motion sickness, new glasses, Menière’s disease, influence of alcohol, vertigo, etc.
What are labyrinth?
- Vestibular system has 5 components: 3 semicircular canals (anterior, posterior, and horizontal) and 2 otoliths (utricle and saccule)
- there are 3 because they are oriented in 3 dimensions of space
- It is a complex structure in the inner ear which contains the organs of hearing and balance.
- It consists of bony cavities (the bony labyrinth ) filled with fluid and lined with sensitive membranes (the membranous labyrinth).
Receptor hair cells
- Stereocilia: oriented in a step like fashion from small to tall; there is still kinocilia on top
- when the stereocilia is bent towards to the tall stereocilia there is depolarization; if bent toward the small there is hyperpolarization
- the ion channels is a nonspecific mechanically gated ion channel; they are based in endolymph
- Type I hair cell
- Type II hair cell
- support cell
- calix
- afferent nerve ending
- receptor potential
What is the function of the kinocilia?
only there to ensure cell polarity that
Receptor systems: semicircular canals, otoliths:
What is covered over the other: bony labyrinth, membranous labyrinth?
membranous labyrinth is encased over with the bony labyrinth; the fluid in between the two is perilymph; the labyrinths themselves are filled with endolymph
Semicircular canals are oriented with their own coordinate system. How do the horizontal semicircular canal orient themselves?
on direction of the horizontal semicircular canal is backward rotation that will cause endolymph current towards the receptor system which is the cupula
Otoliths also have hair cells and are overlaid by the otolithic membrane which contains calcium and carbonate crystals? What is important about the crystals?
-they are necessary for sensory function
When we move our head, the endolymph in the semicircular canals have a so called endolymph current? What happens when this current pushes against the cupula?
we call it ampullopetal flow
In the horizontal canal, ampullopetal flow is necessary for hair-cell stimulation, whereas ampullofugal flow (flee) is necessary for the anterior and posterior canals.
When you tilt the head what do the otoliths do?
they will slide through the hair cells and bend the stereocilia in a certain direction or another which will cause opening of the ion channels
the otoconia slide across hair cells and thus exert a shear force which constitutes the adequate stimulus
What is the on direction of the horizontal and vertical canals?
- HORIZONTAL canals on direction (excitatory): ampullo-PETAL (towards the crista ampullaris)
- VERTICAL canals on direction (excitatory): ampullo-FUGAL (away from the crista ampullaris)
Semicircular canal activation is initiated by the so called what?
endolymph current
it is actually better to say the endolymph stays in place and the head turns around it
What is the push pull mechanism of the semicircular canals?
The semicircular canals are oriented in a way that each canal on each side of the head is mirrored with its counterpart on the opposite side. Each of these three pairs works in a push and pull way.
The stimulation of one canal (for example left side) results in the inhibition of the other on the other side of the head (for example right side). This mechanism makes it possible to be able to sense all directions of rotation when making movements with the head. It is important that both sides work in this push and pull mechanism but if there is a pathological predisposition to the canals it may result in affecting this mechanism as in those seen in vertigo.
How are the hair cells arranged in the semicircular canals?
The hair cells in the canals are arranged in such a way that they project into a gelatinous membrane called the cupula. When you turn your head in the plane of the canal, the endolymph causes it to splash against the cupula which then results in the deflection of the hair cells. But if you were to keep turning in circles without stopping, the endolymph fluid would catch up with the canal, and there would be no more pressure on the cupula. If you stopped spinning, the moving fluid would splash against a suddenly still cupula, and you would feel as though you were turning in the other direction.
What are the displacements of the otoliths?
- utriculus involves with displacements out of the horizontal plane
- sacculus involves with vertical linear displacements
the otoliths also project in a push-pull fashion with on and off directions
When happens to the utricles when you bend your head backwards?
the otoliths move backward and bend the stereocilia
The actual force hair cells are responding to are what?
