Abducent nerve
Encyclopedia
The abducens nerve or abducent nerve (the sixth cranial nerve, also called the sixth nerve or simply VI) is a somatic
efferent
nerve that controls the movement of a single muscle, the lateral rectus muscle
of the eye, in humans. In most other mammals it also innervates the musculus retractor bulbi, which can retract the eye for protection. Homologous abducens nerves are found in all vertebrates except lampreys and hagfishes.
The human CN VI is derived from the basal plate
of the embryo
nic pons
.
officially recognizes two different English translations: abducent nerve and abducens nerve. Either term is correct.
“Abducens” is more common in recent literature, while “abducent” predominates in the older literature. The United States National Library of Medicine
uses “abducens nerve” in its Medical Subject Heading (MeSH
) vocabulary to index the vast MEDLINE
and PubMed
biomedical databases. The 39th edition of Gray’s Anatomy (2005) also prefers “abducens nerve.”
The abducens nerve leaves the brainstem at the junction of the pons
and the medulla
, medial to the facial nerve
. In order to reach the eye, it runs upward (superiorly) and then bends forward (anteriorly).
The nerve enters the subarachnoid space
when it emerges from the brainstem. It runs upward between the pons and the clivus
, and then pierces the dura mater
to run between the dura and the skull. At the tip of the petrous temporal bone
it makes a sharp turn forward to enter the cavernous sinus
. In the cavernous sinus it runs alongside the internal carotid artery
. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle
of the eye.
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 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 abducens nucleus is located in the pons, on the floor of the fourth ventricle
, at the level of the facial colliculus
. 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 and trochlear nuclei).
Motor axons leaving the abducens nucleus run ventrally and caudally through the pons. They pass lateral to the corticospinal tract
(which runs longitudinally through the pons at this level) before exiting the brainstem at the pontomedullary junction.
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.
(double vision), due to the unopposed action of the medial rectus muscle
. The affected eye is pulled medially. In order to see without double vision, patients will rotate their heads 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
), cavernous sinus diseases and various neuropathies. Perhaps the most common overall cause of sixth nerve impairment is diabetic neuropathy
.
Rare causes of isolated sixth nerve damage include Wernicke-Korsakoff syndrome
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
, hydrocephalus
, 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.” Isolated sixth nerve palsies in children are assumed to be due to brain tumors until proven otherwise.
The control of conjugate gaze is mediated in the brainstem by the medial longitudinal fasciculus
(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
(INO).
) involves conjugate gaze, not unilateral eye movement.
Somatic
The term somatic means 'of the body',, relating to the body. In medicine, somatic illness is bodily, not mental, illness. The term is often used in biology to refer to the cells of the body in contrast to the germ line cells which usually give rise to the gametes...
efferent
Efferent
Efferent is an anatomical term with the following meanings:*Conveying away from a center, for example the efferent arterioles conveying blood away from the Bowman's capsule in the kidney. Opposite to afferent....
nerve that controls the movement of a single muscle, the lateral rectus muscle
Lateral rectus muscle
The lateral rectus muscle is a muscle in the orbit. It is one of six extraocular muscles that control the movements of the eye and the only muscle innervated by the abducens nerve, cranial nerve VI....
of the eye, in humans. In most other mammals it also innervates the musculus retractor bulbi, which can retract the eye for protection. Homologous abducens nerves are found in all vertebrates except lampreys and hagfishes.
The human CN VI is derived from the basal plate
Basal plate (neural tube)
In the developing nervous system, the basal plate is the region of the neural tube ventral to the sulcus limitans. It extends from the rostral mesencephalon to the end of the spinal cord and contains primarily motor neurons, whereas neurons found in the alar plate are primarily associated with...
of the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
.
"Abducens" and "Abducent"
The Latin name for the sixth cranial nerve is nervus abducens. The Terminologia AnatomicaTerminologia Anatomica
Terminologia Anatomica is the international standard on human anatomic terminology. It was developed by the Federative Committee on Anatomical Terminology and the International Federation of Associations of Anatomists and was released in 1998. It supersedes the previous standard, Nomina...
officially recognizes two different English translations: abducent nerve and abducens nerve. Either term is correct.
