Vestibular System Flashcards
What are the normal functions of the vestibular system?
Subserve perception of movement in space and tilt with respect to gravity. Provide reflex balance reactions to sudden instability of gait or posture ‘vestibulo-spinal reflexes.’
Stabilise the eyes on earth fixed targets preserving visual acuity during head movements ‘vestibular-ocular reflexes’. Assist control of blood pressure and heart rate during rapid up-down tilts. Assist synchronisation of respiration with body reorientations. Provokes motion sickness when stimulated in unusual motion environments. Provide a reference of absolute motion in space which helps interpret the relativistic signals of the other senses in creating a perception of spatial orientation.
What are the inputs and outputs of the vestibular system?
Input - visual, rotation/gravity and pressure
Output - postural control and ocular reflex which prevents falling
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 is special about the vestibular system?
only sensory organ specialised to transduce absolute motion in space.
List the disorders of the vestibular system
False perception of movement in space – ‘vertigo’.
Instability of gait and posture. – ‘vestibular ataxia’.
Inability to stabilise the eyes – ‘vestibular nystagmus’ eyes start moving in direction of lesion in unilateral lesions;
‘oscillopsia’ during head movement in bilateral vestibular lesions - everything appears to be shaking stabilisation of eyes lost
Slight impairment of orthostatic control in the acute phase of vestibular loss.
Severe nausea and vomiting in the acute phase of unilateral vestibular loss.
Loss of co-ordination on directional reorientation; motion intolerance, oversensitivity to visual motion in the environment.
What are the two otolith organs?
Saccule
Utricle
Summarise the basic mechanism of physiology
Hair cell - synapses with the primary neuron dendrite (cell body in scarpas ganglion) projecting to vestibular nuclei in brain
Define vertigo
False perception of movement in a space
Define vestibular ataxia
Instability of gait and posture
Define vestibular nystagmus
Inability to stabilise the eyes in unilateral lesions, eyes move towards the lesion.
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, especially during head movement.
How is hair cell stimulated?
Deflection of inertial resistance to acceleration in in otoliths and endolymphatic fluid rotation (canals)
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 cilia of these hair cells.
There is one kinocilium and the rest are stereocilia
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
How do the otolith organs give a signal of linear acceleration in all 3-dimensional directions?
Vector sum of utricular and saccular stimulation patterns gives signal of linear acceleration in all 3-dimensional 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
How are the 5 organs of the inner ear connected?
Saccule to cochlea
Utricle to saccule through utricosaccular duct
All 3 semi-circular canals are connected to utricle (ant. lateral and post. )
Where are endolymph and perilymph found?
Endolymph is within the structure where as perilymph is between organ and bone. Function of the fluid is auditory to move hairs for hair cell transduction.
How are the canals and otolith organs stimulated differently?
Otolith organs - linear acceleration
Canals - angular acceleration signal of approximate angular velocity.
How does the labyrinth organ /inner ear sit in the skull?
REFER TO DIAGRAM on printout.
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)
What are the types of hair cells?
Type 1 - more in number, different afferents, indirect efferent (inner ear)
Type 2 - direct afferents and efferents
They are not segregated and not as differentiated as in hearing.
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?
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 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
What is BPPV and how is it treated?
Benign paroxysmal positional vertigo
It is caused by otoconial debris in the canals and is provoked by head movement
Debris floating in the canal stimulates the ampulla and generates false signals of head rotation
Cured by turning the head vigorously in the opposite direction to that which provokes the vertigo, through 360 degrees, flushing out the debris
Treatment name: particle repositioning manoeuvre
• Dizziness?
an illusion of self- and/or environmental motion
