The trochlear nerve (cranial nerve IV) leaves the back of the midbrain just below the inferior colliculi, winds forward round the brainstem, runs in the outer wall of the cavernous sinus and enters the eye socket through the superior orbital fissure. It works a single eye muscle, the superior oblique, and has the fewest fibres of any cranial nerve. [1]
Lit: the whole trochlear nerve on each side as the atlas draws it (its “left” and “right trochlear nerve” shapes): from beside the midbrain, level with the inferior colliculi, forward round the brainstem into the eye socket. [2]
Where it starts: the midbrain. Each nerve comes from a trochlear nucleus in the middle of the midbrain and leaves from its back surface, just below the inferior colliculus. [1]
The 3D brain needs JavaScript. The text below still says where the trochlear nerve is and what it does.
Slice
Opens cut by a top (axial) slice through the trochlear nerve. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
Takes part in: eye movements — see every part of each lit together.
Where it is
Each nerve starts in a trochlear nucleus in the midbrain. Its fibres run backward round the cerebral aqueduct, through the grey matter around it, and cross to the other side inside the midbrain before they come out. [1]
It is the only cranial nerve that leaves the back (dorsal side) of the brainstem, just below the inferior colliculus; from there it curves to the side and forward around the brainstem. [1][3]
On its way forward it passes between the posterior cerebral and superior cerebellar arteries, pierces the dura just under the free edge of the tentorium cerebelli and runs in the outer wall of the cavernous sinus. [1]
It enters the eye socket through the superior orbital fissure and ends in the superior oblique muscle. [1]
What it does
It is a motor nerve for one muscle only, the superior oblique, which turns the eye down and in and rotates it; its tendon runs through a loop of cartilage near the inner corner of the eye socket, the trochlea. [1][4][5]
Because its fibres cross inside the midbrain, each trochlear nucleus works the superior oblique of the eye on the other side. [1]
It is the smallest cranial nerve by number of nerve fibres (axons). [1]
It is one of three nerves that move the eye: the oculomotor (III) works four of the six eye muscles, the trochlear (IV) the superior oblique and the abducens (VI) the lateral rectus. [6][7]
Connections
From: the trochlear nucleus in the midbrain. To: the superior oblique muscle of the eye on the opposite side to that nucleus. [1]
Other names
Cranial nerve IV (CN IV), fourth cranial nerve; nervus trochlearis (Latin). “Trochlea” is Latin for pulley, after the pulley-like loop its muscle’s tendon runs through. [1]
Size
On this model the left trochlear nerve measures about 79 mm front to back, 12 mm top to bottom and 9 mm side to side (the box it fits in, not its volume); the right one is its mirror image. [2]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.