Vestibular Systems Flashcards
Which structures in the body are responsible for angular (rotational) motion of the head?
Semi-circular canals
Which structures in the body sense the acceleration of the head and the strength of gravity?
Otolith organs
What are the two otolith organs?
Saccule
Utricle
Define vertigo.
False perception of movement in space
Define vestibular ataxia
Instability of gait or posture
What are some other consequences of vestibular loss?
Slight impairment of orthostatic control
Severe nausea and vomiting
Loss of coordination on directional reorientation, motion intolerance, oversensitivity to visual motion in the environment
What type of cell is involved in the detection of movement in the vestibular system?
Hair cells
Describe the cilia of these hair cells.
There is one kinocilium and the rest are stereocilia
What does the hair cell fibre synapse with and where does it project?
It synapses with a primary neurone dendrite (cell body in Scarpa’s ganglion)
They project to the vestibular nuclei in the brainstem
What stimulates hair cells?
In Otoliths: deflection by forces of inertial resistance to acceleration
In Semi-circular Canals: endolymphatic fluid rotation
Describe how the hair cell receptor potential can be changed.
Depolarisation = movement towards the kinocilium Hyperpolarisation = movement away from the kinocilium
Describe how ganglion cell discharge can be changed.
Towards the kinocilium = increased firing frequency
Away from the kinocilium = decreased firing frequency
Describe the orientation and sensitivities of the saccule.
Saccule is oriented vertically with the hair cells projecting normal to the plane
They are sensitive in all combinations of vertical and antero-posterior directions
Describe the orientation and sensitivities of the utricle.
Utricle is oriented almost horizontally with the hair cells projecting vertically
Directional sensitivities in all combinations of lateral and antero-posterior directions
Describe the structure and function of the semi-circular canals.
Hair cells project from the ampulla in the wall of the canal and are uni-directionally oriented so that acceleration to a particular side stimulates the canals on that side (e.g. rotation of the head to the right stimulates the right canal, rotation in the other direction inhibits the right canal activity)
When head rotation decelerates to a stop, the canal on the other side (left side) is stimulated
Describe the firing of the canals when the head is still.
Each canal has a tonic firing rate so that they equal out when the head is still
What are the effects of loss of canal function on one side?
There is unopposed signal coming from the intact side meaning that there is partial impairment of sensitivity to rotation in the ‘on’ direction of the defunct canal
Why would a unilateral canal lesion cause vertigo?
The unopposed tonus of the intact canal gives a signal as if the head is rotating to the intact side.
Patient may feel like they’re spinning even though they’re not.
Why would acute unilateral vestibular disorder cause vestibular nystagmus?
Unopposed tonus of the intact canal causes the eyes to be driven to the lesioned side – this is a vestibulo-ocular reflex (because it thinks that your head is rotating towards the intact side)
Where do superior and medial vestibular neurones project?
They project to the motor nuclei supplying extraocular muscles.
Describe the path of medial vestibular neurones.
The axons of medial vestibular neurones cross the midline and project to the contralateral abducens (VI) nucleus to abduct the eye on the opposite side (in the opposite direction to head rotation)
Axons from the abducens nucleus ascend in the MLF to the contralateral oculomotor nucleus (III) to adduct the other eye (in theopposite direction to head rotation)
Describe the path of superior vestibular neurones.
Project ipsilaterally to the oculomotor and trochlear nuclei to generate
VERTICAL vestibulo-occular reflexes
What is oscillopsia?
Everything appears to be oscillating
This is due to marked loss of vestibular function impairing eye stabilisation during rapid head movements.
The vestibulo-ocular reflex is lost.
How would you test if a patient has oscillopsia?
Tell the subject to look at a fixed target and then rapidly move their head.
If they have bilateral loss of vestibular function then their eyes will be taken off target by the head swing.
What are the effects of bilateral vestibular disorder on gait?
Mild gait ataxia
What are the effects of unilateral vestibular disorder on gait?
Tendency for the body and head to lean or fall to the lesioned side
Describe the path and function of the lateral vestibulo-spinal tract.
Descends ipsilaterally in the ventral funiculus of the spinal cord
Axons terminate in lateral part of ventral horn Influence motor neurones to limb muscles
Describe the path and function of the medial vestibulo-spinal tract.
