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

• 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

• 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 swinging-torch

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

• 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!