Abducens nerve (VI)

At a glance

English

Abducens nerve (VI)

Latin (TA2)

Nervus abducens (VI)

System

Nerve

Region

Head

Side

Paired, left and right

About the abducens nerve (vi)

Cranial nerve VI, the abducens or abducent nerve, is a pure somatic motor nerve with a single target, the lateral rectus muscle, which turns the eye away from the nose. In the embryo it grows out of the basal plate of the developing pons.

Its nucleus lies in the lower pons near the midline, beneath the fourth ventricle's floor. Fibres of the facial nerve loop over the back of the nucleus, raising a small bump in the ventricular floor called the facial colliculus. From the nucleus, axons run forwards and downwards through the pons, passing beside the corticospinal fibres, and emerge in the groove between pons and medulla, on the inner side of the facial nerve. The nerve then has an unusually long intracranial course. It climbs in the subarachnoid space along the clivus, pierces the dura and passes under the petrosphenoidal ligament through Dorello's canal, bends sharply forwards over the tip of the petrous temporal bone and enters the cavernous sinus. There it lies inside the sinus itself, beside the internal carotid artery rather than in the wall, and finally passes into the orbit by way of the superior orbital fissure, within the common tendinous ring.

The nucleus holds two kinds of cells. Motor neurons supply the lateral rectus on the same side, while interneurons cross the midline and ascend in the medial longitudinal fasciculus to the oculomotor neurons for the opposite medial rectus.

A lesion of the nerve itself leaves the eye turned in, because medial rectus is unopposed. Double vision side by side worsens when looking towards the affected side, and patients turn the head that way to compensate. The long course explains its many causes: petrous fractures, carotid aneurysms in the cavernous sinus, diabetes, inflammation, meningitis including tuberculous meningitis, and injury when a halo brace is fitted. Raised pressure that pushes the brainstem down stretches the nerve over the petrous apex, so a sixth nerve palsy can appear on either side and is a classic false localising sign; in a child it calls for a search for a tumour. Damage to the nucleus causes a gaze palsy affecting both eyes, a dorsal pontine lesion may add a facial palsy on the same side, and a ventral pontine lesion may add weakness of the opposite limbs.

Course and supply

Arises from

The abducens nucleus in the dorsal pons; the nerve emerges at the pontomedullary junction, medial to the facial nerve.

Ends in

Reaches the orbit via the superior orbital fissure, inside the common tendinous ring, and ends in the inner surface of the lateral rectus.

Supplies

Motor to the lateral rectus, which abducts the eye; through interneurons of its nucleus it also coordinates horizontal gaze of both eyes.