Which cranial nerve is responsible for downward gaze and inward rotation of the eye?

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

Answer: (B)

Downward gaze with inward rotation is achieved by the superior oblique muscle, which depresses and intorts the eye when it is adducted. The nerve that innervates this muscle is the trochlear nerve. So, the trochlear nerve is responsible for that specific combination of movements. Clinically, a CN IV (trochlear) palsy causes vertical diplopia that worsens when looking down and in, and people often compensate by tilting their head. The other nerves control different eye movements: the oculomotor nerve handles most of the remaining eye muscles and eyelid, the abducens nerve abducts the eye, and the trigeminal nerve is not involved in motor control of the extraocular muscles.