ACCELERATION
-not constant speed
Semicircular canal hair cells respond to what type of accelerations?
angular accelerations (rotations)
Otolith hair cells respond to what type of accelerations?
translations
The ubiquitous linear acceleration force on earth is gravity which is?
9.82 m/sec^2
Experiments show that the system really detects acceleration.
A person is very rapid accelerated and a period of constant velocity.
cupula reacts to the acceleration not constant velocity
Tip link structure:
Cadherin 23 and Protocadherin 15 (configurs a spring-like structure)
Adaptation (motor): actin/myosin 1c
Fine thread-like tip links connect to trap doors in the adjacent cilium. Bending the hair cells stretches the tip link, causing an influx of K+ ions and the generation of neural impulses in the VIIIth cranial nerve.
What are tip link structures good for?
reflexes: eye movements and postural control
Postural reflexes
- tonic labyrinths reflexes (upon the extremities)
- tonic neck reflexes (upon the extremities)
Righting reflexes
Stato-kinetic reaction
to keep our neck upright
Stato-kinetic reaction: I can walk without falling over
postural reflexes go down the spinal cord
What are the two main vestibulospinal tracts that we have?
medial and lateral vestibulospinal tracts
Lateral vestibulospinal tract
lateral vestibular nucleus “Dieters” via ventrolateral medulla and spinal cord to ventral funiculus (LUMBO-SACRAL segments)
Medial vestibulospinal tract
medial, lateral, inferior, vestibular nuclei, bilateral projection via descending medial longitudinal fasciculus to CERVICAL segments
Reticulospinal tract
medial, pontomedullary reticular formation, bilateral projection via the lateral and ventral funiculi to lumbar segments
Vestibulo-collic reflexes
- they work on the whole head movement
- shows that when you stimulate the semicircular canals the cat will make head movements in the plane of a particular canal
Stimulation of one canal provokes compensatory eye and head movements in the spatial plane of that canal.
compensatory eye movements occur in these canal coordinates
Compensatory eye movements are also called?
vestibulo-ocular reflex (VOR)
When you move around you think everything is nice and stable but the actual head movements are bobbing up and down left and right. Why don’t we see that?
we have slight eye movements that compensate for these movements
What is oscillopsia?
Oscillopsia is a visual disturbance in which objects in the visual field appear to oscillate.
There are no compensatory eye movements.
The severity of the effect may range from a mild blurring to rapid and periodic jumping. Oscillopsia is an incapacitating condition experienced by many patients with neurological disorders.
How do they get to the eyes
they have vestibulo-ocular projections that go mainly through the superior and medial vestibular nucleus
there are also excitatory and inhibitory components
they go through the ascending medial longitudinal fasciculus and go to the ocular motor nuclei
What is nystagmus?
consists of slow (compensatory) and quick (Resetting) phases. The direction of nystagmus is defined according to the direction of the quick phase.
the oscillating eye movements that consist of slow and fast movements; the direction of nystagmus is defined according to the quick phase
If I move my head to the right my eyes will move to the left over the equidistant angle. So if I move my head 10 degrees to the right, my eye will move in what direction and how many degrees?
eyes will move 10 degrees to the left
Visual input alters the vestibular-derived postural information
????
What is gaze?
eye and head coordination
Classical vestibular cortical areas, input via which nucleus of the thalamus?
ventroposterior inferior nucleus of the thalamus
What is spatial hemineglect?
Hemineglect, also known as unilateral neglect, hemispatial neglect or spatial neglect, is a common and disabling condition following brain damage in which patients fail to be aware of items to one side of space.
