Neuro 11: Vestibular system Flashcards
What are the inputs to the vestibular system
Visual,
Rotation and gravity (inner ear.. movement of head)
Pressure in heal
What are the outputs
Ocular reflex/postural control (spinal reflex) (+nausea!)
Where is the central processing of vestibular system
Brainstem (vestibular nucleus)
Which part of the ear is important in balance
The membranous labyrinth ….
Note…. the bony labyrith is the bony outer wall of the inner ear…. vestibule (=saccule + utricle), SCC and cochlea, and is hollowed out of the temporal bone….
Inside this boney cavity is perilymph…. inside the perilymph, there is a ‘membranous labyrinth’ containing endolymph… (i.e. a membranous portion inside the bony structure)… so this membranous structure is the SCC, the utricle, the saccule, and also the scala medius. The Scala vestibule and scala tympani are outside this membranous labyrinth and have perilymph.
Name the 6 organs in the membranous labyrinth
Which organ connects the SCC to the cochlea
Anterior semicurcular canal, posterior semicircular canal and lateral semicircular canal,
the utricle and the saccule
and the cochlea
saccule connects
Where do all semicircular canals join
To the utricle
Which 2 semicircular canals join each other
anterior and posterior semiciruclar canal
What is the saccule connected to
The utricle and the chochlea
What is inside and outside of the membranous labyrinth
Outside: perilympth
Inside: endolymph
Outline innervation in thhis area
From cochlea: spiral ganglia from around the cochlea merge to form the cochlear nerve
From vestibule + SCCs: various nerves merge toform the scarpa ganglia, which then forms the vestibular nerve.
Look at the positioning of the labyrinth in the head.
Look at the diagram in the scull
State the cell types in the vestibular system (different to hearing inner and outer hair cells!)
Type I:
More in number
Direct aferents, indirect eferents
Type II:
Direct aferents and
eferents
What are the two types of organs in the membranous labyrinth
Static labyrinth=otolith organs= utricle + saccule
Called otoliths because they have carbonate crystals which move (not endolymph fluid moving like in the SCC)
Kinetic labyrinth= semiciruclar canals
Where do neurons from hair cells synapse and where do fibres project from here
Scarpas ganglion… then project to vestibular nuclei
Define kinocilium
Special type of cilia on the surface of hair cells in this region
What stimulates hair cells
In otoliths: deflection of forces or inertial resistance to acceleration (gravity)
In SCC: enolymphatic fluid rotation
What generates action potential in the hair cell… and how does this affect the firing rate to scarpas ganglion
If force is towards the kinoclium, depolarisation (increasing firing rate)
If force is away from kinocilium, hyperpolarisation (decreasing firing rate)
Outline the blood supply of the inner ear
The anterior inferior cerebellar artery (AICA) from the basillar artery…. which can affect ear
Where do primary afferents terminate from this area
Primary afferents end in vestibular nuclei and in cerebellum
Where do primary afferents from the vestibular system terminate generally, and for each type of labyrinth
Terminate in vestibular nuclei and in cerebellum
Static- lateral and inferior
Kinetic- superior and medial
Where are the hair cells contained in the otolithic organs… describe the structure here
In the MACULA….
you have hair cells + supporting cells,
On top of which you have the ‘otolithic membrane’, i.e. a gelatinous matrix
On top of this you have the ‘otoconia’ which are the carbonate crystals.
What is the function of the saccule and utricle and how do the macula achieve this
Saccule= detect movement in the VERTICLE and anteroposterior direction. Saccule oriented in the verticle plane, with hair cells at right angle to it. so it senses up and down movement
Utricle= detect movement in the LATERAL and anteroposterior directions.
Where are the vestibular nuclei projecting to
spinal cord
nuclei of the extraocular muscles
Cerebellum
Centers for cardiovascular + respiratory control
What are the vestibulo cerebellar pathways
Movement coordination
Posture regulation
VOR (eye movements) modulation
Outline the pathway of signals from the vestibular system to thalamus and cortex
Vestibular nuclei: project to thalamus
Thalamic nuclei: project to the head region of the primary somatosensory cortex
Also to superior parietal cortex: ‘vestibular cortex’ concerned with spatial orientation.
Cortical projections may account for feeling of dizziness during certain kinds of vestibular stimulation
Functin of vestibular system
To detect and inform about head movements.
To keep images fixed in the retina during head movements.
Postural control
T/f when not moving there is no firing of neurons in this area
F… there is a resting discharge….
When hair bundles are reflected towards X there is excitation and increased impule frequency
When reflected away, decreased impulse frequency (hyperpolarisation)
Are there opposing striola in the macula
YES….
So when moving head in one direction, then the crystals will move forward…. within the maculae, some hair bundles will depolarise and some will hyperpolarise
State the movement type sensed in the utricle vs saccule vs semi circular canals
Angular (rotatory) motion (of the head) is sensed by the semicircular canals
Acceleration
of the head and strength and direction of gravity are sensed by the otolith organs.
utricle is sensitive to a change in horizontal movement, and the
saccule gives information about vertical acceleration
NB: Vector sum of utricular and saccular stimulation patterns give signal of linear acceleration in all 3xD directions (I.E. THE SCC DON’T TELL ABOUT ACCELERATION, BUT INSTEAD ABOUT ANGULAR MOTION)
The pattern of stimulation from all canals on both sides signals rotation in all 3XD directions.
