CN IV. Trochlear Nerve
The trochlear nerve is purely a motor nerve and is the only cranial nerve to exit the brain dorsally. The trochlear nerve supplies one muscle: the superior oblique. The cell bodies that originate the fourth cranial nerve are located in ventral part of the brainstem in the trochlear nucleus. The trochlear nucleus gives rise to nerves that cross (decussate) to the other side of the brainstem just prior to exiting the brainstem. Thus, each superior oblique muscle is supplied by nerve fibers from the trochlear nucleus of the opposite side. The trochlear nerve fibers curve forward and enter the dura mater at the angle between the free and attached border of the tentorium cerebelli. The nerve travels in the lateral wall of the cavernous sinus and then enters the orbit via the superior orbital fissure. The nerve travels medially and diagonally across the levator palpebrae superioris and superior rectus muscle to innervate the superior oblique muscle.