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Ewald’s laws, explained simply
Three short rules connect endolymph flow in each semicircular canal to the eye movements you see. Learn them once and every positional test starts to make sense.
- Say whether a canal is excited or inhibited from the direction of endolymph flow.
- Explain why the lateral and vertical canals follow opposite rules.
- Use canal pairs to understand the head impulse test.
First, the 30-second physiology
Each semicircular canal is a fluid-filled loop with a swelling at one end, the ampulla. Inside it, a gelatinous flap called the cupula sits over the hair cells.
When the head turns, the endolymph lags behind because of inertia, so it bends the cupula. Vestibular nerve fibres fire all the time (their resting tone), so bending the cupula one way raises the firing rate (excitation) and bending it the other way lowers it (inhibition).
The rules
Same plane
Stimulating a canal moves the eyes in that canal’s plane. That’s why each canal has its own nystagmus “signature”.
Lateral canal
Flow towards the ampulla (ampullopetal) excites. Flow away inhibits.
Anterior and posterior canals
The reverse: flow away from the ampulla (ampullofugal) excites. Flow towards it inhibits.
Excitation beats inhibition. A canal can raise its firing rate much more than it can lower it, because the rate can’t fall below zero. So for strong stimuli, the excited canal drives the response.
Textbooks number Ewald’s laws differently. The physiology above is the same whichever numbering you use.
Why are the vertical canals the other way round?
Hair cells are excited when their stereocilia bend towards the tallest one, the kinocilium. In the lateral canal the kinocilia sit on the utricle side of the cupula; in the anterior and posterior canals they sit on the canal side. Same hair cells, opposite orientation.
Canals work in push–pull pairs
The six canals form three pairs that lie in the same plane: the two lateral canals, left anterior with right posterior (LARP), and right anterior with left posterior (RALP). Any head movement in a pair’s plane excites one canal and inhibits its partner, and the brain reads the difference.
In the clinic: the head impulse test
Turn the head quickly to one side. Normally the canal on that side is excited and the eyes stay on target. If that canal is weak, only its partner’s inhibition is left, which is too weak a signal for a fast turn. The eyes move with the head, then a catch-up saccade brings them back. The weak side is the side you turned towards.
Try it with your model
- Hold the model upright, nose forward, and turn it briskly to the left.
- Endolymph in the left lateral canal flows towards its ampulla. Find the + on that ampulla: it’s excited.
- On the right, the flow is away from the ampulla: find the −. That canal is inhibited.
- Now find the left anterior and right posterior canals and check they line up in one plane (LARP).
- Head movement → endolymph lags → cupula bends → the firing rate rises or falls from resting tone.
- Lateral canal: towards the ampulla excites. Anterior and posterior canals: away from the ampulla excites.
- Excitation produces a bigger response than inhibition.
- Canals work in three coplanar pairs: lateral–lateral, LARP and RALP.
Check yourself
The head turns quickly to the right. Which lateral canal is excited?
The right. The endolymph lags, so relative to the right lateral canal it flows towards the ampulla (ampullopetal), which excites a lateral canal.
In the posterior canal, does flow towards the ampulla excite or inhibit?
It inhibits. The vertical canals follow the reverse rule: flow away from the ampulla excites.
A patient makes a catch-up saccade when you turn their head quickly to the left. Which side is weak?
The left, the side you turned towards. Its lateral canal can’t be excited, and the right canal’s inhibition alone isn’t enough.
For education only, not medical advice. Reviewed by a Consultant Neurologist and Neuro‑Otologist.