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Trochlear nerve (cranial nerve IV)

Recommended video: Oculomotor, trochlear and abducens nerves [13:48]
Course of the oculomotor, trochlear and abducens nerves.

The trochlear nerve is the fourth paired cranial nerve (CN IV). Along with the oculomotor nerve (CN III) and the abducens nerve (CN VI), it is responsible for controlling movement of the eyeball. Interestingly, the trochlear nerve has the longest intracranial course despite being the smallest of all cranial nerves (by number of axons).

The tendon of the muscle supplied by the trochlear nerve (superior oblique muscle) passes through a cartilaginous pulley-like structure known as the trochlea (Latin: trochlea = pulley), which grants the nerve its name.

This article will delve deeper into the anatomy and function of the trochlear nerve, while reviewing the origin and course of the nerve, its field of innervation and clinical significance.

Key facts about the trochlear nerve
Type General somatic efferent (GSE) / Motor nerve
Origin Dorsal midbrain
Innervation Superior oblique muscle
Contents
  1. Origin and course
  2. Innervation
  3. Clinical relations
    1. Diplopia
  4. Sources
+ Show all

Origin and course

The trochlear nerve (CN IV) arises from the trochlear nucleus located within the periaqueductal gray substance of the midbrain and emerges on the dorsal midbrain caudal to the inferior colliculus. It then crosses the midline, before passing anteriorly around the contralateral side of the brainstem.

Once curved around the midbrain, the trochlear nerve pierces the dura mater at the inferior margin of the tentorium cerebelli to run anteriorly within the lateral wall of the cavernous sinus.

It then ultimately enters the orbit through the superior orbital fissure, external to the common tendinous ring.

Innervation

The trochlear nerve is a purely motor nerve, responsible for providing general somatic efferent (GSE)/motor innervation to just one muscle, the superior oblique muscle of the eye, on the contralateral side of its associated nucleus. This extraocular muscle attaches to the superolateral aspect of the eyeball, but before doing so, its tendon passes through a fibrocartilaginous pulley (the trochlea of superior oblique muscle) at the nasal part of the frontal bone (roof of orbit) and courses posterolaterally, deep to the superior rectus muscle.

Thus, contraction of the superior oblique initiates an inferolateral deviation of the pupil by abducting, depressing and internally rotating the eyeball.

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Learn more about the trochlear nerve in the following study unit!

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