Vestibular Systems Flashcards
Which structures in the body are responsible for angular rotation of the head?
3 Semi-circular canals
Which structures in the body sense the acceleration of the head and the strength of gravity?
Otolith organs
What are the 2 otolith organs?
Saccule
Utricle
What are 6 main functions of the vestibular system?
Perception of movement in space + tilt with respect to gravity
Provide reflex balance reactions to sudden instability of gait/posture (vestibulo-spinal reflexes)
Preserve visual acuity during head movement (vestibulo-ocular reflexes)
Assist in control of HR + BP during rapid up-down tilts
Assist synchronisation of respiration with body orientation
Provide a reference for absolute motion in space
Define vertigo.
False perception of movement in space
Define vestibular ataxia
Instability of gait or posture
What happens to the ability of the brain to stabilise the eyes in unilateral vestibular lesions?
Vestibular nystagmus
Eyes start moving in the direction of the lesion
Intermittent “resetting” of eye position with fast saccades
What happens to the ability of the brain to stabilise the eyes in bilateral vestibular lesions?
Oscillopsia
Everything appears to be shaking – the ability to stabilise the eyes is lost
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
Describe the saccule
Bed of sensory cells situated in inner ear
Orientated in vertical plane
Detect linear acceleration + head tilt in vertical plane
Describe the structure of otolith hair cells
Have an overlaying layer of Otoconia
Project normal to the plane of the saccule
Have 1 kinocilia per cell, the rest are stereocillia
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 sensitivities of the saccule
Directional sensitivities in all combinations of vertical + antero-posterior directions
Describe the structure of the utricle and its sensitivities.
Oriented horizontally with the hair cells projecting vertically
Directional sensitivities in all combinations of lateral + antero-posterior directions
How do the otolith organs give a signal of linear acceleration in all 3-dimensional directions?
Vector sum of utricular + saccular stimulation patterns gives signal of linear acceleration in all 3D directions
Describe the structure and function of the semi-circular canals.
Hair cells project from the ampulla in the wall of the canal + are uni-directionally oriented so acceleration to a particular side stimulates canals on that side
(e.g. rotation right stimulates right canal, rotation left inhibits the right canal activity)
When head rotation decelerates to a stop, canal on other side (left) 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?
Unopposed signal coming from the intact side
= partial impairment of sensitivity for movement in accelerator direction of that canal
Why would a unilateral canal lesion cause vertigo?
Unopposed tonus of the intact canal gives a signal as if the head is rotating to the intact side.
Gives feeling of intense spinning
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 the opposite 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?
Inability to stabilise eyes during head movement
Can be caused by unilateral or bilateral loss of vestibular function
Vestibulo-ocular reflex is impaired
Objects in visual field appear to oscillate
How would you test if a patient has oscillopsia?
Tell the subject to look at a fixed target + 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
Seconds: Benign Paroxysmal Positional Vertigo (BPPV)
Minutes: Vertebrobasilar insufficiency
Hours: Meniere’s Syndrome
Days: Vestibular neuritis
Fluctuating/continuous : Uncompensated vestibular lesion
Silent: Acoustic neuroma
What is BPPV and how is it treated?
Benign paroxysmal positional vertigo
Caused by otoconial debris in the canals + is provoked by head movement
Debris floating in the canal stimulates the ampulla + generates false signals of head rotation
Treatment= particle repositioning manoeuvre:
Turning the head vigorously in the opposite direction to that which provokes the vertigo, through 360 degrees, flushing out the debris
List 3 inputs to the vestibular system
Visual system Inner ear (info. on rotation + gravity) Proprioception in joints + limbs
List 3 outputs of the vestibular system
Ocular reflex
Postural control
Nausea
Where is the Labyrinth located?
In the petrous part of the temporal bone (well protected)
Name the fluids found inside and outside the structures of the ear. What is the function of the fluid?
Inside: Endolymph
Outside: Perilymph
Fluid helps move the hairs
Which 2 semi-circular canals are joined on their non-ampulla side?
Anterior/ Superior + Posterior/ Inferior
Describe orientation of the semi-circular canals
Anterior: 45 degrees antero-superiorly
Posterior: 45 degrees postero-inferiorly
Lateral: “arm” points externally, perpendicular
What 2 types of hair cells are found in the vestibular system? What type of receptors are these?
Type 1: More numerous, more afferents, less efferents, round shape
Type 2: Less afferents, more efferents, similar shape to outer hair cells
Mechanoreceptors
What are Otoliths? Where are they found?
Carbonate crystals
On top of the gelatinous matrix that lies on the surface of the hair cells
How does movement effect Otoliths?
Crystals move, generating pressure, moving the “jelly” + deflecting hair cells
How are the hair cells organised from the striola?
They have opposing orientations
Thus 1 stimuli causes both excitation + inhibition in the same organ
