Trochlear nerve. From Wikipedia, the free encyclopedia. The trochlear nerve, also called the fourth cranial nerve or cranial nerve IV, is a motor nerve (a somatic efferent nerve) that innervates only a single muscle: the superior oblique muscle of the eye, which operates through the pulley-like trochlea.he trochlear nerve is unique among the cranial nerves in several respects: 1  It is the smallest nerve in terms of the number of axons it contains. 2  It has the greatest intracranial length.