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Vestibulocochlear : An overview

Vestibulocochlear : An overview. Ken Wu ken.wu09@ imperial.ac.uk Thursday 17 th November 2011. Disclaimer. This tutorial is a simple and conceptual guide to the vestibulocochlear system If there are any conflicts between my slides and the lecturers, THE LECTURER IS ALWAYS RIGHT…

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Vestibulocochlear : An overview

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  1. Vestibulocochlear: An overview Ken Wu ken.wu09@imperial.ac.uk Thursday 17th November 2011

  2. Disclaimer • This tutorial is a simple and conceptual guide to the vestibulocochlear system • 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. The Vestibular System • Located in the inner ear • Semicircular canals • Anterior vertical • Posterior vertical • Horizontal • Otolith organs • Utricule • Saccule

  4. 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)

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

  6. 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

  7. 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

  8. 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

  9. 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

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

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

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

  13. 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

  14. Ear • Sound conduction • Sound transduction • Sound pathways

  15. Sound conduction • Outer ear • Pinna, external acoustic meatus • Sound collection and conduction • Middle ear • Air filled chamber in bone • Malleus • Incus • Stapes (smallest bone in the body) • Sound amplification

  16. Middle ear protection • Reflex contraction of muscles dampens amplitude • Tensor tympani – malleus • Stapedius – stapes • Stapedius supplied by VII, thus Bell’s palsy causes hyperacusis • Eustacian tube allows pressure equalisation

  17. Conductive deafness • Wax • Otitis media • Otosclerosis of ossicles • Perforated tympanic membrane • Congenital malformations

  18. Cochlear • Pressure equalisation by oval and round window movements

  19. Sound transduction • Sound waves causes vibration of vestibular and basilar membranes

  20. Organ of Corti • Basilar membrane vibration • Tectorial membrance provides shear force • Stereocilia displaced away from modiolus (central axis of cochlea • K channels open - depolarisation • Stereocilia displaced towards modiolus • K channels closed - hyperpolarisation • Endolymph provides the ions

  21. Demo

  22. Pitch • Higher frequencies towards the base of basilar membrane • Lower frequencies towards the apex of basilar membrane

  23. Auditory pathways • Bilateral • Tonotropy • Pattern of pitch is preserved • Lateral inhibition • Inferior collicus • To Reticular Activating System • Startle reflex

  24. Auditory cortex • Primary • In temporal lobe near central sulcus • Subdivided areas according to frequencies • Analyses duration, intensity and sound patterns • Secondary • Complex sound patterns • Higher functions e.g. speech

  25. Weber and Rinne • Rinne • Pinna vs mastoid process • Rinne +ve = pinna > mastoid • Normal!!! • Sensorineural deafness • Rinne –ve = mastoid vs pinna • Conductive deafness • Weber • Midline of forehead • Equally loud = normal • L > R • R sensorineural deafness • L conductive deafness

  26. Any questions? • Email me at ken.wu09@imperial.ac.uk • Good luck!

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