trochlear nerve

Also known as cranial nerve IV, the trochlear nerve is a purely motor nerve that innervates the superior oblique muscle of the eye. The “Trochlea” refers to the pulley-like structure the superior oblique tendon passes through, giving the nerve its name.
Anatomically, it emerges from the dorsal aspect of the brainstem, decussates to the opposite side, and innervates the contralateral superior oblique muscle—no other cranial nerves innervate a muscle contralateral of their nucleus.