Cranial nerve IV, also known as the trochlear nerve, is one of the twelve pairs of cranial nerves. It is primarily responsible for controlling the movement of the superior oblique muscle, which helps rotate the eye downward and laterally. This nerve is unique because it is the only cranial nerve that emerges from the dorsal aspect of the brainstem. The trochlear nerve is a motor nerve, meaning it carries signals from the brain to the muscles. Damage to cranial nerve IV can result in double vision and difficulty with eye movement, particularly when looking down. Proper functioning of this nerve is essential for coordinated eye movements.