Nucleus nervi trochlearis

The nucleus of the trochlear nerve is located in the midbrain, at an intercollicular level between the superior colliculus and inferior colliculus.

It is a motor nucleus, and so is located near the midline, embedded within the medial longitudinal fasciculus (see diagram at right).

The oculomotor nerve and trochlear nerve are the only two cranial nerves with nuclei in the midbrain, other than the trigeminal nerve, which has a midbrain nucleus called the mesencephalic nucleus of trigeminal nerve, which functions in preserving dentition.

Oddly, fibers from the trochlear nucleus cross over in the trochlear decussation of the midbrain, located in the superior medullary velum to exit dorsally, the only cranial nerve to do so.

The trochlear nerve then goes around the midbrain, and is visible coming out of the sides.