The abducens nerve or abducent nerve, also known as the sixth cranial nerve, cranial nerve VI, or simply CN VI, is a
cranial nerve
Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
in humans and various other animals that controls the movement of the
lateral rectus muscle
Lateral is a geometric term of location which may refer to:
Healthcare
*Lateral (anatomy), an anatomical direction
* Lateral cricoarytenoid muscle
* Lateral release (surgery), a surgical procedure on the side of a kneecap
Phonetics
*Lateral co ...
, one of the
extraocular muscles
The extraocular muscles (extrinsic ocular muscles), are the seven extrinsic muscles of the human eye. Six of the extraocular muscles, the four recti muscles, and the superior and inferior oblique muscles, control movement of the eye and the ot ...
responsible for outward
gaze
In critical theory, sociology, and psychoanalysis, the gaze (French ''le regard''), in the philosophical and figurative sense, is an individual's (or a group's) awareness and perception of other individuals, other groups, or oneself. The concept ...
. It is a
somatic efferent nerve
Efferent nerve fibers refer to axonal projections that ''exit'' a particular region; as opposed to afferent projections that ''arrive'' at the region. These terms have a slightly different meaning in the context of the peripheral nervous system ...
.
Structure
Nucleus
The
abducens nucleus is located in the pons, on the floor of the
fourth ventricle
The fourth ventricle is one of the four connected fluid-filled cavities within the human brain. These cavities, known collectively as the ventricular system, consist of the left and right lateral ventricles, the third ventricle, and the fourth ...
, at the level of the
facial colliculus
The facial colliculus is an elevated area located in the pontine tegmentum (dorsal pons), within the floor of the fourth ventricle (i.e. the rhomboid fossa
The rhomboid fossa is a rhombus-shaped depression that is the anterior part of the fou ...
. 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
The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of ...
and
trochlear nuclei).
Motor axons leaving the abducens nucleus run ventrally and caudally through the pons. They pass lateral to the
corticospinal tract
The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neuro ...
(which runs longitudinally through the pons at this level) before exiting the brainstem at the pontomedullary junction.
Course
The abducens nerve emerges from the
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...
at the junction of the
pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of ...
and the
medulla, superior to the
medullary pyramid,
and medial to the
facial nerve
The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of ta ...
. It runs upwards and forwards from this position to reach the eye.
The nerve enters the
subarachnoid space
In anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is locate ...
(more precisely, the
pontine cistern
The pontine cistern, also cisterna pontis and cisterna pontocerebellaris is a notable subarachnoid cistern on the ventral aspect of the pons.
It contains the basilar artery, and is continuous behind with the subarachnoid space of the spinal cord ...
) when it emerges from the brainstem. It runs upward between the pons and the
clivus, and then pierces the
dura mater
In neuroanatomy, dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous syste ...
to run between the dura and the skull through
Dorello's canal. At the apex of the
petrous part of the temporal bone
The petrous part of the temporal bone is pyramid-shaped and is wedged in at the base of the skull between the sphenoid and occipital bones. Directed medially, forward, and a little upward, it presents a base, an apex, three surfaces, and three an ...
, it makes a sharp turn forward to enter the
cavernous sinus
The cavernous sinus within the human head is one of the dural venous sinuses creating a cavity called the lateral sellar compartment bordered by the temporal bone of the skull and the sphenoid bone, lateral to the sella turcica.
Structure
The c ...
. In the cavernous sinus, it runs anterior-ward alongside (inferolateral to) the
internal carotid artery. It enters the orbit through (medial end of) the
superior orbital fissure
The superior orbital fissure is a foramen or cleft of the skull between the lesser and greater wings of the sphenoid bone. It gives passage to multiple structures, including the oculomotor nerve, trochlear nerve, ophthalmic nerve, abducens ...
