A8: Vestibular System

Question 1

Functions of the vestibular system

Answer 1

Mediates position of eyes and body relative to the external environment

Question 2

Where are the vestibular sensory cells located?

Answer 2

Maculae: in utricle and saccule of inner ear labrynth

Crista Ampullaris: in ampullas of semicircular ducts

Question 3

Function and basic anatomy of Maculae

Answer 3

Detect linea motion of the head with respect to gravity
Apical stereocilia and kinocilia, on sensory cells of the maculae, extend into the Otolithic Membrane: gelatinous material containing calcium crystals, denser than endolymph

Question 4

How does tilting the head affect the maculae and crista ampullaris?

Answer 4

Tilting of head -> movement of otolithic membrane -> deflection of hairs. Deflection of stereocilia towad the kinoclium results in action potential.

Tilting head forwards -> detected by hair cells of utricle macula

Tilting head to side -> detected by hair cells of saccule macula

Question 5

Describe the cellular events following deflection of (vestibular: maculae/crista ampulla sensory cell) stereocilia in the direction of the kinocilium

Answer 5

Tip links under high tension -> opening of mechanically gated ion channels -> influx of K+ -> Opening of voltage-gated Ca++ channels -> release of Glutamate or Aspartate -> triggers AP in afferent bipolar cells (whose cell bodies are in Scarpa’s/Vestibular ganglion) -> central processes forming vestibular nerve -> joins with cochlea nerve to form vestibulocochlear nerve (CNVIII)

Question 6

Describe the location, basic anatomy and function of the Crista Ampullaris

Answer 6

Located in the ampullas of the semicircular ducts

Hair cells of CA sensory cells embedded in ‘Cupula’ - gelatinous mass, same density as endolymph - extends somewhat into the ampulla. Moved about by movement of endolymph.

Detect rotational movement in any direction. This is because of the specific spacial orientation of the lateral (hoizontal) SC duct, anterior (superior) SC duct and posterior SC ducts.

Question 7

How is movement detected by crista ampulla?

Answer 7

Upon initiation of Rotation Movement -> Endolymph lags because of ‘inertia’ (phenomenon of resistance of matter to movement)

The lag of the endolymph distorts the shape of the cupula, causing deflection of hair cells.

As rotation ends -> endolymph keeps moving for a bit -> cupula deflects in opposite direction

Question 8

Describe the relative orientations of the semicircular ducts:

Answer 8

Horizontal canal: tilted backwards 30 degrees

Anterior canal on one side runs parallel to the Posterior canal on the opposite (contralateral) side

Question 9

Explain the response of vestibular hair cells to movement of the head on the horizontal plane

Answer 9

Head turns left -> Endolymph in left and right SC canals flows to left -> receptos in LEFT SC canals depolarise. Receptors in right SC canals are inhibited.

And vice versa.

Question 10

Describe anatomy of Vestibular (scarpa’s) ganglion

Answer 10

Contains cell bodies of bipolar afferent neurons from crista ampulla (of SC canals) and maculae (of utricles and saccules)

Central processes form vestibular component of CNVIII

Which terminates in Vestibular Nuclei

Question 11

Aside from vestibular component of VIII, what are the other inputs to vestibular nuclei?

Answer 11

• Reciprocal vestibular projections
• Cerebellum (flocculonodular lobe)
• Spinal cord (some fibres from posterior spinocerebellar tract)

Question 12

List the Outputs from the Vestibular Nuclei

Answer 12

Ipsilaterally: Lateral VestibuloSpinal Tract -> for postural adjustments (e.g. spinning around -> dizziness, staggering, etc. this is due to exaggerated activity of the LVST)

Contralaterally: Medial VestibuloSpinal Tract. Compensatory neck movements.

Output goes to: cerebellum, CNIII, IV and VI nuclei, thalamus, reticular formation, emetic centre (control of vomiting)

Question 13

What is the Vestibuloocular Reflex? (VOR)

Answer 13

The VOR generates eye movements to compensate for head movements. I.e. to remain focussed on stimulus whilst moving head.

