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Trochlear nerve course
Which cranial nerve is the only one to emerge from the dorsal surface of the brainstem and to cross to the opposite side before reaching its target?
- A. Oculomotor nerve
- B. Abducent nerve
- correctC. Trochlear nerve
- D. Trigeminal nerve
Explanation
Key fact
The trochlear nerve is the only cranial nerve that leaves the brainstem dorsally and decussates. Both facts follow from where its nucleus sits.
The trochlear nerve. It emerges behind the midbrain, wraps round it, and supplies the superior oblique of the *opposite* side.
Why
Its nucleus lies in the midbrain, but the fibres run backwards and cross.
They emerge below the inferior colliculus, on the dorsal surface.
Each nerve therefore supplies the superior oblique of the other side.
That long, exposed course makes it the most easily injured in head trauma.
The trochlear nerve leaves the brainstem from behind
Why not the others
- A. Oculomotor nerve — emerges ventrally from the interpeduncular fossa. Part of its nucleus is crossed, but the nerve itself is not.
- B. Abducent nerve — emerges ventrally at the pontomedullary junction and does not cross. It has a long course, but an anterior one.
- D. Trigeminal nerve — emerges from the lateral aspect of the pons and supplies its own side. It is the largest cranial nerve, not the most crossed.
Remember
Fourth is the odd one: out the back, and over the other side.
Go deeper
A trochlear palsy gives vertical diplopia that is worst on looking down and in, so the patient tilts the head away from the affected side - the Bielschowsky test.
Because the nerve is thin and travels far, it is the commonest cranial nerve palsy after closed head injury.
The diabetic third nerve palsy in the seed spares the pupil, because the ischaemia hits the central fibres and leaves the superficial parasympathetics intact.
Snell's Clinical Anatomy by Regions, 11th ed. (Wolters Kluwer, 2024); Gray's Anatomy for Students, 5th ed. (Elsevier, 2023).