How do the semicircular canals detect head movement?

• A. They are tubes filled with fluid; when the fluid moves, it senses the change in axis
• B. They respond to light cues
• C. They detect gravity directly
• D. They are tubes filled with air; when the air moves, it senses the pressure changes

Answer: (A)

Semicircular canals detect head movement through the inertia of the fluid inside them. Each canal is a loop filled with endolymph; when you rotate, the fluid tends to stay in its previous motion while the canal moves around it. That relative motion pushes on the gelatinous cupula at the end of the canal, bending the hair-cell stereocilia on the crista inside the ampulla. This bending changes the hair cells’ electrical signals, altering the firing rate of the vestibular nerve and conveying information about angular velocity to the brain. There are three canals in different planes, so you can detect rotation about any axis. Gravity and linear acceleration aren’t sensed by these canals themselves—those are detected by the otolith organs—so the canals aren’t responding to light cues or air pressure.