Answer: The trochlear nerve is a cranial nerve that sends motor information to the superior oblique muscle of the eye.
Of the 12 cranial nerves that exit out of the central nervous system, the trochlear nerve (CN IV) is responsible for only one function. It is a somatic efferent nerve that innervates the superior oblique muscle, which is important for normal movement of the eyeball.
The superior oblique muscle has three primary functions. It's main function is to rotate the eyeball inwards towards the midline. This is called intorsion. It also abducts and depresses the eyeball, rotating laterally and downward.
An injury to cranial nerve IV results in the eyeball turning inward and upward. As a result, patients with trochlear nerve palsy may have difficulty walking down stairs. The eyes are often out of alignment in the vertical axis. As a results, patients with cranial nerve IV injuries may experience diplopia, or double vision. To counteract this, they often tuck their chin towards the chest, looking out from the top half of their eyes. This corrects the double vision.
Closed head injuries may result in damage to the fourth cranial nerve. Stretching of the nerve can result in blurry vision due to displacement of the nerve along the axonal tract. These minor injuries generally repair themselves within months.
More severe injuries such as diabetic neuropathy may result in more long term damage to the trochlear nerve. It is also possible that congenital birth defects result in an improperly functioning nerve.
The trochlear nerve is the only cranial nerve to exit via the dorsal side of the mesencephalon. As such, neurons in the fourth cranial nerve have the farther to travel within the skull to reach its target muscle. It is also unique in the sense that it is the smallest cranial nerve, containing the fewest number of axons.
As it travels from the mesencephalon anteriorly toward the eye, it remains in the subarachnoid space. It joins with the two other cranial nerves involved with control of the eyeball, the oculomotor (cranial nerve 3; CN III) and the abducens (cranial nerve 6; CN VI). It also joins with two of the three branches of the trigeminal nerve (CN V), the ophthalmic branch and the maxillary branch.