Abducens nerve (cranial nerve VI)
The abducens (or abducent) nerve is the sixth paired cranial nerve (CN VI). Along with the oculomotor nerve (CN III) and the trochlear nerve (CN IV), it is a purely motor nerve responsible for controlling movement of the eyeball.
Similar to oculomotor and trochlear nerves, the abducens nerve attributes its name to its function. It supplies the muscle (lateral rectus muscle) involved in abduction of the eye i.e. drawing the eye away from the midline.
This article will delve deeper into the anatomy and function of the abducens nerve by reviewing the origin and course of the nerve, its field of innervation and clinical significance.
|General somatic efferent (GSE) / Motor nerve
|Lateral rectus muscle
Origin and course
The abducens nerve (CN VI) arises from its associated nucleus (nucleus of abducens nerve) located beneath the floor of the fourth ventricle in the caudal pons. Nerve fibers pass anteriorly through the pons to emerge from the medullopontine sulcus at the junction between the pons and the pyramid of the medulla (pontomedullary junction).
It reaches the superior margin of the petrous part of the temporal bone to enter the cavernous sinus, accompanying the internal carotid artery, the oculomotor nerve (CN III), trochlear nerve (CN IV) and the ophthalmic branch of the trigeminal nerve (CN V1) in their course. The abducens nerve concludes its path by exiting the cavernous sinus to enter the orbit via the superior orbital fissure and then pass through the common tendinous ring (Anulus of Zinn).
The abducens nerve is a purely motor nerve, responsible for providing general somatic efferent (GSE)/ motor innervation to just one muscle, the lateral rectus muscle of the eye.
The muscle arises from the common tendinous ring and passes forward to attach to the lateral part of the anterior eyeball, posterior to the corneoscleral junction.
Contraction of the lateral rectus leads to abduction of the eyeball in the horizontal plane.
It is worth highlighting that the lateral rectus muscle of the left eye would abduct the eye to the left, while the muscle on the right eye would move it to the right (always directing the gaze laterally along the horizontal plane).
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All extraocular muscles of the eye work in a synergistic manner to move the eyeball. However, the abducens nerve can get easily compressed due to a lesion or rise in intracranial pressure, particularly along its course when it stretches while sharply curving at the petrous part of the temporal bone. Compression of the abducens nerve would cause paralysis of the lateral rectus muscle and lead to a medial deviation of the affected eye. As a result, the patient will have a fully adducted eye at rest and will demonstrate an inability to abduct their eye.
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