When turning your head horiaontally what happens to the endolymph
In the lateral SCC, endolymph does not move whereas the canal does, so the endolympoh moves in a different direction relative to the SCC
How are otolith hair cells stimulated
By inertial resistance of the otoconial mass to linear head acceleration (huh?)
Basically, the crystals create weights on different areas of the hair cells depending on the movement of head turning and this will stimulate different subsets of cells
What is the striola
Basically in the centre of the macula in the otoliths there is almost a mirror line…. the kinocilia on one side of this line point in an opposite direction to those on the opposite side.
As a result, if moving in a certain direction, some of the hair cells will be hyperpolarised and the other depolarised so important for redundancy
VECTOR SUM of utricular and saccular stimulation patterns give signal of linear acceleration
Where are hairs contained in the SCC
In the ampulla (the widening of the canals near the utricle and saccule) in a layer called the crista (NOT TO BE CONFUSED WITH THE CARBONATE CRYSTALS IN THE OTOLITH ORGANS….. there are no crystals here!!!)
Differentiate the orientation of hair cells in the saccule and utricle vs in the SCC
In saccule and utricle, the hair cells are oriented in opposite directions depending on which side of the striola within the macula.
In SCC, all hair cells are orinted in the SAME direction (different directions between different canals but SAME DIRECTION within each canal). Also same direction on each side of the head
What is the name of the gelatianous projection from the crista in SCC
Cupula (gel is less dense than the gelitanous layer in the otoliths) it closes the ampulla
What is the point of the cupula
When the endolymph in the SCC moves and pushes against the cupula (which seals the ampulla), the cupula will be deflected, causing increased firing or inhibition of the hair cells underlying the cupula depending on the side that the cupula is being pressed from
Rule for which canal is stimulated and which inhibited upon head rotation
If turning head to the right, the right canal is stimulated and the left inhibited and vice versa.
When head rotation decelerates to stop (ie acceleration in the opposite direction) the canal on the opposite side is stimulated. Eg stopping a rightwards rotation stimulates left canal.
T/F at rest, each canal does not fire
F: Each canal has a tonic firing rate so that when the head is still, the tonuses from the right and left canals balance out
T/f loss of canal function on one side gives a permanent partial impairment of sensitivity to rotation in the ‘on’ direction of the impaired canal (i.e. damage to right canal will give loss of right turning sensation), which is the same with damage to the otoliths
F: Everything true except not with otoliths as they are omnidirectional (so can be damaged on one side—> as the striola makes for different hair cell directions)
What happens if the right canal is defunct
At rest, there will be more firing coming from the left (basal firing or ‘tonus’) canal than the right (which is damaged so not firing at all), so it will feel like your head is tuning to the left
What is vertigo
In unilateral canal lesion, you (i.e. see card 40), head feels like it is spinning in one direction (illusory rotation to intact side)- or feeling like on a boat or ground is unsteady = SYMPTOM CALLED VERTIGO
Explain vestibular nystagmus
When you normally turn your head to one side, the eyes turn (in an ocular reflex) to the opposite side (so that the eyes would stay in the same position even if head turns).
When turning head to the left, the eyes move to the right.
So if you have an acute unilateral vestibular disorder, and a right canal is not working, then there is unopposed tonus from the left ear, so the head thinks we are turning to the left. As a result, it will turn the eyes to the right in an vestibulo-ocular reflex.
Drifting of eyes picked up by the brainstem which intermittently resets eye position with fast saccades generating pattern of vestibular nystagmus which beats back to the to the intact side.
Why is vestibular nystagmus only observed in acute stage of vestibular dysfunction
minimised by visual suppression mechanisms
Outline where balance is perceived in the cortex and how it gets there (i.e. the afferents)
vestibular projections via thalamus to tempero-parietal ‘spatial’ cortex subserve perception of motion in space
Explain the vestular ocular reflex
Superior and medial vestibular neurons project onto motor nuclei supplying extraocular muscles
What is the mechanism for movement of the eyes in the opposite direction to the turning of the head (horizontal)
Imagine turning head to the left…. eyes need to turn right. So right eye will need to move away from nose (abduct) and the left eye will need to move towards the nose (adduct)…. how is this achieved:
Axons of medial vestibular nucleus crossing the midline and projecting to CONTRALATERAL abducens nucleus to abduct the eye (via the lateral rectus muscle)
Axons from the VI cross and ascend in the MLF, and excite the contralateral oculomotor (III) nucleus to adduct the other eye (in the opposite direction to head rotation)
eye and head rotations cancel each other so that the direction of fixation of the eyes remains stabilised on the visual target
How is vertical vestibulo-ocular reflex generated
Superior vestibular neurones from the vertical canals project ipsilaterally to the IIIrd and IVth nuclei to generate vertical vestibular-ocular reflexes.