Descend bilaterally in MLF to cervical and upper thoracic spinal cord
Axons terminate in medial part of ventral horn
Influence motor neurones to back and neck muscles
State a common cause of vestibular vertigo that lasts:
a. Seconds
b. Minutes
c. Hours
d. Days
e. Fluctuating/continuous
f. Silent
a. Seconds Benign Paroxysmal Positional Vertigo (BPPV) b. Minutes Vertebrobasilar insufficiency c. Hours Meniere’s Syndrome d. Days Vestibular neuritis e. Fluctuating/continuous Uncompensated vestibular lesion f. Silent Acoustic neuroma
Inputs to vestibular system
Vision
Pressure in feet
Inner ear for info on rotation and gravity
Outputs of vestibular system
Ocular reflex
Posture control
What bone is inner ear in
Petrus part of temporal as hardest
How many organs to vestibular system per ear
5
Otilith organs of vestibular system
Sacculus
Utricals
What does sacculus connect to
Superiorly utricle and inferiorly cochlea
What are 3 other organs to system
Lateral
Posterior
Superior canals
What do 3 canals connect to
Utricle
What fluid is in vestibular organs
Endolymph
What fluid is between organ and bone
Perilymph
What do each semi circular canals have
They have 2 canals leading to utricle
For each defined canal there is one wide and one narrow canal
What is name given to wider canals
Ampulla
Comparison of anterior and posterior canal planes
90 degrees
Purposefully orientated
Types of hair cells in vestibular organ
Type 1 and 2
Differences between type 1 and 2
More of type 1
Type 1 has indirect efferent but type 2 has direct
Afferents all direct
Where are hair cells in sacula
Side part nearest cochlea
Where are hair cells in utricle
At bottom
What plane does utricle deal with
Horizontal
What plane does sacula deal with
Vertical
What are otoliths
Carbonate cristals
Name given to functional component of otoliths
Maculae
Components of maculae
Hair cells
Gelatinous matrix
Otoliths
Where is gelatinous matrix
On top of hair cells
Purpose of gelatinous matrix
AIDS hair cell movement
Where are otoliths
On matrix
How are hairs stimulated
If moved in one way they trigger stimulatory pathway and in other an inhibitory pathway
What is striola
Central part of maculae
Either side of it the orientation of cilia in opposite direction
Where are hair cells in semicircular canals
Ampulla
Where is cupula
Ampulla
What is the cupula
A gelatinous mixture more dense than endolymph
Direction of hair cells in ampulla
All in one direction unlike otoliths
Mirroring instead occurs between ears
Name of functional part of ampulla
Crista
Supply of blood to vestibular system in inner ear
Basilar artery-> anterior inferior cerebrellar artery -> labyrinthine
Divisions of vestibular nerve
Inferior and superior
Where does vestibular nerve go
Vestibular nuclei
Organisation of vestibular nuclei
Lateral and inferior deal with static labyrinth
Medial and superior deal with kinetic labyrinth
Where do vestibular nuclei project
Spinal chord
Cerebellum
Nuclei of extraocular muscles
Cardiovascular and respiratory centres
Tracts for vestibulospinal reflexes and place innervated
Medial vestibulospinal tract- upper back and neck
Lateral vestibulospinal tract- limbs and trunk
Vestibulocerebrellar reflex goes where and via what
Vestibulo cerebellum via inferior cerebrellar peduncle
Where does vestibulo-ocular reflex travel to and via what
Oculumotor, abducens and trochlear nucleus via medial longtitudal fasiculus
Route from vestibular nuclei to vestibular cortex
Medial leminiscus to ventroposterior nucleus to vestibular cortex via internal capsule
Physiology of hair cell potentials
Tips open k+ channels depolarising the cell, leads to ca2+ influx which binds to vesicles
Resting discharge of hair cells
Tonic activity of hair cells
If excited then more frequent impulses due to depolarisation
If inhibited then less frequent impulses due to hyperpolarisation
Same applies to ones in cochlea
Effect of acceleration on hair cells
Acceleration causes hairs to shift in opposite direction to acceleration as endolymph moves gelatinous matrix
How semicircular canals work between ears
They are paired
Pairings of left and right semicircular canals
Lateral both together
Left anterior and right posterior
Right anterior and left posterior
LARP and RALP
Way of remembering parings
Ralp and Larp
Lateral vestibulospinal pathway
Lateral vestibular nucleus down lateral vestibulospinal tract to influence ipsilateral limb movements
Medial vestibulospinal tract pathway
From medial vestibular nucleus down medial vestibular tract to upper thoracic and cervical levels as supplies trunk and neck muscles
Differences between lateral and medial vestibulospinal pathways
Medial is bilateral (ipsilateral pathway more dense)
Purpose of having hairs in 2 directions
Can send same signal twice but in different way
One being that the hairs have been stimulated and other being that they have been inhibited
Vestibuloocular reflex features
Aims to keep image fixed so ensure eye movements in opposite direction to head movement
Fastest reflex in the body
Vestibulo ocular pathway horizontal
Lateral SCC-> scarpas ganglion->vestibular nucleus-> abducens nucleus-> up medial longtitudal fusiculus-> occulomotor nucleus-> lateral and medial rectus
Vestibulo ocular pathway vertical
Anterior SCC-> scarpas ganglion->vestibular nucleus-> abducens nucleus-> up medial longtitudal fusiculus-> occulomotor nucleus-> superior oblique
Common symptoms of vestibular issues
Vertigo
Dizziness
Unsteadiness
Issues of movement perception
