1. Vestibular systems and the eyes: an overview
Ken Wu

2. Disclaimer
This tutorial is a simple and conceptual guide to the vestibular system and the eyes
If there are any conflicts between my slides and the lecturers, THE LECTURER IS ALWAYS RIGHT…
…maybe not always but they set your exams so if in doubt, refer back to their teaching

3. Eye Structure + circulation Movement Light detection Reflexes
Learn from slides/anatomy textbook
Movement
Light detection
Reflexes

4. Intrinsic muscles of the eye
Superior/inferior oblique
Abduct + depress + internal rotate/adduct + depress + external rotate eyes
Medial/lateral rectus
Adduct/Abduct eyes
Superior/inferior rectus
Elevates/Depress eyes
SO4LR6AO3

5. Testing the intrinsic muscles
Superior oblique
Adduct then depress
Inferior oblique
Abduct then depress

6. Eye movements Saccade vs smooth pursuit Duction Version
Movement in one eye
Version
Movement in both eyes in same direction
Innervation to both eyes
Relaxation of antagonist muscles
Convergence vs Divergence

7. Light detection ‘Backwards’ retina Visual acuity Colour
Sclera, photoreceptors, bipolar cells, ganglion cells
Visual acuity
20/20 means you can read at 20 feet what a person with normal acuity can read at 20 feet
Colour
3 cones: Red, Green, Blue

8. Primary visual pathway
Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus
Optic radiation
Primary visual cortex
Know your visual field defects!!!

10. Reflexes Pupillary light reflex The near response
Vestibulo-ocular reflex

11. Afferent pupillary light reflex
Rods and cones
Retinal ganglion
Optic nerve
Optic chiasm
Left and right optic tracts
Left and right pretectal nucleus
Edinger-Westphal nucleus

12. Efferent pupillary light reflex
Edinger-Westphal nucleus
Along oculomotor nerves
Ciliary ganglion
Synapse with parasympathetic
Iris constrictor muscle

13. Near response Triad Efferent = oculomotor nerve Pupillary miosis
Convergence
Accomodation
Efferent = oculomotor nerve

14. Reflex defects Optic (afferent) damage Oculomotor (efferent) damage
Loss of ipsilateral direct and contralateral consensual
Oculomotor (efferent) damage
Loss of ipsilateral direct and consensual
Relative Afferent Pupillary Defect (Marcus Gunn pupil)
Partial optic nerve lesion
Pupil in affected eye appears to dilate in swinging-torch test
Argyll-Robertson pupil
Can accommodate but cannot react

15. The Vestibular System Located in the inner ear Semicircular canals
Anterior vertical
Posterior vertical
Horizontal
Otolith organs
Utricule
Saccule

16. Semicircular Canals Detect angular acceleration
3 sets covering rotation in the 3 planes
Anterior vertical = coronal (head – shoulder)
Posterior vertical = sagittal (head nodding)
Horizontal = transverse (head shaking)

17. Mechanism - Demo Cilia of hair cells within endolymph
Inertia movement of endolymph causes hair cells to deform
Displacement of hair cells causes depolarisation

18. Angular acceleration Bilateral stimulation
Rotation to one side stimulates the same side AND inhibits the opposite side
There is a tonic firing rate – normally the left and right balance out

19. Otolith organs Detects gravity and linear acceleration Saccule
Arranged in vertical plane
Hair cells are horizontal
Therefore detects vertical movement
Utricule
Arranged in horizontal plane
Hair cells are vertical
Therefore detects horizontal movement

20. Mechanism - Demo
Cilia of hair cells covered by otoliths – a gelatinous matrix containing calcium carbonate crystals
Inertia of otoliths cause hair cells to deform
Displacement of hair cells cause depolarisation

21. Gravity and Linear acceleration
Once hair cells are displaced, they stay displaced
Therefore tonic firing exists in the saccule due to the presence of gravity
It also means head tilt and lying down are also detected even after several hours (e.g. sleep)
Inertia of the otoliths in linear acceleration temporarily causes displacement of the utricule hair cells

22. Vestibular pathways Vestibular nerve ganglion (Scarpa’s ganglion)
Vestibulocochlear nerve
Vestibular nuclei
In the brainstem, at the floor of the 4th ventricle

23. Vestibulo-ocular
Vestibular nucleus stimulates contralateral VI nucleus
VI nucleus
Abduct eye
Stimulates contralateral III nucleus – adducts opposite eye
Causes vestibulo-ocular reflex
Head rotate left, eyes moves right to maintain gaze
Function is to maintain gaze

25. Vestibulo-spinal Lateral vestibulo spinal tract
Ipsilateral
Influence limb muscles
Medial vestibulo spinal tract
Bilateral
Influence neck and back muscles

26. Vestibular disorders Vestibular nystagmus Vestibular ataxia
Unopposed tonus of intact canal
Eyes driven to lesioned side
Fast saccade beat to intact side
Vestibular ataxia
Body/head fall towards lesioned side

27. Any questions?
me at or
Visit the ICSM Year 1+2 past paper bank Facebook group
Good luck!