Vestibular system

Question 1

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

Answer 1

Semi-circular canals

Question 2

Which structures in the body sense the acceleration of the headand the strength of gravity?

Answer 2

Otolith organs

Question 3

What are the two otolith organs?

Answer 3

Saccule

Utricle

Question 4

What are the main functions of the vestibular system?

Answer 4

Subserve perception of motion in space and tilt (with respect to gravity) Provide reflex balance reactions to sudden instability of gait/posture (vestibulo-spinal reflexes) Stabilise the eyes on fixed targets during head movement, preserving acuity (vestibulo-ocular reflexes) Assist in control of heart rate and blood pressure during rapid up-down tilts Assist synchronisation of respiration with body reorientations Provokes motion sickness

Question 5

Define vertigo.

Answer 5

False perception of movement in space

Question 6

Define vestibular ataxia

Answer 6

Instability of gait or posture

Question 7

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

Answer 7

Vestibular nystagmus

The eyes start moving in the direction of the lesion

Question 8

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

Answer 8

Oscillopsia

Everything appears to be shaking – the ability to stabilise the eyes is lost

Question 9

What are some other consequences of vestibular loss?

Answer 9

Slight impairment of orthostatic control Severe nausea and vomiting Loss of coordination on directional reorientation, motion intolerance, oversensitivity to visual motion in the environment

Question 10

What type of cell is involved in the detection of movement in the vestibular system?

Answer 10

Hair cells

Question 11

Describe the cilia of these hair cells.

Answer 11

There is one kinocilium and the rest are stereocilia

Question 12

What does the hair cell fibre synapse with and where does it project?

Answer 12

It synapses with a primary neurone dendrite (cell body in Scarpa’s ganglion) They project to the vestibular nuclei in the brainstem

Question 13

What stimulates hair cells?

Answer 13

In Otoliths: deflection by forces of inertial resistance to acceleration
In Semi-circular Canals: endolymphatic fluid rotation

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 orientation and sensitivities of the saccule.

Answer 16

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 17

Describe the orientation and sensitivities of the utricle.

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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 20

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

Answer 20

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

Question 21

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

Answer 21

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 22

Why would a unilateral canal lesion cause vertigo?

Answer 22

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 23

Why would acute unilateral vestibular disorder cause vestibular nystagmus?

Answer 23

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 24

Where do superior and medial vestibular neurones project?

Answer 24

They project to the motor nuclei supplying extraocular muscles.

Question 25

Describe the path of medial vestibular neurones.

Answer 25

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)

Question 26

Describe the path of superior vestibular neurones.

Answer 26

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

Question 27

What is oscillopsia?

Answer 27

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 28

How would you test if a patient has oscillopsia?

Answer 28

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 29

What are the effects of bilateral vestibular disorder on gait?

Answer 29

Mild gait ataxia

Question 30

What are the effects of unilateral vestibular disorder on gait?

Answer 30

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

Question 31

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

Answer 31

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 32

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

Answer 32

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 33

State a common cause of vestibular vertigo that lasts: a. Seconds b. Minutes c. Hours d. Days e. Fluctuating/continuous f. Silent

Answer 33

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 34

What is BPPV and how is it treated?

Answer 34

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 falsesignals 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