The trochlear nerve
Trochlear nerve (IV) · nervus trochlearis
The thinnest cranial nerve, controlling one eye muscle that turns the eye down and in.
What it does
The trochlear nerve supplies a single eye muscle, the superior oblique, which loops through a small pulley before attaching to the eye. That muscle helps you look down and inwards, the movement you make reading or walking downstairs.
Surprising fact
It is the only cranial nerve that leaves from the back of the brainstem, and it has the longest path inside the skull of any cranial nerve.
Why it matters
Damage to it causes double vision when looking down, so people often tilt their head to compensate and have trouble on stairs.
Nearby
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Sources
Drafted with AI help; awaiting a human fact-check against these sources.
Open in the 3D atlas · Suggest a correction · Not medical advice.
3D models: Z-Anatomy & BodyParts3D (CC BY-SA 4.0 / 2.1 JP). Card text CC BY 4.0.