Cranial Nerve Six – Anatomy, Nerve and Blood Supply

Cranial Nerve Six/Cranial nerve six (CN VI) also known as the abducens nerve, is one of the nerves responsible for the extraocular motor functions of the eye, along with the oculomotor nerve (CN III) and the trochlear nerve (CN IV).

The sixth cranial nerve runs a long course from the brainstem to the lateral rectus muscle. Based on the location of an abnormality, other neurologic structures may be involved with the pathology related to this nerve. Sixth nerve palsy is frequently due to a benign? process with full recovery within weeks, yet caution is warranted as it may portend a serious neurologic process. Hence, early diagnosis? is often critical for some conditions that present with sixth nerve palsy. This article outlines a simple clinical approach to sixth nerve palsy based on its anatomy.

The abducens (also called abducent) nerve (CN VI) is the last of the three visual-motor nerves. It is a brainstem structure that is located in the dorsal aspect of the pons, deep to the facial colliculus in the 4th ventricle (rhomboid fossa). As the fibers of CN VI emerge from the pons, they travel ventrally, to leave the brain parenchyma. It eventually arrives at the preoptic region where it will innervate the lateral rectus muscle, which arises from the third preoptic myotome.

Structure of Cranial Nerve Six

Unlike the oculomotor nerve and the trochlear nerve, the abducens nerve is a purely somatic nerve, meaning the nerve has no sensory function. Its main function is to carry general somatic efferent nerve axons to innervate the lateral rectus muscle which then abducts the eye on the ipsilateral side. It is also secondarily involved in innervation of the contralateral medial rectus muscle by way of the medial longitudinal fasciculus so that both eyes move laterally in a coordinated manner.[rx][rx][rx]

The abducens nerve leaves the brainstem at the junction of the pons and the medulla, medial to the facial nerve. It runs upwards and forwards from this position to reach the eye.

The nerve enters the subarachnoid space when it emerges from the brainstem. It runs upward between the pons and the clivus and then pierces the dura mater to run between the dura and the skull through Dorello’s canal. At the tip of the petrous part of the temporal bone, it makes a sharp turn forward to enter the cavernous sinus.^[1] In the cavernous sinus, it runs alongside the internal carotid artery. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye.

Nucleus

The abducens nucleus is located in the pons, on the floor of the fourth ventricle, at the level of the facial colliculus. Axons from the facial nerve loop around the abducens nucleus, creating a slight bulge (the facial colliculus) that is visible on the dorsal surface of the floor of the fourth ventricle. The abducens nucleus is close to the midline, like the other motor nuclei that control eye movements (the oculomotor and trochlear nuclei).

Motor axons leaving the abducens nucleus run ventrally and caudally through the pons. They pass lateral to the corticospinal tract (which runs longitudinally through the pons at this level) before exiting the brainstem at the pontomedullary junction.

Nerves of Cranial Nerve Six

The nerve itself can be divided into four distinct portions: the nucleus, the cisternal portion, the cavernous sinus portion, and the orbital portion. The abducens nucleus resides in the dorsal pons, ventral to the floor of the fourth ventricle, and just lateral to the medial longitudinal fasciculus. About forty percent of the axons project through the ipsilateral medial longitudinal fasciculus to cross over to the contralateral medial rectus subnucleus to eventually innervate the contralateral medial rectus muscle. The abducens nucleus is supplied by the pontine branches of the basilar artery.

Of all the cranial nerves, the abducens nerve has the longest intracranial course. It is located in the pons on the floor of the fourth ventricle, at the same level as the facial colliculus. In fact, the axons of the facial nerve loop around the posterior aspect of the abducens nucleus. This will be of clinical significance later. The nerve originates from the caudal, dorsal pontine below the fourth ventricle. After the fibers emerge from the nucleus, they course superiorly and then anteriorly before the majority of the axons leave the brainstem at the junction of the pontine and the medulla (i.e., the pontomedullary groove) caudal and medial to both the facial nerve and the vestibulocochlear nerve in most cases.

The nerve then travels through the subarachnoid space and crosses the upper edge of the tip of the petrous part of the temporal bone towards the clivus within a fibrous sheath called Dorello’s canal and enters the dura inferior to the posterior clinoid process. Because it is anchored in Dorello’s canal, the nerve is prone to stretching when intracranial pressure is increased due to multiple causes discussed later. It then enters the cavernous sinus (along with the oculomotor nerve, the trochlear nerve, and the first branch of the trigeminal nerve (V1), following lateral to the internal carotid artery and medial to a lateral wall of the sinus before following the sphenoidal fissure and entering the orbit through the superior orbital fissure within the tendinous ring to reach its destination at the lateral rectus muscle.

Muscles of Cranial Nerve Six

The abducens nerve functions to innervate the ipsilateral lateral rectus muscle and partially innervate the contralateral medial rectus muscle (at the level of the nucleus – via the medial longitudinal fasciculus).