The Trochlear Nerve (CN IV)
The Trochlear Nerve (CN IV)
- Last updated November 23, 2025
✅ Objectives
By the end of these revision notes, you should be able to:
- Know the anatomy and path of the trochlear nerve
- Understand the examination and function of the trochlear nerve
The Basics
The trochlear nerve (CN IV) is the fourth cranial nerve and the only nerve to exist from the posterior aspect of the brainstem.
It only supplies a single extraocular muscle – the superior oblique.
Anatomy of the Trochlear Nerve
Origin
The trochlear nerve arises from the trochlear nucleus in the midbrain. It is the only nerve that crosses over entirely before exiting the brainstem, with each nerve innervating the contralateral superior oblique muscle.
Course
The nerve, after decussating (crossing over), exits the brainstem and runs anteriorly. It passes through the cavernous sinus (on the lateral wall) and through the superior orbital fissure. Once in the orbit, it can supply the superior oblique muscle.
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