, passing through the
common tendinous ring to reach and innervate the
lateral rectus muscle
Lateral is a geometric term of location which may refer to:
Healthcare
*Lateral (anatomy), an anatomical direction
* Lateral cricoarytenoid muscle
* Lateral release (surgery), a surgical procedure on the side of a kneecap
Phonetics
*Lateral co ...
of the eye.
Development
The human abducens nerve is derived from the
basal plate of the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
nic
pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of ...
.
Function
The abducens nerve supplies the
lateral rectus muscle
Lateral is a geometric term of location which may refer to:
Healthcare
*Lateral (anatomy), an anatomical direction
* Lateral cricoarytenoid muscle
* Lateral release (surgery), a surgical procedure on the side of a kneecap
Phonetics
*Lateral co ...
of the
human eye. This muscle is responsible for outward gaze. The abducens nerve carries axons of type GSE,
general somatic efferent.
Clinical significance
Damage
Damage to the peripheral part of the abducens nerve will cause double vision (
diplopia
Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally or vertically in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often ...
), due to the unopposed muscle tone of the
medial rectus muscle
The medial rectus muscle is a muscle in the orbit near the eye. It is one of the extraocular muscles. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of t ...
. The affected eye is pulled to look towards the midline. In order to see without double vision, patients will rotate their heads so that both eyes are toward the temple. Partial damage to the abducens nerve causes weak or incomplete abduction of the affected eye. The diplopia is worse on attempts at looking laterally.
The long course of the abducens nerve between the brainstem and the eye makes it vulnerable to injury at many levels. For example, fractures of the petrous temporal bone can selectively damage the nerve, as can
aneurysms
An aneurysm is an outward bulging, likened to a bubble or balloon, caused by a localized, abnormal, weak spot on a blood vessel wall. Aneurysms may be a result of a hereditary condition or an acquired disease. Aneurysms can also be a nidus ( ...
of the intracavernous carotid artery. Mass lesions that push the brainstem downward can damage the nerve by stretching it between the point where it emerges from the pons and the point where it hooks over the petrous temporal bone.
The central anatomy of the sixth nerve predicts (correctly) that infarcts affecting the dorsal pons at the level of the abducens nucleus can also affect the facial nerve, producing an ipsilateral facial palsy together with a lateral rectus palsy. The anatomy also predicts (correctly) that infarcts involving the ventral pons can affect the sixth nerve and the corticospinal tract simultaneously, producing a lateral rectus palsy associated with a contralateral hemiparesis. These rare syndromes are of interest primarily as useful summaries of the anatomy of the brainstem.
Peripheral lesions
Complete interruption of the peripheral sixth nerve causes
diplopia
Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally or vertically in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often ...
(double vision), due to the unopposed action of the
medial rectus muscle
The medial rectus muscle is a muscle in the orbit near the eye. It is one of the extraocular muscles. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of t ...
. The affected eye is pulled medially. In order to see without double vision, patients will turn their heads sideways so that both eyes are looking sideways. On formal testing, the affected eye cannot abduct past the midline – it cannot look sideways, toward the temple. Partial damage to the sixth nerve causes weak or incomplete abduction of the affected eye. The diplopia is worse on attempted lateral gaze, as would be expected (since the lateral gaze muscle is impaired).
Peripheral sixth nerve damage can be caused by tumors, aneurysms, or fractures – anything that directly compresses or stretches the nerve. Other processes that can damage the sixth nerve include strokes (infarctions), demyelination, infections (e.g.
meningitis
Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion ...
), cavernous sinus diseases and various neuropathies. Perhaps the most common overall cause of sixth nerve impairment is
diabetic neuropathy
Diabetic neuropathy is various types of nerve damage associated with diabetes mellitus. Symptoms depend on the site of nerve damage and can include motor changes such as weakness; sensory symptoms such as numbness, tingling, or pain; or autonomic c ...
. Iatrogenic injury is also known to occur, with the abducens nerve being the most commonly injured cranial nerve in halo orthosis placement. The resultant palsy is identified through loss of lateral gaze after application of the orthosis.
