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Abducens nerve: Anatomy and function of the sixth cranial nerve


Abducens nerve: Anatomy and function of the sixth cranial nerve

The abducens nerve, also known as the sixth cranial nerve or CN VI, is a nerve that controls the movement of the lateral rectus muscle of the eye. This muscle is responsible for outward gaze, or abduction of the eye. The abducens nerve is a purely motor nerve that originates from the brainstem and travels through various structures to reach the orbit.

Origin and course of the abducens nerve

The abducens nerve arises from its associated nucleus, the abducens nucleus, located beneath the floor of the fourth ventricle in the caudal pons. The 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. The nerve then enters the subarachnoid space and runs upward between the pons and the clivus, a bony part of the skull base. It pierces the dura mater, the outermost layer of the meninges, to run between the dura and the skull through Dorello’s canal. At the apex of the petrous part of the temporal bone, it makes a sharp turn forward to enter the cavernous sinus. In the cavernous sinus, it runs alongside (inferolateral to) the internal carotid artery. It enters the orbit through (medial end of) the superior orbital fissure, passing through the common tendinous ring (Anulus of Zinn) to reach and innervate the lateral rectus muscle of the eye.

Innervation and function of the abducens nerve


Origin and course of the abducens nerve

The abducens nerve has a general somatic efferent (GSE) function, meaning that it carries motor impulses from the central nervous system to skeletal muscles. The only muscle that it innervates is the lateral rectus muscle of the eye. This 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 this muscle leads to abduction of the eyeball in the horizontal plane, or outward gaze. For example, when looking to the left, both eyes need to abduct. The left eye is controlled by its ipsilateral (same side) abducens nerve, while the right eye is controlled by its contralateral (opposite side) oculomotor nerve via its medial rectus muscle. The coordination of eye movements is achieved by a complex neural pathway called medial longitudinal fasciculus (MLF), which connects various cranial nerve nuclei in the brainstem.

Clinical relevance of the abducens nerve


Innervation and function of the abducens nerve

The abducens nerve can be affected by various pathological conditions that can cause damage or compression along its course. Some common causes include head trauma, increased intracranial pressure, brainstem stroke, cavernous sinus thrombosis, aneurysm of internal carotid artery, tumor or infection in orbit or skull base. The main clinical sign of abducens nerve palsy is diplopia (double vision), due to inability to abduct one or both eyes. The affected eye may also show esotropia (inward deviation) when looking straight ahead. Other symptoms may include headache, nausea, vomiting, dizziness or blurred vision depending on
the underlying cause. Treatment depends on identifying and managing
the cause of nerve damage. In some cases, surgery may be required to decompress
the nerve or repair its injury. In other cases, conservative measures such as
eye patching, prism glasses or botulinum toxin injection may be used to improve
vision and reduce diplopia.

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