How do the semicircular canals encode head rotation and how does canal plane orientation support 3D motion sensing?

• A. Each canal detects angular velocity about its axis; three mutually orthogonal canals enable detection of complex 3D rotations.
• B. They detect only static orientation; orientation is from otoliths.
• C. They sense linear acceleration in one plane only.
• D. They code head tilt via gravity rather than rotation.

Answer: (B)

The semicircular canals function as angular velocity sensors. Each canal is oriented in a distinct plane, and when the head rotates, inertia of the endolymph causes it to lag and push against the cupula, bending the hair cells and changing the firing rate of the vestibular nerve. Because there are three canals arranged in three mutually perpendicular planes, the brain receives velocity signals about rotation around each axis. By combining these signals, it can reconstruct the full 3D rotation vector, distinguishing yaw, pitch, and roll.

Otolith organs (the utricle and saccule) handle gravity and linear acceleration, signaling static head orientation and translational movement. That’s why rotation is not detected by the otoliths, and why the idea that the canals only sense static orientation isn’t correct.