Semicircular canals

Canales semicirculares ossei; ductus semicirculares

Also known as: semicircular ducts, ampulla, crista ampullaris, cupula, BPPV, anterior canal, posterior canal, lateral canal, horizontal canal

Three mutually perpendicular bony canals (anterior, posterior, lateral) containing membranous ducts whose ampullae house the cristae ampullares, which detect angular (rotational) acceleration of the head.

Show on body map Ear — Coronal Section

Overview

Number3 per ear (anterior, posterior, lateral)

Arc≈2/3 of a circle each

Lateral canal tilt≈30° above horizontal anteriorly

Openings into vestibule5

Each bony canal forms about two-thirds of a circle and opens into the vestibule at both ends; one end of each is dilated as the ampulla. The anterior (superior) and posterior canals are vertical and join to form a common crus, so there are five openings into the vestibule. The lateral (horizontal) canal is tilted ≈30° from the horizontal (anterior end up) when the head is upright; it bulges into the medial wall of the middle ear (where cholesteatoma may erode it).

Each canal contains a membranous semicircular duct filled with endolymph. In the ampulla a saddle-shaped ridge, the crista ampullaris, bears hair cells whose bundles project into a gelatinous cupula that spans the ampulla like a swinging door. Unlike the otolith organs, the cupula has no otoconia and the same density as endolymph, so it is insensitive to gravity. When the head starts to rotate, the endolymph lags behind (inertia), deflecting the cupula and hair bundles; during sustained constant-velocity rotation the cupula returns to rest within ≈10–20 s, and when rotation stops it is deflected the opposite way (post-rotatory vertigo and nystagmus).

The canals work in coplanar pairs with push–pull signalling: right and left lateral; right anterior with left posterior (RALP); left anterior with right posterior (LARP). In the lateral canal, endolymph flow towards the ampulla (ampullopetal) is excitatory; in the vertical canals, flow away from the ampulla (ampullofugal) is excitatory, reflecting the orientation of the kinocilia.

Location: Posterosuperior part of the bony labyrinth in the petrous temporal bone; the anterior canal raises the arcuate eminence on the floor of the middle cranial fossa.

Parts & subdivisions

Anterior (superior) canal
Vertical; ≈perpendicular to the long axis of the petrous bone; paired with the contralateral posterior canal. Dehiscence of its bony roof causes superior canal dehiscence syndrome (sound- or pressure-induced vertigo — Tullio phenomenon — autophony).
Posterior canal
Vertical; parallel to the long axis of the petrous bone; its ampulla is lowest when upright — the commonest site of BPPV.
Lateral (horizontal) canal
≈30° to the horizontal; tested by caloric irrigation and the head-impulse test.
Ampulla and crista ampullaris
Dilated end with the sensory ridge of hair cells.
Cupula
Gelatinous dome over the crista; deflected by endolymph inertia.
Common crus
Shared limb of anterior and posterior canals.

Anatomy

Innervation: Superior vestibular nerve — anterior and lateral ampullae; inferior vestibular nerve (posterior ampullary/singular nerve) — posterior ampulla.
Arterial supply: Anterior and posterior vestibular arteries (labyrinthine artery).

Function

Detect angular acceleration of the head in any plane (three orthogonal components)
Drive the vestibulo-ocular reflex to stabilise gaze during head movement
Contribute to balance and spatial orientation

Physiology: Primary afferents fire spontaneously (≈90 spikes/s), so each canal can signal rotation in both directions. Effective sensitivity to angular velocity in the range of natural head movements (≈0.1–10 Hz) because of cupula–endolymph mechanics (an "integrating" accelerometer).

Clinical significance

Benign paroxysmal positional vertigo (BPPV)

Free-floating otoconia (canalithiasis), usually in the posterior canal, move with gravity when the head is repositioned and drag endolymph, deflecting the cupula. Brief (<1 min) vertigo on lying down, rolling over or looking up. Dix–Hallpike test: head turned 45° to the affected side and brought rapidly into a supine head-hanging position → after a few seconds' latency, torsional up-beating nystagmus (top pole towards the lower ear) that fatigues. Treated by the Epley particle-repositioning manoeuvre, which returns otoconia to the utricle.

Caloric testing — COWS

Irrigating the ear with cold water induces endolymph convection in the lateral canal (patient supine with the head raised 30°, which makes the canal vertical): cold → nystagmus fast phase to the Opposite side; warm → Same side. In a comatose patient with an intact brainstem, cold water causes tonic deviation of the eyes towards the irrigated ear (no fast phase); absent responses are part of brainstem death testing.

Head-impulse test

A rapid head turn towards a side with a failing lateral canal produces a corrective "catch-up" saccade — positive in vestibular neuritis; a normal test in acute continuous vertigo suggests a central (cerebellar/brainstem) cause.

Mnemonics

Canal pairs

"LARP and RALP" — Left Anterior with Right Posterior; Right Anterior with Left Posterior; the lateral canals pair with each other.

Test yourself

Brief vertigo on rolling over in bed, with torsional up-beating nystagmus after a few seconds' latency on the Dix–Hallpike test, is most likely due to:

Ménière disease
Vestibular neuritis
Benign paroxysmal positional vertigo of the posterior canal
Vestibular schwannoma

Answer: Benign paroxysmal positional vertigo of the posterior canal

Otoconia displaced from the utricle into the posterior semicircular canal (canalithiasis) cause BPPV — brief positional vertigo with latent, fatiguing torsional up-beating nystagmus. It is treated with the Epley manoeuvre.

Which structure detects angular acceleration of the head?

Macula of the utricle
Crista ampullaris
Macula of the saccule
Spiral organ

Answer: Crista ampullaris

The cristae ampullares in the ampullae of the three semicircular ducts sense angular acceleration via endolymph inertia deflecting the cupula. The otolith maculae sense gravity and linear acceleration.