Trochlear nerve (cranial nerve IV)
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.
|Type||General somatic efferent (GSE) / Motor nerve|
|Innervation||Superior oblique muscle|
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.
It then ultimately enters the orbit through the superior orbital fissure, external to the common tendinous ring.
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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All extraocular muscles of the eye work in a synergistic manner to move the eyeball. Therefore, patients rarely present with an isolated injury to the trochlear nerve. Diplopia (double vision) usually signifies trochlear nerve injury, particularly in downward or lateral gaze. This clinical presentation usually occurs due to the superior oblique muscle being hindered from assisting the inferior rectus muscle in depressing the eyeball, when the eye is in an abducted position.
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