HOME

TheInfoList



OR:

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 ...
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 cons ...
, 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 othe ...
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 syste ...
.


Structure


Nucleus

The
abducens nucleus 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 Anatomical terms of location, caudal portion of the pons, Anatomical t ...
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 ve ...
, 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). It is formed by fibres from the facial motor nucleus looping over the abducens nucleus. ...
. 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 (anatomy), orbit through the superior orbital fissure and innervates extraocular muscles that enable mo ...
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 neur ...
(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 cont ...
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 Va ...
and the
medulla Medulla or Medullary may refer to: Science * Medulla oblongata, a part of the brain stem * Renal medulla, a part of the kidney * Adrenal medulla, a part of the adrenal gland * Medulla of ovary, a stroma in the center of the ovary * Medulla of t ...
, 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 tas ...
. 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 located in th ...
(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 system. ...
to run between the dura and the skull through
Dorello's canal Dorello's canal is the bow-shaped bony enclosure at the tip of the temporal bone. It surrounds the abducens nerve (CN VI) and the inferior petrosal sinus as the two structures merge with the cavernous sinus. Structure Dorello's canal is a bow- ...
. 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 cave ...
. In the cavernous sinus, it runs anterior-ward alongside (inferolateral to) the
internal carotid artery The internal carotid artery (Latin: arteria carotis interna) is an artery in the neck which supplies the anterior circulation of the brain. In human anatomy, the internal and external carotids arise from the common carotid arteries, where these b ...
. 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 ner ...
, passing through the
common tendinous ring The common tendinous ring, also known as the annulus of Zinn, or annular tendon, is a ring of fibrous tissue surrounding the optic nerve at its entrance at the apex of the orbit. It is the common origin of the four recti muscles of the group of e ...
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 cons ...
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 spe ...
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 Va ...
.


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 cons ...
of the
human eye The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm. ...
. This muscle is responsible for outward gaze. The abducens nerve carries axons of type GSE,
general somatic efferent The general (spinal) somatic efferent neurons (GSE, somatomotor, or somatic motor fibers), arise from motor neuron cell bodies in the ventral horns of the gray matter within the spinal cord. They exit the spinal cord through the ventral roots, carr ...
.


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 v ...
), 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 (s ...
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 v ...
(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 or ...
), cavernous sinus diseases and various neuropathies. Perhaps the most common overall cause of sixth nerve impairment is diabetic neuropathy. 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 syndrome ...
and
Tolosa–Hunt syndrome Tolosa–Hunt syndrome is a rare disorder characterized by severe and unilateral headaches with orbital pain, along with weakness and paralysis (ophthalmoplegia) of certain eye muscles ( extraocular palsies). In 2004, the International Headache S ...
. 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 seconda ...
,
hydrocephalus Hydrocephalus is a condition in which an accumulation of cerebrospinal fluid (CSF) occurs within the brain. This typically causes increased intracranial pressure, pressure inside the skull. Older people may have headaches, double vision, poor ...
, 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 Conjugate gaze palsies are neurological disorder (medicine), disorders affecting the ability to move both eyes in the same direction. These palsies can affect Gaze (physiology), gaze in a horizontal, upward, or downward direction. These entities o ...
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 The term gaze is frequently used in physiology to describe coordinated motion of the eyes and neck. The lateral gaze is controlled by the paramedian pontine reticular formation (PPRF). The vertical gaze is controlled by the rostral interstitial nucl ...
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 ari ...
(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 minimall ...
(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 A saccade ( , French for ''jerk'') is a quick, simultaneous movement of both eyes Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling s ...
, smooth pursuit, accommodation) involves
conjugate gaze The term gaze is frequently used in physiology to describe coordinated motion of the eyes and neck. The lateral gaze is controlled by the paramedian pontine reticular formation (PPRF). The vertical gaze is controlled by the rostral interstitial nucl ...
, 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, ex ...
) 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 the ...
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 cons ...
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), fur or ...
it also innervates the musculus retractor bulbi, 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, ...
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 s ...
and hagfishes.


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