The trochlear nerve (the fourth cranial nerve) (from Ancient Greek τροχιλέα trokhiléa, “pulley; block-and-tackle equipment”) is a motor nerve (a “somatic efferent” nerve) that only innervates one single muscle: the superior oblique muscle of the eye.The trochlear nerve is unique among the cranial nerves in several respects:It is the smallest nerve in terms of the number of axons it contains. It has the greatest intracranial length.