Abducens nerve

Number: VI

Name: Abducens

Sensory, motor, or both: Mainly motor

Origin/Target: Nuclei lying under the floor of the fourth ventricle, Pons

Nuclei: Abducens nucleus

Function: Innervates the lateral rectus, which abducts the eye.

Description:

The abducent nerve [VI] supplies the Rectus lateralis oculi.

The abducens nerve, also known as Cranial Nerve VI, is one of the twelve cranial nerves that emerge from the brain. This nerve is exclusively a motor nerve and specifically innervates the lateral rectus muscle of the eye, playing a key role in moving the eyeball laterally, or away from the midline of the body.

Nucleus:

The nucleus of the abducens nerve is a small motor structure located under the floor of the fourth ventricle in the brainstem, close to the midline underneath the facial colliculus. Click here to read more about the abducent nerve nucleus.

This nucleus receives several types of input:

• Corticonuclear fibers–from both cerebral hemispheres allow for voluntary control of eye movements.

• Tectobulbar fibers–from the superior colliculus connect visual stimuli with the movement of the eye.

• medial longitudinal fasciculus fibers–link the abducens nucleus with the nuclei of cranial nerves III (oculomotor), IV (trochlear), and VIII (vestibulocochlear), coordinating conjugate eye movement and vestibular responses.

Course of the Abducent Nerve:

After exiting its nucleus, fibers of the abducens nerve traverse anteriorly through the pons and surface at the junction between the pons and medulla oblongata i.e. the pontomedullary junction. From there, the nerve courses forward, entering the cavernous sinus. In this space, it lies inferolateral to the internal carotid artery. Upon reaching the orbit, the abducens nerve continues its journey through the superior orbital fissure, finally reaching its target—the lateral rectus muscle.

Innervation and Function:

The primary role of the abducens nerve is to innervate the lateral rectus muscle. This muscle's contraction abducts the eye—turning it outward laterally. Proper functioning of the abducens nerve is essential for coordinating the movement of both eyes, which allows for binocular vision.

Clinical significance and dysfunction:

A lesion or injury to the abducens nerve can lead to abducens nerve palsy, characterized by the inability of the affected eye to move laterally, resulting in diplopia, or double vision. The affected eye may remain in a medial position when attempting to look straight ahead or to the side, resulting in a convergent strabismus. For instance, in a left abducens nerve palsy, the left eye is pulled inward due to the unaffected medial rectus muscle. This inward pull causes the left eye to turn towards the nose. Patients may also experience horizontal double vision, especially when looking leftwards, because the left lateral rectus muscle fails to abduct the eye properly.