Can be overriden voluntarily by the Flocculus of the Cerebellum

Question 14

What is the Oculocephalic reflex? How do you test for it?

Answer 14

(Doll’s eye movement)
A form of VOR
Suggestive of damage to the brainstem in comatosed patients.
To test in comatosed patient, gently turn head side to side, and check for eye movements with turning.
Absent dolls sign = eyes remain fixed in mid position.

Question 15

Describe Nystagmus

What can nystagmus assess?

Answer 15

Fast eye movement, involving signals from the reticular formation.
Very rapid movement of the eye in the opposite direction to that in VOR.
Crista ampullaris of SC canals are being stimulated while head is not in motion.

Nystagmus and VOR can be sued to assess level of brainstem damage in patients

Question 16

Describe Optokinetic Nystagmus

Answer 16

When there are Slow ‘smooth persuits’ of the visual stimulus (e.g. looking at a tree as you drive past it), coupled with rapid corrective phases ‘saccades’ to catch up with actual movement.

I.e. imagine looking at passing by countryside through train window

Question 17

Describe Caloric Nystagmus & how to test

Answer 17

Endolymphatic convection induced by cool water or air

Head is tilted back at 60 degrees

Idea: cool water in right ear ilicits similar response as rotation of the head to the left

Mnemonic: COWS: cool, opposite; warm, same.

COLD = head turn and rapid eye movement (nystagmus) occurs in the OPPOSITE direction to ear in which cold water is put. Then slow eye movement (VOR) occurs towards the side of cold water.

WARM = head turn and nystagmus is in SAME direction as side of warm water. Then slow eye movement (VOR) occurs in opposite direction.

Question 18

What symptoms would occur due to tumour involving the vestibular nerve?

Answer 18

Dizziness
Nausea
Spacial Disorientation

Question 19

What symptoms would occue due to tumour at cerebellopontine angle?

Answer 19

Facial (VII) and Vestibulocochlear (VIII) nerve involvement…

Question 20

Loss of pain and temperature from ipsilateral (left) side of the face: where is lesion? Due to?

Answer 20

Left Spinal Trigeminal Nucleus and Tract

issues with posterior inferior cerebellar artery

Question 21

Dysarthria and Dysphagia: lesion is? due to?

Answer 21

Nucleus Ambiguous

Posterior inferior cerebellar artery

Question 22

Loss of PAIN and TEMPERATURE on the contralateral (right) side of the body: lesion is where? due to

Answer 22

Left Spinothalamic Tract

posterior inferior cerebellar artery

Question 23

GAIT ATAXIA on the ipsilateral (right) side of the body: lesion is where? due to

Answer 23

Right Spinocerebellar Tracts

Posterior inferior cerebellar artery

Question 24

Vertigo, Abnormal Nystagmus: lesion?

Due to?

Answer 24

Vestibular Nucleus

Posterior inferior cerebellar artery

Question 25

Stroke of Posterior Inferior Cerebellar Artery causes what type of general syndrome?

Answer 25

Lateral Medullary Syndromes.

Question 26

Basic Functions of the different areas of the inner ear:

Answer 26

Cochlea Duct - hearing
Utricle and Saccule (of vestibule) - Linear Acceleration/Orientation of the head, relative to gravity
Semicircular Ducts (anterior or superior, posterior and lateral) - Angular Acceleration

Question 27

What are the 2 sealed openings between the tympanic cavity and inner ear? Where are they situated?

Answer 27

Oval Window - vestibular - Closed by Stapes

Round Window - cochlear - closed by the secondary typmpanic membrane

Both reside on Medial Wall (between tympanic canity and inner ear)

Question 28

How does Lateral Medullary Syndrome present, and what is principal cause?

Answer 28

Principally caused by occlusion/stroke of Posterior Inferior Cerebellar Artery (supplies cerebellum and choroid plexus of the 4th ventricle)

Diagnostic presentation for LMS is:
Loss of pain/temp and sensation to trunk and extremities on contralateral side to stroke/occlusion. And loss of pain/temp and sensation to face on ipsilateral side.