Trochlear nerve

The trochlear nerve (IV) is a motor cranial nerve that innervates the dorsal oblique muscle of the contralateral eye. It is unique because it is the only cranial nerve that emerges dorsally from the brainstem and the only one whose fibers completely cross before reaching their target muscle.

Number : IV

Sensory, motor, or both : motor

Origin : trochlear nucleus in the caudal mesencephalon, at the level of the caudal colliculi, close to the midline and adjacent to the mesencephalic aqueduct.

Route / Reports : the fibers course dorsally around the mesencephalic aqueduct, enter the rostral medullary velum, and completely cross at the trochlear decussation. The nerve then emerges caudal to the contralateral caudal colliculus, passes rostroventrally along the lateral surface of the mesencephalon, and leaves the cranial cavity through the orbital fissure.

Distribution : dorsal oblique muscle of the contralateral eyeball.

Description : The trochlear nerve originates in the caudal mesencephalon, curves dorsally around the mesencephalic aqueduct, and completely decussates in the rostral medullary velum. It then emerges from the dorsal surface of the brainstem, passes rostroventrally along the mesencephalon, enters the orbit through the orbital fissure, and terminates in the dorsal oblique muscle of the opposite eye.