What is the main action of CN IV - Trochlear Nerve?

The main action of the Trochlear Nerve, which is cranial nerve IV, is to provide motor innervation specifically to the superior oblique muscle of the eye. The superior oblique muscle is responsible for actions including depression, abduction, and internal rotation of the eyeball.

This nerve is unique among the cranial nerves because it is the only one that exits the brainstem from its dorsal aspect, and it innervates the muscle that allows for the downward movement of the eye when it is adducted and contributes to the overall coordination of eye movements. Understanding its function is vital for recognizing how eye movements are controlled and the potential implications of trochlear nerve damage, which could lead to issues like vertical diplopia (double vision).