Which nerve innervates the superior oblique muscle?

• A. Trochlear nerve (CN IV)
• B. Oculomotor nerve (CN III)
• C. Trigeminal nerve (CN V)
• D. Abducens nerve (CN VI)

Answer: (B)

The muscle that moves the eye downward and rotates it when it is adducted is controlled by the trochlear nerve, which is cranial nerve IV. This nerve is unique because it is the only nerve that innervates the superior oblique muscle. The superior oblique passes through a pulley-like structure called the trochlea, and the tendon then inserts on the eye to produce depression and intorsion when the eye is adducted. The nerve originates in the dorsal midbrain, exits the brainstem, and travels to the orbit to reach this muscle. In contrast, the oculomotor nerve (cranial nerve III) supplies most of the other extraocular muscles and the levator palpebrae, the abducens nerve (cranial nerve VI) supplies the lateral rectus, and the trigeminal nerve (cranial nerve V) is primarily sensory for the face and also supplies some jaw muscles, not the eye muscles. So the superior oblique is innervated by the trochlear nerve.