What is ‘physiological vestibular nystagmus’
With continuing head rotation frequent saccades reposition the eyes more centrally (by vestibular ocular reflex) to form an overall pattern of normal or ‘physiological vestibular nystagmus’
Describe oscillopsia and how would you test?
Vestibular dysfunction impairs eye fixation when turning head…
vestibulo-ocular reflex is the only mechanism which can drive fast compensatory eye movements.
Patient feels like visual world is bouncing or laggging behind during active or passive head movements
Head shaking test:
-Normal: shaking head patient can fix on target
Bilateral loss of ves. functin: eyes taken off target by head swing and mutliple catch up saccades needed
Loss on one side: turning towards the good side, eyes will seem on target…. on swing to lesioned side, eyes off target and need saccades
Define ataxia
lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes and abnormalities in eye movements
2 types of vestibular ataxia
Bilateral loss of vestibular function= mild gait ataxia worse at speed, rough ground
Unilateral loss= leaning or falling to the lesioned side (due to ipsilaterally of vestibular-spinal projection)
Outline efferent projection of vestibular system to the spinal cord
Lateral vestibulo-spinal tract:
decends ipsilaterally in ventral funiculus of spinal cord. axons terminate in lateral part of ventral horn and influence motor neurons to limb (especially extensor antigravity) muscles
Medial vestibulospinal tract descend bilaterally in medial longitudinal fasciculus (MLF) to cervical and upper thoracic spinal cord
axons terminate in medial part of ventral horn and influence motor neurons to neck and back muscles (so stops in lumbar region)
Where are hair cells located in SCC and otolith organs
Otoliths… Macula: in utricle and saccule
SCC…Crista: of ampulla (swelling of semicircular canals)
The primary afferent neurons of the vestibular system have cell bodies where
The cell bodies of the vestibular nerve fibers are located in Scarpa’s Ganglion, comprising clusters of bipolar cells lodged in the internal auditory meatus of the petrous portion of the temporal bone and distributed within the emergent vestibular nerve fibers
Which otolith organ gives rise to what
Saccule gives rise to cochlear duct and utricle gives rise to semi—circular canals.
How many vestibular nuclei
4
Where do vestibyular nuclei project to within the thalamus
All vestibular nuclei project to ventral posterior and ventral lateral nuclei of the thalamus
When does osscilopsia occur vs when does vestibular nystagmus occur (important, one of learning objectives)
Oscillopsia…. bilateral lesion (during head movement)
Vestibular nystagmus… unilateral lesion
BOTH RELATED TO INABILITY TO STABILISE THE EYES
What can happen during cute phase of unilateral vestibular loss and give an example of such a condition
Severe nausea and vomiting
Vest. nystagmus presents due to unilateral vestibular loss
What type of movement do the utricle and saccule give information of
Vector sum of utricular and saccular stimulation patterns give signal of linear acceleration in all 3xD directions.
UNDERSTAND PHYSIOLOIGCAL NYSTAGMUS IT MAKES SENSE
!!!!
Outline the direction of endolymph movement in SCC that leads to depolaerisation
Superior and Inferior SCC: (+) Away from Utricle
Horizontal SCC: (+) Towards the Utricule
T/f there is large firing of SCC at constant delocity
No, because the SCC respond respond to acceleration not to constant velocity. see slide 29
`Which semicircular canals work together
Horizontals
Left anterior with right posterior
Right anterior with left posterior
Outline the ipsilaterality of the lateral and medial vestibulospinal tract
Lateral: motor neurons to limb muscles… ipsilateral
Medial: moto rneurons to neck and back muscles… bilateral
Where is the lateral vestibulospinal tract located
Within the VENTRAL FUNICULUS
Where are the cells of origin of each vestibulospinal tractq
Medial: cells begin in the medial vestibular nucleus
Lateral: cells begin in the medial vestibular nucleus
NOTE… THE TRACT STARTS IN THE NUCLEI BUT FEEDING ONTO THIS TRACT IS OF COURSE THE AFFERENTS WITH CELL BODIES IN THE SCARPA’S GANGLION FROM THE VESTIBULAR NERVE
What is latency of the vestibuloocular reflex
8mseg latency
this means takes 8 mseg for vestibular afferents to influence extrinsic eye muscles
What is the name of the connection between the 6th nerve nucleus nad the 3rd nerve nucleus
Medial longitudinal fasciculus
https://www.youtube.com/watch?v=m10vi4liwgo
watch from 22 minutes
What is used in diagnosis of vestibular disorders
Anamnesis Balance and Gait assessment Cerebellum Gaze assessment: eye movements. Vestibular tests: Caloric test vHIT VEMP Rotational test Imaging: CT Scan, MRI Subjective assessment (questionnaires)
Types of balance disorder classifie by peripheral and central classification and examples of each
Peripheral vestibular disorders: labyrinth and VIII nerve
eg, vestibular neuritis, BPPV, Meniere’s disease BVF, UVF
Central vestibular disorders: CNS (brainstem/cerebellum)
eg, stroke, MS, tumours
