Trochlear nerve (CN IV)

Nervus trochlearis

NervousNervesHeadLeft and right

View left in 3D View right in 3D

What it is
The thinnest cranial nerve and the only one to leave the back of the brainstem. It winds around the midbrain, runs in the lateral wall of the cavernous sinus and enters the orbit to reach the superior oblique muscle.
Function
Supplies the superior oblique, which turns the eye down and in and rotates it inward (intorsion) - essential for looking down stairs.
Size
About 1 mm thick; the longest intracranial course (~7.5 cm)
Clinical note
Its long, thin course makes it the cranial nerve most often damaged by closed head injury; patients tilt the head away from the weak side to fuse double images.

Nearby nervous structures Abducens nerve (CN VI), Amygdala, Oculomotor nerve (CN III), Oculomotor nerve root (CN III), Optic chiasm, Optic nerve (CN II), intracranial part, Optic tract, Optic tract root (CN II), Trigeminal ganglion, Trigeminal nerve (CN V), root