Cranial nerve · one in each half of the brain
Trochlear nerve (IV)
Also called Cranial nerve IV, CN IV, Fourth nerve
The thinnest cranial nerve, running one eye muscle that loops through a tiny pulley.
The trochlear nerve controls just one muscle, the superior oblique, which helps you look down and in toward your nose. That muscle's tendon runs through a little loop of tough tissue in the eye socket, like a rope through a pulley. It is the thinnest of the twelve cranial nerves and has the longest path inside the skull, which makes it easy to injure.
Where it is
It is the only cranial nerve that leaves from the back of the brainstem, just below the inferior colliculus. It then wraps around the side of the brainstem and runs forward to the eye socket.
What it does
Looks down and in
It helps you look down toward your nose, like when you read a book in your lap or walk down stairs.
Keeps the view level
When you tilt your head, it helps twist the eye slightly so the world does not seem to tilt as much.
Teamwork
It works with nerves III and VI so both eyes stay perfectly lined up and you see one picture.
Think of it as a rope running through a pulley: tug the rope and the eye rolls down and inward.
Works with
- MidbrainIts starting cells sit low in the midbrain, and their fibers cross to the opposite side before leaving.
- Inferior colliculusIt pops out of the back of the brainstem just below this hearing relay.
- Oculomotor nerve (III)The oculomotor nerve handles most eye moves, and the trochlear adds the down-and-in and twisting moves.
- Abducens nerve (VI)Together these nerves keep both eyes pointing at the same target.
When it goes wrong
How doctors check it
You "follow my finger" as it moves down and toward your nose, while the doctor watches whether each eye tracks smoothly.
Fourth nerve palsy
Damage causes double vision with one image sitting slightly above the other, worst when reading or walking down stairs. Many people tilt their head to the opposite side to line the images up again.
Easily hurt
Because it is so thin and long, it is easily injured by a blow to the head.
For learning only, not medical advice. If you have a health concern, talk to a doctor.
Surprising facts
- Its fibers all cross to the other side before leaving the brainstem, so the left side of the brainstem runs the right eye's muscle.
- It is the only cranial nerve that comes out of the back of the brainstem.
- Doctors sometimes look through old family photos to spot a long-standing fourth nerve problem, because the person always tilts their head the same way.
By the numbers
Where the name comes from
From the Latin trochlea, "pulley", after the pulley-like loop that its muscle's tendon slides through.
Quick quiz
3 questions. Your answers are saved and come back in your daily review.
Which of these is a job of the trochlear nerve (IV)?
Show the answer
Looks down and in. It helps you look down toward your nose, like when you read a book in your lap or walk down stairs.
Looks down and in. It helps you look down toward your nose, like when you read a book in your lap or walk down stairs.
The trochlear nerve (IV) is part of which larger structure?
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Cranial nerves. It belongs to the cranial nerves.
Cranial nerves. It belongs to the cranial nerves.
Which part fits this description? "Like a rope running through a pulley: tug the rope and the eye rolls down and inward."
Show the answer
Trochlear nerve (IV). It is the trochlear nerve (IV). The thinnest cranial nerve, running one eye muscle that loops through a tiny pulley.
Trochlear nerve (IV). It is the trochlear nerve (IV). The thinnest cranial nerve, running one eye muscle that loops through a tiny pulley.
These parts now come back in your daily review, just before you would forget them.
Practice it and its neighbors in 3DSources
- Neuroanatomy, Cranial Nerve 4 (Trochlear), StatPearls, NCBI Bookshelf
- Trochlear nerve, Radiopaedia
- Trochlear nerve palsy, Radiopaedia
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