Hearing and Vestibular Sense

Outer ear (auricle/pinna, ear canal, tympanic membrane) collects sound → middle ear (malleus, incus, stapes; Eustachian tube equalizes pressure) transmits + amplifies → inner ear (cochlea, basilar membrane, organ of Corti) converts to neural signal.
Auditory transduction: eardrum vibrates → ossicle chain amplifies → stapes pushes oval window → pressure wave in cochlear fluid (round window bulges to let incompressible fluid move) → basilar membrane vibrates → tectorial membrane shears against hair cell stereocilia → K+ channels open → depolarization → cochlear nerve.
Tonotopic organization: high-frequency sounds peak near the BASE of the cochlea; low-frequency sounds peak near the APEX.
Vestibular system: utricle + saccule (otoliths) detect LINEAR acceleration/head position vs. gravity; semicircular canals (ampulla) detect ANGULAR/rotational acceleration.
Cochlear nerve (hearing) + vestibular nerve (balance) join to form the vestibulocochlear nerve = cranial nerve VIII.

Tympanic membrane: The eardrum; vibrates in response to sound, marks outer/middle ear boundary.
Auditory ossicles: Malleus, incus, stapes — smallest bones in the body; transmit and amplify vibrations.
Oval window: Membrane the stapes pushes against, initiating the cochlear pressure wave.
Round window: Membrane that bulges outward to let incompressible cochlear fluid move.
Organ of Corti: Sensory organ for hearing, sitting on the basilar membrane; contains the hair cells.
Otoliths: Calcium carbonate crystals in the utricle/saccule that shift with linear motion/head tilt, bending hair cells.
Ampulla: Enlarged region at the base of each semicircular canal containing hair cells for detecting rotation.

Utricle & Saccule (Otoliths)	Semicircular Canals (Ampulla)
Detect linear acceleration	Detect angular/rotational acceleration
Head position relative to gravity	3 canals in perpendicular planes — any rotation direction detected

1Sound waves travel down the ear canal and vibrate the tympanic membrane (eardrum).
2Vibrations pass through the ossicle chain (malleus → incus → stapes), gaining amplification.
3Stapes pushes the oval window, creating a pressure wave in cochlear fluid; the round window bulges outward to allow fluid movement.
4The pressure wave travels along the basilar membrane, vibrating it; the tectorial membrane's shearing motion bends hair cell stereocilia.
5Mechanically gated ion channels open, potassium flows in (endolymph), hair cells depolarize and release neurotransmitter onto sensory neurons → cochlear nerve.

Oval window = where the stapes pushes IN; round window = bulges OUT to let incompressible fluid move — easy to confuse.
Auditory transduction opens channels that let POTASSIUM (not sodium) flow in, because hair cells sit in endolymph.
Utricle/saccule (linear) vs. semicircular canals (angular/rotational) is a commonly swapped pair.
High-frequency sounds peak near the base of the cochlea; low-frequency near the apex — don't reverse this.

What structure equalizes air pressure across the eardrum?

Which ossicle attaches directly to the eardrum?

Why does the round window need to bulge outward during sound transduction?

What cranial nerve carries both hearing and balance information?

Which vestibular structures use otoliths, and what do they detect?

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