Visual scan paths in a normal and neglect patient under spontaneous conditions and during left-sided vestibular stimulation (cold water irrigation). What happens in a patient with spatial hemineglect under the caloric stimulation?
the patient’s visual search is restored momentarily under the caloric stimulation
Stimulating illusory own-body perceptions
he got into an accident and all of a sudden he was seeing himself above the accident site
Where did the labyrinth (auditory system) come from embryologically?
otic placode in the hindbrain
comes from otocyste–> vestibular pouch–> ganglion–> semicircular canal–> fully developed labyrinth
What are the genes important in regulating ear morphogenesis?
neurogenin 1 (ngn1): makes primary neurons
atonal homologue 1: makes hair cells
these are all originally Drosophila genes
Otx1: for development of horizontal semicircular canals
When you knock out Otx1 what is the result?
horizontal semicircular canals will not be made
When you knock out ngn1 what is the result?
cannot make primary neurons
What is a hemi-labyrinthectomy? When would you perform it?
vestibular schwannoma to take out one labyrinth
there has to be equilibrium between the two sides
one labyrinth it puts out 10 APs per sec, the other puts out 20 APs; the animal will always turn to the side of the lesion
What is the characteristic movement of the animal when there is global vestibular lesion thus needing to perform a hemi-labyrinthectomy?
Animal rolls towards the side of the lesion (plus eye movement symptoms; nystagmus)
What is the term for disappearance of lesion symptoms?
compensation
If a second labyrinth is lesioned after compensation, we observe what phenomenon?
Bechterew-Phenomenon
If there are similar lesion symptoms as in the case of the lesion of the first labyrinth, but now towards the side of the lesioned second labyrinth, what does this prove?
proves that tonic activity has been restored in the vestibular nuclei of the first labyrinthine lesion
What is the radio-controlled human?
Electrically stimulating the vestibular nerve can influence human movement – and may create better virtual-reality devices and prosthetics.
What is the vestibular and optokinetic stimulation (vertical axis)?
rotating chair???
What is the Halmagyi-Curthoys head jerk?
- the eyes come back very quickly in normal pt
- there is a lag of the eye movement towards the side of the labyrinth
What is the vestibulo-ocular reflex suppression test?
the pt should fixate the nails of her thumbs and do combined movement of the head and the target with the same angular velocity
the doctor sees if the eyes stays on the target (normal)
if there is lag then the pt is abnormal
What is the Romberg test?
- checks for proprioception of the lower body primarily
- ask pt to look straight ahead with feet together and hands on side
- watch for excessive swaying and loss of balance
- have pt stand on one leg and then two legs
What is the stepping test?
- Ask the pt to march on the spot with arms extended and close their eyes and keep marching for 50 steps
- 45 degree rotation to the right or left is considered abnormal
- a positive test result may indicate an unilateral vestibular deficit
What is the blind-walking test?
have the pt walk with their eyes closed
-always make sure the pt does not FALL!!!
For all the tests to examine vestibular function, which direction do patient turn if there is a deficit?
- pt moves towards the side of the lesion for all those exams
- because the other side (normal) pushes towards the abnormal side
What is the past pointing test?
- past-pointing or falling is towards the lesioned side, i.e. the direction of the slow phase of nystagmus
- you have an imbalance so you tell the pt to move the arm up and down with eyes closed, if there is a lesion the arm will point more and more towards the lesioned side
What is the direction of the nystagmus with cold water stimulation during the caloric test?
- fast phase contralateral
- slow phase ipsilateral
What is the direction of the nystagmus with warm water stimulation during the caloric test?
- fast phase ipsilateral
- slow phase contralateral
What is posturography?
- you place pt on a platform to which you disturb
- you check to see if pt can stay upright during the disturbance
What is vertigo?
unpleasant sensation in connection with disorientation, misinterpretation of the direction of gravity and misinterpretation of passive and self-motion
-per se not a disorder but a consequence of many pathological processes
What is alcohol induced nystagmus and vertigo?
