Module 5 of 12

Semicircular Canals

Three orthogonal canals, the crista and cupula, coplanar push-pull pairing, and canal hydrodynamics.

The semicircular canals are the sensors of angular acceleration. Their geometry and hydrodynamics let the labyrinth report head rotation about any axis in space.

There are three canals on each side, set at roughly right angles to one another, so between them they cover every direction a head can turn. When the head rotates, the fluid inside a canal lags behind and pushes on a flexible flap, the cupula. Bending the cupula bends the hair cells beneath it, and the canal reports the turn.

The three canals — horizontal, anterior, and posterior — lie in approximately orthogonal planes. The horizontal canal is tilted about 30° up from the true horizontal, which is why the head is pitched forward for caloric testing ⁹. At the base of each canal the crista ampullaris carries the hair cells, and the cupula seals the ampullary lumen above them. Canals are paired coplanarly between the ears, so each works against a partner on the opposite side, enabling push-pull signalling: excitation on one side is mirrored by inhibition on the other ¹.

Canal dynamics are described by two time constants — a short constant for the cupula’s immediate deflection and a longer constant, roughly 5–10 s, for its return to rest. Central processing in the vestibular nuclei and nodulus extends the response through the velocity-storage mechanism, so per-rotational nystagmus can persist for 18–30 s after a sustained turn¹⁰. Velocity storage is itself a clinically relevant target: it shapes caloric responses and is modulated by the cerebellar nodulus.

The three canals lie in approximately orthogonal planes. The horizontal canal is tilted ~30° above the true horizontal — the reason the head is pitched forward 30° to align it with gravity for caloric testing.

The three coplanar pairings across the two ears: the two horizontal canals together, the left anterior with the right posterior, and the left posterior with the right anterior. Excitation on one side mirrors inhibition on the other — the push-pull foundation of vestibular signalling.

Inside each ampulla, hair cells of the crista project into the gelatinous cupula, which seals the ampullary lumen. Endolymph movement bends the cupula and, with it, the hair-cell stereocilia.

Coplanar canals signal as a push-pull pair. A head turn excites one ampulla and inhibits its contralateral partner; the brainstem reads the difference. At rest both fire near 90 spikes/s.

Two time constants describe canal mechanics. T₁ is the short constant of cupular deflection at rotation onset — fast enough to follow rapid head movement. T₂ is the long constant of passive return to rest, roughly 5–10 seconds. Central velocity storage extends the effective response well beyond T₂.

Canal hydrodynamics & velocity storage

Because the cupula has the same density as the surrounding endolymph, it responds to angular acceleration rather than to gravity. The velocity-storage mechanism then integrates and prolongs that signal, improving spatial orientation during sustained motion and extending the window over which gaze can be stabilized ¹⁰.