AUDITORY AND VESTIBULAR SYSTEMS Key points Relations of external, middle and inner ear Understanding the relationship between the cochlear and vestibular apparatus in the bony labyrinth Structure of cochlear duct and organ of Corti Receptor deformation and transduction Central auditory pathways Tonopicity – from basilar membrane to cortex Basic knowledge of efferent pathways, auditory reflexes Structure of vestibular apparatus - utricle and saccule (static), semicircular canals (kinetic) Knowledge of major central vestibular pathways Lesions and disturbances of the auditory and vestibular systems Reading Nolte (4th Ed), pgs 325-357 Fitzgerald, pgs 130-140 Kiernan (7th Ed), pgs 382-405

External, middle and inner ear – the bony labyrinth

Cochlear duct and the organ of Corti scala vestibuli, cochlear duct, scala tympani, Reissner’s membrane, basilar membrane, organ of Corti Note changes in width of basilar membrane

Cochlear duct contains endolymph: high K+, low Na+ The organ of Corti

Relations between the middle and inner ear Changes in the width of the basilar membrane – vibration and frequency response

Shearing forces in the organ of Corti – mechanical deformation and signal transduction

Ascending (central) auditory pathways Tonotopicity, lesions, feedback at just about all levels including efferent projections back to the peripheral receptors in the cochlear, reflexes

Balance, equilibrium, head movements Vestibular system Balance, equilibrium, head movements

Utricle and saccule ‘STATIC LABYRINTH’ Semicircular canal (inner ear) ‘KINETIC LABYRINTH’ Receptors and associated specializations

Cilia and kinocilium – embedded in inertial mass Excitation when deformation towards kinocilium Inhibition when deformation away from kinocilium

View of the horizontal ducts from above, showing how paired semicircular canals work together to provide a bilateral indication of head movement

Central pathways – vestibular nerve and nuclei, medial longitudinal fasciculus, cerebellum, vestibulospinal pathways, reticular formation, cortex Disturbances of vestibular system – motion sickness, vertigo, Meniere’s disease

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