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NS44 Hearing + Vestibular Function (i) (auditory + vestibular systems…
NS44 Hearing + Vestibular Function (i)
auditory + vestibular systems
share components of the bony + membranous labyrinths
hair cells: mechanical transducers
info transmitted through vestibulocochlear N (CN VIII)
cochlear/auditory part transduces sound
vestibular part transducers info about position + movement of head in space
ear consists of 3 parts: outer (ext), middle, inner
Middle ear
contains air
tympanic membrane (eardrum)
moves in due to sound pressure
connected to chain of ossicles: malleus, incus, stapes
stapes connect to oval window (opening to inner ear)
round window (2nd opening to inner ear) is adjacent to oval window
inner ear
contains fluid
perilymph in scala vestibuli + tympani
endolymph in scala media (cochlear duct)
contains bony + membranous labyrinths
form cochlea + vestibular apparatus
characteristics of sound
longitudinal air vibrations
specific frequency (Hz, cycles/s) + intensity (dB)
humans can detect between 20-16000 Hz
optimum = 100-3000 Hz (speech)
presbyacusis: decreased ability to detect higher frequencies (normal part of ageing)
Sound transduction
vibrations @ tympanic membrane
ossicles
footplate of stapes
oval window
fluid displaced in scala vestibuli
pressure wave transmitted through basilar membrane
scala tympani
impedance matching
occurs as sounds moves from air (middle ear) to fluid (inner ear)
depends on...
ratio of tympanic membrane SA: oval window SA
mechanical advantage of lever system in ossicle chain
improves hearing by 10-20 dB
audiometry
ear presented with different frequencies + intensities
audiogram plotted to show thresholds pf each ear + hence what frequencies + intensities they can hear
= audibility curve
Meniere's Disease
unilateral N deafness
tinnitus, nystagmus, vertigo, ataxia, vomiting
cause unknown (thought to be endolymph buildup or decreased perfusion to inner ear)
treat tinnitus by injecting ototoxic drugs into inner affected ear (results in deafness)