Final answer:
The gelatinous cap surrounding the stereocilia and kinocilium of the hair cells within the ampullae of the semicircular ducts is known as the cupula.
Step-by-step explanation:
The term that refers to the gelatinous cap surrounding the stereocilia and kinocilium of the hair cells within the ampullae of the semicircular ducts is cupula.
The cupula is a structure that plays a critical role in our sense of balance. Within the ampulla at the base of each semicircular canal in the ear, the stereocilia of hair cells extend into this gelatinous cap.
As the head rotates, the fluid in the semicircular canals moves, causing the cupula to bend and deflect, in turn bending the embedded stereocilia. This action stimulates the hair cells, sending signals to the brain that help it interpret the head's rotation in three-dimensional space.
The other terms provided, saccule, macula, and crista, relate to different structures within the vestibular system but are not directly associated with the function of the cupula in the ampullae of semicircular canals.