Describe the semicircular canals and how they detect angular motion via endolymph movement and hair cell deflection.

• A. The semicircular canals are oriented in three planes; movement of endolymph within the canal bends the hair cells in the crista ampullaris, altering firing rates and signaling angular head acceleration.
• B. The semicircular canals detect linear acceleration.
• C. Endolymph movement within the canal bends the cochlear hair cells to signal sound.
• D. Hair cells in the macula detect angular acceleration.

Answer: (B)

Angular motion is detected by the semicircular canals through endolymph movement that deflects hair cells in the crista ampullaris. Each canal is oriented in a different plane (three orthogonal axes), so together they form a three-dimensional map of head rotation. When the head rotates, the endolymph lags behind due to inertia and flows within the canal, bending the gelatinous cupula that sits over the crista ampullaris. This cupular deflection bends the stereocilia and kinocilium of the hair cells, altering their neurotransmitter release and changing the firing rate of the vestibular nerve. Movement toward the kinocilium increases activity; movement away decreases it. The brain compares signals from all three canals to determine the direction and speed of angular head movement and to drive reflexes that stabilize vision and balance. Linear acceleration and head position relative to gravity are detected by the otolith organs, not the semicircular canals.