Trochlear nerve (IV)

The trochlear nerve, also called the fourth cranial nerve or CN IV, is a motor nerve (a somatic efferent nerve) that innervates just one muscle: the superior oblique muscle of the eye, which operates through the pulley-like trochlea.

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.

It is the only cranial nerve that exits from the dorsal (rear) aspect of the brainstem.

It innervates a muscle, the superior oblique muscle, on the opposite side (contralateral) from its nucleus.

The trochlear nerve decussates within the brainstem before emerging on the contralateral side of the brainstem (at the level of the inferior colliculus).

An injury to the trochlear nucleus in the brainstem will result in an contralateral superior oblique muscle palsy, whereas an injury to the trochlear nerve (after it has emerged from the brainstem) results in an ipsilateral superior oblique muscle palsy.Homologous trochlear nerves are found in all jawed vertebrates.

The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.The human trochlear nerve is derived from the basal plate of the embryonic midbrain.

The words trochlea and trochlear come from Ancient Greek τροχιλέα trokhiléa, “pulley; block-and-tackle equipment”.