Vestibular Systems

Question 1

Which structures in the body are responsible for angular rotation of the head?

Answer 1

3 Semi-circular canals

Question 2

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

Answer 2

Otolith organs

Question 3

What are the 2 otolith organs?

Answer 3

Saccule

Utricle

Question 4

What are 6 main functions of the vestibular system?

Answer 4

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

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
Eyes start moving in the direction of the lesion
Intermittent “resetting” of eye position with fast saccades

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

Describe the saccule

Answer 10

Bed of sensory cells situated in inner ear
Orientated in vertical plane
Detect linear acceleration + head tilt in vertical plane

Question 11

Describe the structure of otolith hair cells

Answer 11

Have an overlaying layer of Otoconia
Project normal to the plane of the saccule
Have 1 kinocilia per cell, the rest are stereocillia

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 sensitivities of the saccule

Answer 16

Directional sensitivities in all combinations of vertical + antero-posterior directions

Question 17

Describe the structure of the utricle and its sensitivities.

Answer 17

Oriented horizontally with the hair cells projecting vertically
Directional sensitivities in all combinations of lateral + 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 + saccular stimulation patterns gives signal of linear acceleration in all 3D 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 + 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

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

Unopposed signal coming from the intact side

= partial impairment of sensitivity for movement in accelerator direction of that canal

Question 22

Why would a unilateral canal lesion cause vertigo?

Answer 22

Unopposed tonus of the intact canal gives a signal as if the head is rotating to the intact side.
Gives feeling of intense spinning

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 the opposite 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

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

Question 28

How would you test if a patient has oscillopsia?

Answer 28

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.

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

Seconds: Benign Paroxysmal Positional Vertigo (BPPV) Minutes: Vertebrobasilar insufficiency Hours: Meniere’s Syndrome Days: Vestibular neuritis Fluctuating/continuous : Uncompensated vestibular lesion Silent: Acoustic neuroma

Question 34

What is BPPV and how is it treated?

Answer 34

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

Question 35

List 3 inputs to the vestibular system

Answer 35

``` Visual system Inner ear (info. on rotation + gravity) Proprioception in joints + limbs ```

Question 36

List 3 outputs of the vestibular system

Answer 36

Ocular reflex Postural control Nausea

Question 37

Where is the Labyrinth located?

Answer 37

In the petrous part of the temporal bone (well protected)

Question 38

Name the fluids found inside and outside the structures of the ear. What is the function of the fluid?

Answer 38

Inside: Endolymph Outside: Perilymph Fluid helps move the hairs

Question 39

Which 2 semi-circular canals are joined on their non-ampulla side?

Answer 39

Anterior/ Superior + Posterior/ Inferior

Question 40

Describe orientation of the semi-circular canals

Answer 40

Anterior: 45 degrees antero-superiorly Posterior: 45 degrees postero-inferiorly Lateral: "arm" points externally, perpendicular

Question 41

What 2 types of hair cells are found in the vestibular system? What type of receptors are these?

Answer 41

Type 1: More numerous, more afferents, less efferents, round shape Type 2: Less afferents, more efferents, similar shape to outer hair cells Mechanoreceptors

Question 42

What are Otoliths? Where are they found?

Answer 42

Carbonate crystals | On top of the gelatinous matrix that lies on the surface of the hair cells

Question 43

How does movement effect Otoliths?

Answer 43

Crystals move, generating pressure, moving the "jelly" + deflecting hair cells

Question 44

How are the hair cells organised from the striola?

Answer 44

They have opposing orientations | Thus 1 stimuli causes both excitation + inhibition in the same organ