The nucleus of the trochlear nerve is located in the midbrain, at the level of the inferior colliculus. It is a motor nucleus, so located near the midline.
Oddly, fibers from the trochlear nucleus cross over in the midbrain at the superior medullary velum and exit posteriorly, (this is the only cranial nerve to do so). The nerve then goes around the midbrain, and is visible coming out the sides.
