Final answer:
The cupula, located at the apex of the ampulla, moves with fluid in the ear to transduce head movements into neural signals for balance.
Step-by-step explanation:
The cupula is the apex of the ampulla and gets moved either toward or away from the semicircular canals by the endolymph fluid within the ear when the head rotates. This movement bends the stereocilia of the hair cells, essential for the transduction of rotational head movements into neural signals that the brain interprets, allowing us to maintain our balance and spatial orientation.