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
Caudal may refer to:
Anatomy
* Caudal (anatomical term) (from Latin ''cauda''; tail), used to describe how close something is to the trailing end of an organism
* Caudal artery, the portion of the dorsal aorta of a vertebrate that passes into the ...
portion of the
pons,
medial to the
sulcus limitans.
The abducens nucleus along with the
internal genu
The geniculate ganglion (from Latin ''genu'', for "knee") is a collection of pseudounipolar sensory neurons of the facial nerve located in the facial canal of the head. It receives fibers from the facial nerve. It sends fibers that supply the lac ...
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
Two primary neuron types are located in the abducens nucleus: motorneurons 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.
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 Syndrome 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.
[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.]
Damage to the abducens nerve causes monocular ipsilateral lateral
ophthalmoparesis
Ophthalmoparesis refers to weakness (-paresis) or paralysis (-plegia) of one or more extraocular muscles which are responsible for eye movements. It is a physical finding in certain neurologic, ophthalmologic, and endocrine disease.
Internal o ...
: 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
The medial longitudinal fasciculus (MLF) is an area of crossed over tracts, on each side of the brainstem. These bundles of axons are situated near the midline of the brainstem. They are made up of both ascending and descending fibers that ari ...
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
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.
References
External links
*
*
Template (look for "GSE")*
NIF Search - Abducens Nucleusvia the
Neuroscience Information Framework
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Cranial nerve nuclei
Nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucle ...