Which structure extends into the endolymph and transmits force to the hair cells when rotated?

• A. Otolith
• B. Reissner's membrane
• C. Basilar membrane
• D. Cupula

Answer: (D)

Rotational movement is detected by the semicircular canals because the endolymph lags behind the moving inner ear structures. This flow pushes on the gelatinous cupula, which extends into the ampulla and spans the canal’s lumen. When the cupula deflects, the hair-cell bundles in the crista ampullaris bend, changing the hair cells’ receptor potential and sending signals about angular acceleration to the brain. The cupula is the structure that extends into the endolymph and directly transmits force to the hair cells during rotation. By contrast, otoliths in the utricle and saccule respond to linear acceleration and gravity, Reissner’s membrane is a cochlear separator not involved in rotation detection, and the basilar membrane is keyed to sound-induced vibration in the cochlea.