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How do the semicircular canals detect rotational head movements?

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Diagram showing auditory mechanisms in insects, including labeled illustrations of a scolophore organ, the mosquito ear with antenna shaft and scolophores, the cicada ear with auditory capsule and tympanic membrane, and the grasshopper ear with sensory body and tympanic membrane. — sound reception Auditory mechanisms in insects. (Left) A scolophore organ. (Top right) The mosquito ear. (Centre right) The ear of the cicada *Magicicada septendecim*. (Bottom right) The ear of the grasshopper.

The semicircular canals detect rotational head movements through a mechanism based on the inertia of the fluid they contain, known as endolymph.21

Because the three canals are oriented at approximately right angles to one another, they are capable of detecting movement in three-dimensional space. When the head begins to rotate, the endolymph lags behind due to inertia, exerting pressure that deflects a gelatinous structure called the cupula.234

This deflection of the cupula acts like a swinging door, bending the stereocilia of the hair cells located on a ridge known as the crista. Depending on the direction of the bend, the hair cells are either depolarized or hyperpolarized, which increases or decreases the frequency of nerve impulses sent to the brain. This system functions as a rotary accelerometer, allowing the body to perceive and respond to angular acceleration.214