Rare causes of isolated sixth nerve damage include
Wernicke–Korsakoff syndrome
Wernicke–Korsakoff syndrome (WKS) is the combined presence of Wernicke encephalopathy (WE) and Korsakoff syndrome. Due to the close relationship between these two disorders, people with either are usually diagnosed with WKS as a single syndro ...
and
Tolosa–Hunt syndrome. Wernicke–Korsakoff syndrome is caused by thiamine deficiency, classically due to alcoholism. The characteristic ocular abnormalities are nystagmus and lateral rectus weakness. Tolosa-Hunt syndrome is an idiopathic granulomatous disease that causes painful oculomotor (especially sixth nerve) palsies.
Indirect damage to the sixth nerve can be caused by any process (
brain tumor
A brain tumor occurs when abnormal cells form within the brain. There are two main types of tumors: malignant tumors and benign (non-cancerous) tumors. These can be further classified as primary tumors, which start within the brain, and secon ...
,
hydrocephalus
Hydrocephalus is a condition in which an accumulation of cerebrospinal fluid (CSF) occurs within the brain. This typically causes increased pressure inside the skull. Older people may have headaches, double vision, poor balance, urinary in ...
,
pseudotumor cerebri, hemorrhage, edema) that exerts downward pressure on the brainstem, causing the nerve to stretch along the clivus. This type of traction injury can affect either side first. A right-sided brain tumor can produce either a right-sided or a left-sided sixth nerve palsy as an initial sign. Thus a right-sided sixth nerve palsy does not necessarily imply a right-sided cause. Sixth nerve palsies are infamous as "false localizing signs." Neurological signs are described as "false localizing" if they reflect dysfunction distant or remote from the expected anatomical location of pathology. Isolated sixth nerve palsies in children are assumed to be due to brain tumors until proven otherwise.
Nuclear lesions
Damage to the abducens nucleus does not produce an isolated
sixth nerve palsy
Sixth nerve palsy, or abducens nerve palsy, is a disorder associated with dysfunction of cranial nerve VI (the abducens nerve), which is responsible for causing contraction of the lateral rectus muscle to abduct (i.e., turn out) the eye. The ina ...
, but rather a horizontal
gaze palsy that affects both eyes simultaneously. The abducens nucleus contains two types of cells: motor neurons that control the lateral rectus muscle on the same side, and interneurons that cross the midline and connect to the contralateral oculomotor nucleus (which controls the medial rectus muscle of the opposite eye). In normal vision, lateral movement of one eye (lateral rectus muscle) is precisely coupled to medial movement of the other eye (medial rectus muscle), so that both eyes remain fixed on the same object.
The control of
conjugate gaze is mediated in the brainstem by the
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 ar ...
(MLF), a nerve tract that connects the three extraocular motor nuclei (abducens, trochlear and oculomotor) into a single functional unit. Lesions of the abducens nucleus and the MLF produce observable sixth nerve problems, most notably
internuclear ophthalmoplegia
Internuclear ophthalmoplegia (INO) is a disorder of conjugate lateral gaze in which the affected eye shows impairment of adduction. When an attempt is made to gaze contralaterally (relative to the affected eye), the affected eye adducts minimally ...
(INO).
Supranuclear lesions
The sixth nerve is one of the final common pathways for numerous cortical systems that control eye movement in general. Cortical control of eye movement (
saccades, smooth pursuit,
accommodation) involves
conjugate gaze, not unilateral eye movement.
Tuberculosis
15–40% of people with tuberculosis have some resulting cranial nerve deficit. The sixth nerve is the most commonly affected cranial nerve in immunocompetent people with tuberculosis.
History
Etymology
The Latin name for the sixth cranial nerve is "nervus abducens". The
Terminologia Anatomica
''Terminologia Anatomica'' is the international standard for human anatomical terminology. It is developed by the Federative International Programme on Anatomical Terminology, a program of the International Federation of Associations of Anatomis ...
officially recognizes two different English translations: "abducent nerve" and "abducens nerve".
