Cochlea: Spiral Organ of Hearing and Balance

The cochlea is a bony, spiral-shaped (2.5 turns) chamber in the inner ear (temporal bone) filled with perilymph and endolymph.

Structure

• Scala vestibuli (perilymph) – receives stapes vibration.
• Scala media (endolymph) – contains Organ of Corti with hair cells.
• Scala tympani (perilymph) – terminates at round window.
• Separated by Reissner's membrane and basilar membrane.

Function

1. Stapes vibration → perilymph wave.
2. Traveling wave on basilar membrane – frequency-specific (base: high, apex: low).
3. Hair cell stereocilia deflection → K⁺ influx → depolarization → afferent signal via CN VIII.

Balance Role

Cochlea shares fluid space with vestibular system; pressure changes influence vestibular function. Primary balance organs are semicircular canals, utricle, saccule.