Vestibular System

Question 1

What are the normal functions of the vestibular system?

Answer 1

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.

Question 2

What are the inputs and outputs of the vestibular system?

Answer 2

Input - visual, rotation/gravity and pressure

Output - postural control and ocular reflex which prevents falling

Question 3

Which structures in the body are responsible for angular (rotational) motion of the head?

Answer 3

Semi-circular canals

Question 4

Which structures in the body sense the acceleration of the head and the strength of gravity?

Answer 4

Otolith organs

Question 5

What is special about the vestibular system?

Answer 5

only sensory organ specialised to transduce absolute motion in space.

Question 6

List the disorders of the vestibular system

Answer 6

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.

Question 7

What are the two otolith organs?

Answer 7

Saccule

Utricle

Question 8

Summarise the basic mechanism of physiology

Answer 8

Hair cell - synapses with the primary neuron dendrite (cell body in scarpas ganglion) projecting to vestibular nuclei in brain

Question 9

Define vertigo

Answer 9

False perception of movement in a space

Question 10

Define vestibular ataxia

Answer 10

Instability of gait and posture

Question 11

Define vestibular nystagmus

Answer 11

Inability to stabilise the eyes in unilateral lesions, eyes move towards the lesion.

Question 12

What happens to the ability of the brain to stabilise the eyes in bilateral vestibular lesions?

Answer 12

Oscillopsia

Everything appears to be shaking – the ability to stabilise the eyes is lost, especially during head movement.

Question 13

How is hair cell stimulated?

Answer 13

Deflection of inertial resistance to acceleration in in otoliths and endolymphatic fluid rotation (canals)

Question 14

Describe how the hair cell receptor potential can be changed.

Answer 14

Depolarisation = movement towards the kinocilium
Hyperpolarisation = movement away from the kinocilium

Question 15

Describe how ganglion cell discharge can be changed.

Answer 15

Towards the kinocilium = increased firing frequency

Away from the kinocilium = decreased firing frequency

Question 16

Describe the cilia of these hair cells.

Answer 16

There is one kinocilium and the rest are stereocilia

Question 17

Describe the orientation and sensitivities of the saccule.

Answer 17

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

Question 18

Describe the orientation and sensitivities of the utricle.

Answer 18

Utricle is oriented almost horizontally with the hair cells projecting vertically
Directional sensitivities in all combinations of lateral and antero-posterior directions

Question 19

How do the otolith organs give a signal of linear acceleration in all 3-dimensional directions?

Answer 19

Vector sum of utricular and saccular stimulation patterns gives signal of linear acceleration in all 3-dimensional directions

Question 20

Describe the structure and function of the semi-circular canals.

Answer 20

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

Question 21

Describe the firing of the canals when the head is still.

Answer 21

Each canal has a tonic firing rate so that they equal out when the head is still

Question 22

How are the 5 organs of the inner ear connected?

Answer 22

Saccule to cochlea
Utricle to saccule through utricosaccular duct
All 3 semi-circular canals are connected to utricle (ant. lateral and post. )

Question 23

Where are endolymph and perilymph found?

Answer 23

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.

Question 24

How are the canals and otolith organs stimulated differently?

Answer 24

Otolith organs - linear acceleration

Canals - angular acceleration signal of approximate angular velocity.

Question 25

How does the labyrinth organ /inner ear sit in the skull?

Answer 25

REFER TO DIAGRAM on printout.

Question 26

What are the effects of loss of canal function on one side?

Answer 26

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

Question 27

Why would a unilateral canal lesion cause vertigo?

Answer 27

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.

Question 28

Why would acute unilateral vestibular disorder cause vestibular nystagmus?

Answer 28

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)

Question 29

What are the types of hair cells?

Answer 29

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.

Question 30

Where do superior and medial vestibular neurones project?

Answer 30

They project to the motor nuclei supplying extraocular muscles.

Question 31

Describe the path of medial vestibular neurones.

Answer 31

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)

Question 32

Describe the path of superior vestibular neurones.

Answer 32

Project ipsilaterally to the oculomotor and trochlear nuclei to generate VERTICAL vestibulo-occular reflexes

Question 33

What is oscillopsia?

Answer 33

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

Question 34

How would you test if a patient has oscillopsia?

Answer 34

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.

Question 35

What are the effects of bilateral vestibular disorder on gait?

Answer 35

Mild gait ataxia

Question 36

What are the effects of unilateral vestibular disorder on gait?

Answer 36

Tendency for the body and head to lean or fall to the lesioned side

Question 37

Describe the path and function of the lateral vestibulo-spinal tract.

Answer 37

Descends ipsilaterally in the ventral funiculus of the spinal cord
Axons terminate in lateral part of ventral horn
Influence motor neurones to limb muscles

Question 38

Describe the path and function of the medial vestibulo-spinal tract.

Answer 38

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

Question 39

State a common cause of vestibular vertigo that lasts:

Answer 39

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

Question 40

What is BPPV and how is it treated?

Answer 40

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

Question 41

• Dizziness?

Answer 41

an illusion of self- and/or environmental motion