Vestibular systems and the eyes:
an overview
Ken Wu
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
Eye
• Structure + circulation
– Learn from slides/anatomy textbook
• Movement
• Light detection
• Reflexes
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
Testing the intrinsic muscles
• Superior oblique
– Adduct then depress
• Inferior oblique
– Abduct then depress
Eye movements
• Saccade vs smooth pursuit
• Duction
– Movement in one eye
• Version
– Movement in both eyes in same direction
– Innervation to both eyes
– Relaxation of antagonist muscles
• Convergence vs Divergence
Light detection
• ‘Backwards’ retina
– 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
Primary visual pathway
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Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus
Optic radiation
Primary visual cortex
Know your visual field defects!!!
Reflexes
• Pupillary light reflex
• The near response
• Vestibulo-ocular reflex
Afferent pupillary light reflex
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Rods and cones
Retinal ganglion
Optic nerve
Optic chiasm
Left and right optic tracts
Left and right pretectal nucleus
Edinger-Westphal nucleus
Efferent pupillary light reflex
• Edinger-Westphal nucleus
• Along oculomotor nerves
• Ciliary ganglion
– Synapse with parasympathetic
• Iris constrictor muscle
Near response
• Triad
– Pupillary miosis
– Convergence
– Accomodation
• Efferent = oculomotor nerve
Reflex defects
• Optic (afferent) 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 swingingtorch test
• Argyll-Robertson pupil
– Can accommodate but cannot react
The Vestibular System
• Located in the inner ear
• Semicircular canals
– Anterior vertical
– Posterior vertical
– Horizontal
• Otolith organs
– Utricule
– Saccule
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)
Mechanism - Demo
• Cilia of hair cells within endolymph
• Inertia movement of endolymph causes hair
cells to deform
• Displacement of hair cells causes
depolarisation
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
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
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
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
Vestibular pathways
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Vestibular nerve ganglion (Scarpa’s ganglion)
Vestibular nerve
Vestibulocochlear nerve
Vestibular nuclei
– In the brainstem, at the floor of the 4th ventricle
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
Vestibulo-spinal
• Lateral vestibulo spinal tract
– Ipsilateral
– Influence limb muscles
• Medial vestibulo spinal tract
– Bilateral
– Influence neck and back muscles
Vestibular disorders
• Vestibular nystagmus
– Unopposed tonus of intact canal
– Eyes driven to lesioned side
– Fast saccade beat to intact side
• Vestibular ataxia
– Unopposed tonus of intact canal
– Body/head fall towards lesioned side
Any questions?
• Email me at ken.wu09@imperial.ac.uk or
icsm.ao12ge@imperial.ac.uk
• Visit the ICSM Year 1+2 past paper bank
Facebook group
• Good luck!