The cupula serves as a receptor-level structure because it does what?

• A. Extends into the endolymph and bends hair cells when endolymph moves
• B. Houses hair cells directly
• C. Detects gravity
• D. Produces endolymph

Answer: (A)

The cupula is the movable, gelatinous lid of the crista ampullaris that extends into the endolymph and deflects when the endolymph moves. When the head rotates, inertia causes the endolymph to lag, pushing against the cupula and bending the hair cells’ stereocilia. That deflection changes the receptors’ ion flow, altering neurotransmitter release and sending signals to the vestibular nerve. This is how mechanical motion is translated into neural information, which is the essence of a receptor-level structure. The hair cells themselves sit in the sensory epithelium, not inside the cupula; gravity detection is done by the otolith organs, and endolymph is produced by other vascular channels, not by the cupula.