What is the function of the ampulla into the ear?

The three semicircular canals (SSCs) within the vestibular organ of each ear contain fluid and hair receptor cells encased inside a fragile membrane called the cupula. The cupula is located in a widened area of each canal called the ampulla. When you move your head, the fluid in the ampulla lags behind, pushing the cupula a very tiny bit which causes the hairs to also bend a very tiny bit. The bending hairs stimulate the hair cells, which in turn trigger sensory impulses in the vestibular nerve going to the brain to "report" the movement. Hair cells are amazingly sensitive. For example, a cupula movement of even a thousandth of an inch is detected by the brain as a big stimulus. The three canals are positioned roughly at right angles to one another in the three planes of space. Thus, the canals react separately and in combination to detect different types of swiveling head movement. They detect when we nod in an up and down motion (pitch), when we tilt our head to the side towards our shoulder (roll), and when we shake our head "no" in a side to side motion (yaw). The semicircular canals are responsible for detecting any kind of rotational motion in the head.