Trochlear nerve

The trochlear nerve (/ˈtrɒklɪər/), (lit. pulley-like nerve) also known as the fourth cranial nerve, cranial nerve IV, or CN IV, is a cranial nerve that innervates a single muscle - the superior oblique muscle of the eye (which operates through the pulley-like trochlea). Unlike most other cranial nerves, the trochlear nerve is exclusively a motor nerve (somatic efferent nerve).

The trochlear nerve is unique among the cranial nerves in several respects:

The superior oblique muscle which the trochlear nerve innervates ends in a tendon that passes through a fibrous loop, the trochlea, located anteriorly on the medial aspect of the orbit. Trochlea means “pulley” in Latin; the fourth nerve is thus also named after this structure. The words trochlea and trochlear (/ˈtrɒkliə/, /ˈtrɒkliər/) come from Ancient Greek τροχιλέα trokhiléa, “pulley; block-and-tackle equipment”.

The trochlear nerve provides motor innervation to the superior oblique muscle of the eye, a skeletal muscle; the trochlear nerve thus carries axons of general somatic efferent type.^{[citation needed]}

Each trochlear nerve originates from a trochlear nucleus in the medial midbrain. From their respective nuclei, the two trochlear nerves then travel dorsal-ward through the substance of the midbrain surrounded by the periaqueductal gray, crossing over (decussating) within the midbrain before emerging from the dorsal midbrain just inferior to the inferior colliculus. Each trochlear nerve thus comes to course on the contralateral side, first passing laterally (to the side) and then anteriorly around the pons, then running forward toward the eye in the subarachnoid space. It passes between the posterior cerebral artery and the superior cerebellar artery. It then pierces the dura just under free margin of the tentorium cerebelli, close to the crossing of the attached margin of the tentorium and within millimeters of the posterior clinoid process. It runs on the outer wall of the cavernous sinus. Finally, it enters the orbit through the superior orbital fissure and to innervate the superior oblique muscle.

The human trochlear nerve is derived from the basal plate of the embryonic midbrain.^{[citation needed]}

Injury to the trochlear nerve cause weakness of downward eye movement with consequent vertical diplopia (double vision). The affected eye drifts upward relative to the normal eye, due to the unopposed actions of the remaining extraocular muscles. The patient sees two visual fields (one from each eye), separated vertically. To compensate for this, patients learn to tilt the head forward (tuck the chin in) in order to bring the fields back together—to fuse the two images into a single visual field. This accounts for the “dejected” appearance of patients with “pathetic nerve” palsies.

Trochlear nerve palsy also affects torsion (rotation of the eyeball in the plane of the face). Torsion is a normal response to tilting the head sideways. The eyes automatically rotate in an equal and opposite direction, so that the orientation of the environment remains unchanged—vertical things remain vertical.