DESCRIPTION and FUNCTION: The name trochlear means "pulley" and it innervates one extrinsic eye muscle that loops through a pulley-shaped ligament in the orbit (Figure 4A). 
The nerve supplies somatic motor fibers to the superior oblique muscle and carries sensory proprioceptor fibers from this muscle to the brain (see III Oculomotor and VI Abducens, which also function to move the eye).
ORIGIN and COURSE: Fibers emerge from the dorsal midbrain and course ventrally around the midbrain to enter the orbit along side the oculomotor nerves. Figure 4B
CLINICAL TESTING: Tested in common with III Oculomotor wherein subject is asked to track objects with the eyes.
HOMEOSTATIC IMBALANCE: Trauma to, or paralysis of, a trochlear nerve results in double vision and reduced ability to rotate eye inferiorlaterally (down and out, or clockwise).