From the grey matter of the aqueduct of Sylvius (aquaeductus cerebri) immediately below the origin of the oculo-motor nerve. From this it passes to the anterior medullary velum where it decussates with its fellow of the opposite side.
It emerges from the upper part of the anterior medullary velum, and passes round the cerebral peduncle.
After piercing the dura mater behind the posterior clinoid process, the trochlear proceeds forwards in the lateral wall of the cavernous sinus, and enters the orbit via the sphenoidal (superior orbital) fissure. It supplies the superior oblique muscle on its upper surface.
These are similar to those of the oculomotor nerve.