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Portrait of Semicircular canals

Anatomy · Eye, ear & senses

Semicircular canals

The rotation sensors of the inner ear

Inner ear · vestibular labyrinth

Three looped, fluid-filled tubes in the bony labyrinth of each inner ear, arranged roughly at right angles to one another so that they can sense rotation of the head in any plane. Each opens into the vestibule and has a swelling, the ampulla, containing a gelatinous cupula that bends with the inertia of the endolymph. Their signals drive the reflexes that hold the eyes steady as the head turns.

How to spot them

  • ▸Three perpendicular loops behind and above the vestibule: anterior (superior), posterior and lateral (horizontal)
  • ▸Each has a bulge at one end, the ampulla, holding a sensory crista and a jelly-like cupula
  • ▸The two lateral canals lie in the same plane; the anterior canal on one side is parallel to the posterior canal on the other
  • ▸Respond to rotation (angular acceleration), not to gravity or straight-line movement

What to know

Loose crystals of calcium carbonate drift into the lowest loop when the head is tipped back, and a brief spin follows

Benign paroxysmal positional vertigo — canalithiasis, most often of the posterior semicircular canal

A sequence of head turns devised by John Epley in 1980 to shift the crystals out of the loop (1980)

The Epley manoeuvre, a treatment for BPPV of the posterior canal

A swelling at one end of each loop containing a ridge of hair cells capped by a gelatinous flap

The ampulla, with its crista ampullaris and cupula

In pigeons, cutting these loops in the 1820s produced head tilting and staggering, in experiments by Pierre Flourens (1824)

Flourens' pigeon experiments on the semicircular canals (published 1824)

Warm or cold water in the ear canal makes the eyes flick, because convection currents in the horizontal loop mimic a head turn

Caloric testing of the lateral semicircular canal

The loop that lies flat in the horizontal plane when the head is tipped forward by about 30 degrees

The lateral (horizontal) semicircular canal

Spinning on the spot then stopping makes the world seem to turn the other way, as the fluid keeps moving

Post-rotational nystagmus and dizziness from the semicircular canals

Key ideas

  • Fluid lags behind the moving bony wall because of inertia, deflecting the cupula. The canals are therefore accelerometers, and the brain reads a steady rotation as it fades after 10–20 seconds.
  • The signals drive the vestibulo-ocular reflex, which rotates the eyes opposite to the head so that the visual image stays still on the retina.

Quick-fire facts

  • •Three on each side, six in all; the three planes are nearly orthogonal
  • •Each canal is about 1 mm in diameter, but the loops themselves are 12–20 mm long
  • •Push-pull pairs: the two lateral canals; the left anterior with the right posterior; and the right anterior with the left posterior
  • •Endolymph inside the membranous canal, perilymph between it and the bony wall
  • •The vestibular nerve, joining the cochlear nerve as the vestibulocochlear (VIII) nerve, carries the signals to vestibular nuclei in the brainstem
  • •Ernst Mach, Josef Breuer and Alexander Crum Brown independently proposed the function of the canals in 1873–74
  • •Ménière's disease, labyrinthitis and vestibular neuritis affect the vestibular labyrinth and cause vertigo
  • •Drinking alcohol makes the cupula lighter than the endolymph for a time, causing positional alcohol nystagmus and the 'spins'

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Sources