“the world is spinning around you”
- if you drink too much alcohol diffuses in the cupula (the cupula and the endolymph are usually at specific equilibrium by weight); the alcohol makes the cupula lighter than the endolymph
- mostly horizontal canal is affected when you lie down, the endolymph presses on the cupula since its heavier; so it is an inadequate stimulus of the labyrinth; the brain interprets this “I’m turning my head” and makes reflex compensatory eye movements
- after excessive alcohol intake, alcohol diffuses into the cupula, thus making it lighter than the endolymph. The now heavier endolymph deflects the cupula, effectively rendering it a graviceptor, leading to nystagmus and a sensation of self-movement
What are some examples of movement disorders?
- seasickness
- airsickness
- carsickness
- space motion sickness
What is perilymph fistula?
rupture of the otic capsule (pressure changes in the labyrinth so it does not work properly anymore), usually at the oval or the round window, causing perilymph leakage and abnormal transfer of pressure changes
What is Tullio phenomenon?
-pathological sound induced vestibular signs and symptoms in patients with perilymph fistula
What is the difference between tinnitus and vertigo?
- tinnitus is “ringing” in the ear
- vertigo: severe dizziness
What is Menière’s disease?
- temporary leakage of endolymph from the labyrinth and acting upon vestibular nerve fibers thus resulting into blockage of nerve impulses
- classical triad of fluctuating hearing loss, tinnitus, and episodic vertigo
What is vestibular neuritis?
acute partial unilateral vestibular nerve loss due to inflammation of the vestibular nerve with rotatory vertigo, nystagmus, postural imbalance, nausea, and vomiting
What is benign paroxysmal positional vertigo (BPPV)?
loose particles, mostly in the posterior semicircular canal, and exert pressure
pts experience vertigo, nausea, and feeling of falling backwards
occurs mostly in the morning after rising from bed or after a nap during the day
displaced otoconia irritates the posterior semicircular canal
occurs mainly in middle aged women
How do you diagnose benign paroxysmal positional vertigo ? How would you treat this?
- through the Dix-Hallpike test (diagnose)
- you lay the pt backwards and see the movement of their eyes
-do the Semont maneuver or Epley maneuver (to treat)
For dizziness from the left ear and side:
Sit on the edge of your bed. Turn your head 45 degrees to the right.
Quickly lie down on your left side. Stay there for 30 seconds.
Quickly move to lie down on the opposite end of your bed. Don’t change the direction of your head. Keep it at a 45-degree angle and lie for 30 seconds. Look at the floor.
Return slowly to sitting and wait a few minutes.
Reverse these moves for the right ear.
Again, do these moves three times a day until you go 24 hours without vertigo.
What is the Epley maneuver?
If your vertigo comes from your left ear and side:
Sit on the edge of your bed. Turn your head 45 degrees to the left (not as far as your left shoulder). Place a pillow under you so when you lie down, it rests between your shoulders rather than under your head.
Quickly lie down on your back, with your head on the bed (still at the 45-degree angle). The pillow should be under your shoulders. Wait 30 seconds (for any vertigo to stop).
Turn your head halfway (90 degrees) to the right without raising it. Wait 30 seconds.
Turn your head and body on its side to the right, so you’re looking at the floor. Wait 30 seconds.
Slowly sit up, but remain on the bed a few minutes.
If the vertigo comes from your right ear, reverse these instructions. Sit on your bed, turn your head 45 degrees to the right, and so on.
Do these movements three times before going to bed each night, until you’ve gone 24 hours without dizziness.
Vestibular input plays important roles in everyday vital functions, however, most of these functions occur without us being aware of them.
Vestibular signals are ubiquitous in movement-related central processing centers. Their existence has been largely neglected until now, and thus remained unknown
Describe VOR with respect to what canals correspond with what extraocular eye muscles.
Anterior semicircular canal:
Excitatory: ipsilateral IO, contralateral SR
Inhibitory: contralateral SO, ipsilateral IR
Posterior semicircular canal:
Excitatory: contralateral SO, ipsilateral IR
Inhibitory: ipsilateral IO, contralateral SR
Horizontal semicircular canal:
Excitatory: ipsilateral MR, contralateral LR
Inhibitory: contralateral MR, ipsilateral IR
