BIO 304 · Human Anatomy & Physiology
Hearing & Equilibrium
Special Senses · Module 8
A reference for the Hearing & Equilibrium video. The ear is a mechanical transducer. Sound vibrates the eardrum, three tiny bones, fluid, and finally hair cells. The vestibular system uses similar hair cells to track head movement.
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- Identify the structures of the outer, middle, and inner ear.
- Trace sound from the air to an action potential on the auditory nerve.
- Compare the roles of the semicircular canals, utricle, and saccule in equilibrium.
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Hearing
Outer ear
- Auricle (pinna)collects and funnels sound
- External auditory canalamplifies and protects
- Tympanic membrane (eardrum)vibrates with sound waves
Middle ear (air-filled)
- Malleus, incus, stapesthree smallest bones; amplify and transmit vibration
- Stapes → oval windowtransmits to fluid-filled inner ear
- Pharyngotympanic (Eustachian) tubeequalizes pressure with throat
Inner ear: cochlea
- Cochleasnail-shaped fluid-filled tube; coiled 2.5 turns
- Scala vestibuliupper chamber; receives stapes vibration
- Scala tympanilower chamber; ends at round window
- Cochlear duct (scala media)middle chamber; contains organ of Corti
- Organ of Cortisits on basilar membrane; hair cells topped by tectorial membrane
- Hair cellsmechanoreceptors; bend → depolarize → release NT
- Cochlear branch of CN VIIIcarries signal to auditory cortex (temporal lobe)
How pitch is coded
- Basilar membrane variesnarrow & stiff at base, wide & flexible at apex
- High-frequency soundsdisplace base of basilar membrane
- Low-frequency soundsdisplace apex
- Tonotopic mappreserved up through auditory cortex
Equilibrium
Vestibular apparatus
- Three semicircular canalsdetect angular (rotational) acceleration; one per plane
- Ampullaenlarged base of each canal; contains crista ampullaris (hair cells in cupula)
- Utricledetects horizontal linear acceleration and head tilt
- Sacculedetects vertical linear acceleration
- Otolithscalcium carbonate crystals in utricle/saccule; weight bends hair cells
Vestibular signaling
- Hair cells bend → depolarizeNT release modulates afferent firing
- Vestibular branch of CN VIIIto vestibular nuclei in brainstem & cerebellum
- Vestibulo-ocular reflex (VOR)eyes move opposite head movement to keep gaze stable
Types of hearing loss
- Conductiveouter/middle ear problem; wax, otitis media, ossicle issue
- Sensorineuralcochlea or CN VIII; loud noise exposure, aging, ototoxic drugs
- Mixedboth components
- Weber testtuning fork on midline; sound lateralizes toward conductive loss or away from sensorineural
- Rinne testair vs bone conduction; AC > BC normally and in sensorineural; BC > AC in conductive
Common pathology
- Meniere diseaseincreased endolymph; vertigo + tinnitus + low-frequency hearing loss
- BPPVdisplaced otoliths in semicircular canal; brief positional vertigo
- Presbycusisage-related sensorineural loss; high frequencies first
- Vestibular neuritisCN VIII inflammation; days of vertigo, then resolves
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