The The abducens nerveabducens nerve (the (the sixth cranial nervesixth cranial nerve, , also called the also called the sixth nervesixth nerve or simply or simply VIVI) is a ) is a motor nerve (a “somatic efferent” nerve) that motor nerve (a “somatic efferent” nerve) that

controls the movement of a single muscle, the controls the movement of a single muscle, the laterallateral rectus rectus muscle muscle of the eye of the eye

Peripheral anatomyPeripheral anatomy

The abducens nerve leaves the brainstem at the junction of The abducens nerve leaves the brainstem at the junction of the the ponspons and the and the medullamedulla, medial to the , medial to the facial nervefacial nerve. In . In order to reach the eye, it runs upward (superiorly) and then order to reach the eye, it runs upward (superiorly) and then bends forward (anteriorly).bends forward (anteriorly).

The nerve enters the The nerve enters the subarachnoidsubarachnoid space space when it emerges when it emerges from the brainstem. It runs upward between the pons and from the brainstem. It runs upward between the pons and the the clivusclivus, and then pierces the , and then pierces the duradura mater mater to run between to run between the dura and the skull. At the tip of the petrous the dura and the skull. At the tip of the petrous temporal bonetemporal bone it makes a sharp turn forward to enter the it makes a sharp turn forward to enter the cavernous sinuscavernous sinus. In the cavernous sinus it runs alongside . In the cavernous sinus it runs alongside the the internal carotid arteryinternal carotid artery. It then enters the orbit through . It then enters the orbit through the the superior orbital fissuresuperior orbital fissure and innervates the lateral rectus and innervates the lateral rectus musclemuscle of of the eye.the eye.

Central anatomyCentral anatomy The abducens nucleus is located in the pons, on the floor of The abducens nucleus is located in the pons, on the floor of

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

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

Clinical syndromesClinical syndromes Complete interruption of the peripheral sixth nerve causes Complete interruption of the peripheral sixth nerve causes

diplopiadiplopia (double vision), due to the unopposed action of the (double vision), due to the unopposed action of the medialmedial rectus rectus muscle muscle. The affected eye is pulled medially. . The affected eye is pulled medially. In order to see without double vision, patients will rotate In order to see without double vision, patients will rotate their heads so that both eyes are looking sideways. On their heads so that both eyes are looking sideways. On formal testing, the affected eye cannot abduct past the formal testing, the affected eye cannot abduct past the midline – it cannot look sideways, toward the temple. Partial midline – it cannot look sideways, toward the temple. Partial damage to the sixth nerve causes weak or incomplete damage to the sixth nerve causes weak or incomplete abduction of the affected eye. The diplopia is worse on abduction of the affected eye. The diplopia is worse on attempted lateral gaze, as would be expected (since the attempted lateral gaze, as would be expected (since the lateral gaze muscle is impaired).lateral gaze muscle is impaired).

Nuclear lesionsNuclear lesions Damage to the abducens Damage to the abducens nucleusnucleus does not produce an does not produce an

isolated sixth nerve palsy, but rather a horizontal isolated sixth nerve palsy, but rather a horizontal gaze palsygaze palsy that affects both eyes simultaneously. The abducens nucleus that affects both eyes simultaneously. The abducens nucleus contains two types of cells: motor neurons that control the contains two types of cells: motor neurons that control the lateral rectus muscle on the same side, and interneurons that lateral rectus muscle on the same side, and interneurons that cross the midline and connect to the contralateral cross the midline and connect to the contralateral oculomotor nucleus (which controls the medial rectus muscle oculomotor nucleus (which controls the medial rectus muscle of the opposite eye). In normal vision, lateral movement of of the opposite eye). In normal vision, lateral movement of one eye (lateral rectus muscle) is precisely coupled to medial one eye (lateral rectus muscle) is precisely coupled to medial movement of the other eye (medial rectus muscle), so that movement of the other eye (medial rectus muscle), so that both eyes remain fixed on the same object.both eyes remain fixed on the same object.

The control of conjugate gaze is mediated in the brainstem The control of conjugate gaze is mediated in the brainstem by the medial longitudinal fasciculus ,a nerve tract that by the medial longitudinal fasciculus ,a nerve tract that connects the three extraocular motor nuclei (abducens, connects the three extraocular motor nuclei (abducens, trochlear and oculomotor) into a single functional unit. trochlear and oculomotor) into a single functional unit. Lesions of the abducens nucleus and the MLF produce Lesions of the abducens nucleus and the MLF produce observable sixth nerve problems, most notably internuclear observable sixth nerve problems, most notably internuclear ophthalmoplegia (INO).ophthalmoplegia (INO).

Supranuclear lesionsSupranuclear lesions

The sixth nerve is one of the final common pathways for The sixth nerve is one of the final common pathways for numerous cortical systems that control eye movement in numerous cortical systems that control eye movement in general. Cortical control of eye movement (saccades, smooth general. Cortical control of eye movement (saccades, smooth pursuit, accommodation) involves pursuit, accommodation) involves conjugate gazeconjugate gaze, not , not unilateral eye movement. Disorders of unilateral eye movement. Disorders of conjugate gazeconjugate gaze are are discussed elsewhere discussed elsewhere