Trochlear nerve
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The fourth of twelve cranial nerves, the trochlear nerve controls the function of the superior oblique muscle, which rotates the eye towards the nose and also moves the eye downward.
This nerve is sometimes called the 'pathetic' nerve as loss of its function paralyses the superior oblique muscle giving the eyes a pathetic look.
Trochlear nerve nucleus
The nucleus of the trochlear nerve is located in the midbrain, at the level of the inferior colliculus. It is a motor nucleus, so located near the midline.
Oddly, fibres from the trochlear nucleus cross over in the midbrain and exit posteriorly, the nerve then goes around the midbrain, and is visible coming out the sides.