Nerve, trochlear: The trochlear nerve controls the superior oblique muscle of the eye, one of the extraocular muscles, the muscles that move the eye. Paralysis of the trochlear nerve results in rotation of the eyeball upward and outward (and, therefore, double vision).
The trochlear nerve is the fourth cranial nerve. (The twelve cranial nerves emerge from or enter the skull as opposed to the spinal nerves which emerge from the vertebral column).
The trochlear nerve is the only cranial nerve that arises from the back of the brain stem. It follows the longest course within the skull of any of the cranial nerves.