The trochlear nerve, which is also called cranial nerve IV (CN IV), is the motor nerve which innervates the superior oblique muscle of the eyeball. The trochlear nerve is composed of axons of neurons situated in the trochlear nucleus, in the ventral part of the brainstem.
In terms of the number of axons that make it up, the cranial nerve IV is the smallest nerve of the twelve cranial nerves, but has the greatest intracranial length. Along with the optic nerve, it is the only cranial nerve that decussates (crosses to the other side) before innervating the superior oblique muscle. Trochlear nerve is also the only cranial nerve that exits from the dorsal aspect of the brainstem.