Download - Vestibulocochlear : An overview
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Vestibulocochlear: An overview
Thursday 17th November 2011
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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
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The Vestibular System
• Located in the inner ear• Semicircular canals– Anterior vertical– Posterior vertical– Horizontal
• Otolith organs– Utricule– Saccule
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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)
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Mechanism - Demo
• Cilia of hair cells within endolymph• Inertia movement of endolymph causes hair
cells to deform• Displacement of hair cells causes
depolarisation
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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
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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
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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
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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
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Vestibular pathways
• Vestibular nerve ganglion (Scarpa’s ganglion)• Vestibular nerve• Vestibulocochlear nerve• Vestibular nuclei– In the brainstem, at the floor of the 4th ventricle
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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
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Vestibulo-spinal
• Lateral vestibulo spinal tract– Ipsilateral– Influence limb muscles
• Medial vestibulo spinal tract– Bilateral– Influence neck and back muscles
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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
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Ear
• Sound conduction• Sound transduction• Sound pathways
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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
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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
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Conductive deafness
• Wax• Otitis media• Otosclerosis of ossicles• Perforated tympanic membrane• Congenital malformations
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Cochlear
• Pressure equalisation by oval and round window movements
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Sound transduction
• Sound waves causes vibration of vestibular and basilar membranes
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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
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Demo
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Pitch
• Higher frequencies towards the base of basilar membrane
• Lower frequencies towards the apex of basilar membrane
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Auditory pathways
• Bilateral• Tonotropy– Pattern of pitch is preserved
• Lateral inhibition• Inferior collicus– To Reticular Activating System• Startle reflex
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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
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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