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Why does fluid lag behind when the head rotates in a semicircular canal?
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Mechanoreception
In human skin, specialized nerve endings known as Meissner's corpuscles (or tactile corpuscles) are sensitive to touch. They are one of several different types of mechanoreceptors found in human skin.
When the head begins to rotate, the fluid within the semicircular canals, known as endolymph, lags behind due to its inertia.12
Because the fluid does not immediately move with the bony walls of the canal, it exerts pressure that deflects the cupula in the direction opposite to the head's rotation. This mechanical displacement bends the stereocilia of the hair cells located within the ampulla, which triggers the transmission of nerve impulses to the brain to signal the movement. Once rotation is maintained at a steady velocity, the endolymph eventually catches up to the speed of the canal, and the stimulation of the hair cells ceases until the rotation stops or changes.12