Module 2 · Anatomy and physiology

Anatomy and physiology

Labyrinth, canals, hair cells, VOR arc
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The six canals

Each labyrinth contains three semicircular canals — lateral, anterior, and posterior — arranged roughly orthogonally. Across the two ears, the six canals form three coplanar pairs that work together. The two lateral canals share a plane (yaw rotation). The right anterior and left posterior canals share a plane that runs 45° from sagittal, called RALP. The left anterior and right posterior canals share the orthogonal LARP plane.

Use the simulator below to rotate each canal and see which head thrust direction activates it. Switch to the "by nerve division" view to see which canals share the superior nerve and which share the inferior nerve — a piece of anatomy that directly explains the classic neuritis patterns you'll meet later.

View:
FrontBackRight earLeft earRARPRLLPLALLRightward yaw
Selected canal
Right lateral
Plane: lateral
Head thrust direction
Head turned rapidly to the right (yaw)
Nerve division
Superior vestibular nerve

Hair cells, cupula, and endolymph

Each canal has an ampulla — a bulge at one end — containing a gelatinous membrane called the cupula. The cupula sits across the lumen of the canal like a swinging door, and embedded in its base are hair-cell stereocilia. The hair cells fire at a resting rate of around 90 Hz even when the head is still. This baseline tone is essential: it lets the system signal motion in either direction by going up or down from baseline.

When the head rotates, the bony canal moves with it but the endolymph inside lags behind due to inertia. The lagging fluid pushes the cupula one way or the other, deflecting the stereocilia and modulating the firing rate of the hair cells. Watch the dynamics in the simulator below.

Head turn:
Front of head (top-down view)Right lateral canalampullaLeft lateral canalampullaRight vestibular nerve90 HzLeft vestibular nerve90 Hz
0°/s
What's happening

At rest, both vestibular nerves fire at their resting tonic rate (~90 Hz). The cupula sits undeflected in the ampulla. This baseline symmetry is what the brain interprets as "no head motion."

Push–pull and Ewald's laws

Because each canal has a contralateral partner in the same plane, every head rotation excites one canal and inhibits the other. This push–pullarrangement gives the brain a differential signal that is more reliable than either canal alone. It also means a single canal can't encode head motion unambiguously: if the right lateral canal increases its firing rate, the brain interprets that as a rightward head turn only because the left lateral has decreased.

Ewald's three laws summarise the relationship:

  1. The slow phase of any vestibular eye movement lies in the plane of the canal that is being stimulated.
  2. For the lateral canal, excitation (ampullopetal flow) produces a larger response than inhibition (ampullofugal flow).
  3. For the vertical canals, inhibition is the stronger stimulus.

Ewald's second law is why a head impulse towardthe affected side reveals an ipsilateral lateral canal lesion more clearly than an impulse away from it — the brain has to rely on the affected canal's excitation, which it cannot deliver.

The vestibular nerve and its two divisions

The vestibular nerve splits into a superior and an inferior division before reaching the brainstem. The superior division carries afferents from the lateral canal, the anterior canal, and the utricle. The inferior division carries afferents from the posterior canal and the saccule. This anatomy is the single most important fact for interpreting vHIT clinically. The superior division is preferentially affected in vestibular neuritis, which is why the classic pattern is reduced gain in lateral and anterior canals with a spared posterior canal on the same side.

Key teaching points

  • Six canals in three coplanar pairs: lateral, LARP, RALP.
  • Hair cells fire at ~90 Hz at rest; the brain reads head motion from the up/down modulation from this baseline.
  • Superior nerve = lateral + anterior canals + utricle. Inferior nerve = posterior canal + saccule. Remember this.
  • vHIT probes the high-frequency, Type I / irregular-afferent system; calorics probe the low-frequency system.

References

  1. Curthoys IS, Halmagyi GM, Manzari L, McGarvie LA, MacDougall HG. A review of the geometrical basis and the principles underlying the use and interpretation of the vHIT in clinical vestibular testing. Frontiers in Neurology 2023;14:1147253. doi:10.3389/fneur.2023.1147253
  2. Halmagyi GM, Chen L, MacDougall HG, Weber KP, McGarvie LA, Curthoys IS. The video head impulse test. Frontiers in Neurology 2017;8:258. doi:10.3389/fneur.2017.00258
  3. MacDougall HG, Weber KP, McGarvie LA, Halmagyi GM, Curthoys IS. The video head impulse test: diagnostic accuracy in peripheral vestibulopathy. Neurology 2009;73:1134–41. doi:10.1212/WNL.0b013e3181bacf85
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