[Federative Committee on Anatomical Terminology. Terminologia Anatomica. Thieme, 1998]
"Abducens" is more common in recent literature, while "abducent" predominates in the older literature. The
United States National Library of Medicine
The United States National Library of Medicine (NLM), operated by the United States federal government, is the world's largest medical library.
Located in Bethesda, Maryland, the NLM is an institute within the National Institutes of Health. Its ...
uses "abducens nerve" in its Medical Subject Heading (
MeSH
A mesh is a barrier made of connected strands of metal, fiber, or other flexible or ductile materials. A mesh is similar to a web or a net in that it has many attached or woven strands.
Types
* A plastic mesh may be extruded, oriented, e ...
) vocabulary to index the vast
MEDLINE
MEDLINE (Medical Literature Analysis and Retrieval System Online, or MEDLARS Online) is a bibliographic database of life sciences and biomedical information. It includes bibliographic information for articles from academic journals covering medic ...
and
PubMed
PubMed is a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine (NLM) at the National Institutes of Health maintain ...
biomedical databases. The 39th edition of ''Gray's Anatomy'' (2005) also prefers "abducens nerve."
Other animals
The abducens nerve controls the movement of a single muscle, the
lateral rectus muscle
Lateral is a geometric term of location which may refer to:
Healthcare
*Lateral (anatomy), an anatomical direction
* Lateral cricoarytenoid muscle
* Lateral release (surgery), a surgical procedure on the side of a kneecap
Phonetics
*Lateral co ...
of the eye. In most other
mammals
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...
it also innervates the
musculus retractor bulbi
Musculus may refer to:
*Andreas Musculus (1514–1581), German Lutheran theologian
*Heinrich Musculus (b. 1868), Swedish-Norwegian businessperson
*Wolfgang Musculus
Wolfgang Musculus, born "Müslin" or "Mauslein", (10 September 1497 – 30 August ...
, which can retract the eye for protection. Homologous abducens nerves are found in all
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, wi ...
except
lampreys
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like ...
and
hagfishes
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, a ...
.
See also
References
* Blumenfeld H. Neuroanatomy Through Clinical Cases. Sinauer Associates, 2002
* Brodal A. Neurological Anatomy in Relation to Clinical Medicine, 3rd ed. Oxford University Press, 1981
* Brodal P. The Central Nervous System, 3rded. Oxford University Press, 2004
* Butler AB, Hodos W. Comparative Vertebrate Neuroanatomy, 2nd ed. Wiley-Interscience, 2005
* Carpenter MB. Core Text of Neuroanatomy, 4th ed. Williams & Wilkins, 1991.
* Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed. McGraw-Hill, 2000
* Martin JH. Neuroanatomy Text and Atlas, 3rd ed. McGraw-Hill, 2003.
* Patten J. Neurological Differential Diagnosis, 2nd ed. Springer, 1996.
* Victor, M, Ropper, AH. Adam's and Victor's Principles of Neurology, 7th ed. McGraw-Hill, 2001.
* Wilson-Pauwels L, Akesson EJ, Stewart PA. Cranial Nerves: Anatomy and Clinical Comments. Decker, 1998.
;Books
*
Additional images
Image:Gray567.png, Dura mater and its processes exposed by removing part of the right half of the skull, and the brain.
Image:Gray689.png, Superficial dissection of brain-stem. Ventral view.
Image:Gray719.png, Hind- and mid-brains; postero-lateral view.
Image:Gray785.png, Figure showing the mode of innervation of the Recti medialis and lateralis of the eye.
Image:Gray787.png, Dissection showing origins of right ocular muscles, and nerves entering by the superior orbital fissure.
File:Slide3PIT.JPG, Cerebrum.Inferior view.Deep dissection
External links
*
*
Animations of extraocular cranial nerve and muscle function and damage (University of Liverpool)* ()
{{Authority control
Cranial nerves