trochlear nerve

The trochlear nerve is the fourth cranial nerve, and it supplies only the superior oblique muscle.¹ The trochlear nerve takes its name from the trochlea, a tube 4 to 6 mm long at the angle between the superior and medial orbital walls, through which the superior oblique passes.^2,3 The nerve is very long and slender, and it has the fewest axons of any cranial nerve.¹

The trochlear nerve is the only cranial nerve to emerge from the dorsal aspect of the brain.¹ Its nucleus lies at the level of the inferior colliculi, and its fibers decussate completely in the anterior medullary velum, so each nucleus innervates the superior oblique of the opposite eye.¹ Apart from the optic nerve, the trochlear is the only cranial nerve that crosses in this way.¹

Crossed innervation also occurs inside the oculomotor nuclear complex, where the superior rectus subnucleus innervates the superior rectus of the opposite eye.⁴ With both pathways crossed, each cyclovertical muscle and its yoke muscle have nuclei on the same side of the brain, and that arrangement lets the corresponding semicircular canal drive the yoked pair directly.⁴