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Vestibular systems and the eyes: a n overview

Vestibular systems and the eyes: a n 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…

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Vestibular systems and the eyes: a n overview

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  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 • 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 • 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 • 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!!!

  9. Reflexes • Pupillary light reflex • The near response • Vestibulo-ocular reflex

  10. 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

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

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

  13. 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

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

  15. 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)

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

  17. 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

  18. 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

  19. 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

  20. 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

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

  22. 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

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

  24. 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

  25. 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!

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