{{Short description|Neuron group in the brainstem}} {{Infobox brain | Name = Abducens nucleus | Latin = nucleus nervi abducentis | Image = Gray696.svg | Width = 200px | Caption = The cranial nerve nuclei schematically represented; dorsal view. Motor nuclei in red; sensory in blue. The olfactory and optic centers are not represented. (Abducens nucleus is VI) | Image2 = | Caption2 = | IsPartOf = Pons | Components = | Artery = Pontine branches of the Basilar artery | Vein = }} The '''abducens nucleus''' is the originating nucleus from which the abducens nerve (VI) emerges—a cranial nerve nucleus. This nucleus is located beneath the fourth ventricle in the caudal portion of the pons near the midline,'''<ref name=":022">{{Cite book |last=Sinnatamby |first=Chummy S. |title=Last's Anatomy |publisher= Elsevier Australia|year=2011 |isbn=978-0-7295-3752-0 |edition=12th |pages=478}}</ref>''' medial to the sulcus limitans.
The abducens nucleus along with the internal genu of the facial nerve make up the facial colliculus, a hump at the caudal end of the medial eminence on the dorsal aspect of the pons.
==Structure== {{Unreferenced section|date=September 2025}} Two primary neuron types are located in the abducens nucleus: motoneurons and interneurons. The former directly drive the contraction of the ipsilateral lateral rectus muscle via the abducens nerve (sixth cranial nerve); contraction of this muscle rotates the eye outward (abduction). The latter relay signals from the abducens nucleus to the contralateral oculomotor nucleus, where motoneurons drive the contraction of the ipsilateral medial rectus muscle (hence, contralateral to the abducens nucleus that issues the command); contraction of this muscle rotates the eye inward (adduction).
==Function== This "wiring" pattern suggests that the main function of the abducens nucleus is to generate coordinated movements of both eyes in the same direction. Indeed, electrical stimulation of the abducens nucleus has been shown to generate conjugate eye movements (i.e. both eyes rotate in the same direction, and by the same angle). Such eye movements occur whenever we look between targets located in the distance. Moreover, lesions to the axonal tract of interneurons (in the medial longitudinal fasciculus) have been shown to disrupt conjugate eye movements through the paralysis of the contralateral eye. Importantly, despite the lesions, this muscle remains functional during convergence eye movements. Finally, experiments where the electrical activity of single neurons in the abducens nucleus has been recorded during slow and fast conjugate eye movements have demonstrated very little differences in the discharge patterns of motoneurons and interneurons.<ref>{{Citation |last=Somani |first=Anisha N. |title=Neuroanatomy, Abducens Nucleus |date=2025 |work=StatPearls |url=http://www.ncbi.nlm.nih.gov/books/NBK544265/ |access-date=2025-12-22 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31334985 |last2=Adesina |first2=Ore-ofe}}</ref>
Altogether, it is now well accepted that the abducens nucleus is a key structure for the conjugated movements of both eyes.
==Clinical significance== Infantile esotropia may originate from problems in the abducens nucleus firing rate when there is poor abduction in an infant. Studies have shown differences in firing rates in Monkeys with induced strabismus.<ref name="Cullen">{{cite journal |last1=Cullen |first1=K. E. |title=Neural Circuits That Drive Binocular Eye Movements: Implications for Understanding and Correcting Strabismus |journal=Investigative Ophthalmology & Visual Science |date=2 January 2015 |volume=56 |issue=1 |pages=20 |doi=10.1167/iovs.14-16177|pmid=25555399 }}</ref><ref>Walton MMG, Mustari MJ, Willoughby CL, McLoon LK. Abnormal activity of neurons in abducens nucleus of strabismic monkeys. Invest Ophthalmol Vis Sci. 2015;56:10–19. {{doi|10.1167/iovs.14-15360}}</ref><ref>Van Horn MR, Waitzman DM, Cullen KE. Vergence neurons identified in the rostral superior colliculus code smooth eye movements in 3D space. J Neurosci. 2013;33:7274–7284.</ref>
Damage to the abducens nerve causes monocular ipsilateral lateral ophthalmoparesis: specifically, loss of the ability to move the ipsilateral eye outward (abduction).
In contrast, damage to the abducens nucleus causes lateral gaze palsy. This is due to damage to both the lower motor neurons that innervate the ipsilateral lateral rectus and internuclear neurons that projecting through the contra lateral medial longitudinal fasciculus to the medial rectus subnucleus of the oculomotor nucleus. Note, however, that the eye contralateral to the lesion can still move in the direction of the lesion during convergence movements.
==Additional images== <gallery> Image:Gray697.png|Nuclei of origin of cranial motor nerves schematically represented; lateral view. Image:Gray774.png|Scheme showing central connections of the optic nerves and optic tracts. Image:Gray785.png|Figure showing the mode of innervation of the Recti medialis and lateralis of the eye. Image:Pons section at facial colliculus.png|Axial section of the Brainstem (Pons) at the level of the Facial Colliculus Image:ThreeNeuronArc.png |Vestibulo-ocular reflex Image:Brain stem sagittal section.svg|Brain stem sagittal section Image:Lower pons horizontal KB.svg|Section through lower pons. Abducens nucleus is labeled #4. </gallery>
==References== {{Reflist}}
==External links== * {{BrainstemWisconsin|13VNAN}} * {{LoyolaMedEd|Neuro/frames/nlBSs/nl27fr.htm}} * [https://web.archive.org/web/20060903225311/http://isc.temple.edu/neuroanatomy/lab/embryo_new/pons/2/ Template (look for "GSE")] * {{BrainMaps|Abducens%20nucleus|Abducens nucleus}} * [https://www.neuinfo.org/mynif/search.php?q=Abducens%20Nucleus&t=data&s=cover&b=0&r=20 NIF Search - Abducens Nucleus]{{dead link|date=October 2016 |bot=InternetArchiveBot |fix-attempted=yes }} via the Neuroscience Information Framework
{{Pons}} {{Cranial nerves}} {{Authority control}}
Category:Cranial nerve nuclei Nucleus