“Abducens” is more common in recent literature, while “abducent” predominates in the older literature. The United States National Library of Medicine
United States National Library of Medicine
The United States National Library of Medicine , operated by the United States federal government, is the world's largest medical library. Located in Bethesda, Maryland, the NLM is a division of the National Institutes of Health...
uses “abducens nerve” in its Medical Subject Heading (MeSH
Medical Subject Headings
Medical Subject Headings is a comprehensive controlled vocabulary for the purpose of indexing journal articles and books in the life sciences; it can also serve as a thesaurus that facilitates searching...
) vocabulary to index the vast MEDLINE
MEDLINE
MEDLINE is a bibliographic database of life sciences and biomedical information. It includes bibliographic information for articles from academic journals covering medicine, nursing, pharmacy, dentistry, veterinary medicine, and health care...
and PubMed
PubMed
PubMed is a free database accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine at the National Institutes of Health maintains the database as part of the Entrez information retrieval system...
biomedical databases. The 39th edition of Gray’s Anatomy (2005) also prefers “abducens nerve.”
Peripheral anatomy
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
and the medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
, medial to the facial nerve
Facial nerve
The facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...
. In order to reach the eye, it runs upward (superiorly) and then bends forward (anteriorly).
The nerve enters the subarachnoid space
Subarachnoid space
In the central nervous system, the subarachnoid cavity is the interval between the arachnoid membrane and pia mater....
when it emerges from the brainstem. It runs upward between the pons and the clivus
Clivus (anatomy)
The clivus is a part of the cranium, a shallow depression behind the dorsum sellæ that slopes obliquely backward. It forms a gradual sloping process at the anterior most portion of the basilar occipital bone at its junction with the sphenoid bone. On axial planes, it sits just posterior to the...
, and then pierces the dura mater
Dura mater
The dura mater , or dura, is the outermost of the three layers of the meninges surrounding the brain and spinal cord. It is derived from Mesoderm. The other two meningeal layers are the pia mater and the arachnoid mater. The dura surrounds the brain and the spinal cord and is responsible for...
to run between the dura and the skull. At the tip of the petrous temporal bone
Temporal bone
The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.The temporal bone supports that part of the face known as the temple.-Parts:The temporal bone consists of four parts:* Squama temporalis...
it makes a sharp turn forward to enter the cavernous sinus
Cavernous sinus
The cavernous sinus , within the human head, is a large collection of thin-walled veins creating a cavity bordered by the temporal bone of the skull and the sphenoid bone, lateral to the sella turcica.-Contents:...
. In the cavernous sinus it runs alongside the internal carotid artery
Internal carotid artery
In human anatomy, the internal carotid arteries are two major arteries, one on each side of the head and neck. They arise from the common carotid arteries where these bifurcate into the internal and external carotid artery, and they supply the brain....
. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle
Lateral rectus muscle
The lateral rectus muscle is a muscle in the orbit. It is one of six extraocular muscles that control the movements of the eye and the only muscle innervated by the abducens nerve, cranial nerve VI....
of the eye.
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 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.
Central anatomy
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 ventricle...
, at the level of the facial colliculus
Facial colliculus
The facial colliculus is an elevated area located on the dorsal pons. In the floor of the 4th ventricle It is formed by motor fibers of the facial nerve as they loop 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 and trochlear nuclei).
Motor axons leaving the abducens nucleus run ventrally and caudally through the pons. They pass lateral to the corticospinal tract
Corticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....
(which runs longitudinally through the pons at this level) before exiting the brainstem at the pontomedullary junction.
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 diplopiaDiplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
(double vision), due to the unopposed action of the medial rectus muscle
Medial rectus muscle
The medial rectus muscle is a muscle in the orbit.As with most of the muscles of the orbit, it is innervated by the inferior division of the oculomotor nerve ....
. The affected eye is pulled medially. In order to see without double vision, patients will rotate their heads 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
Meningitis is inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges. The inflammation may be caused by infection with viruses, bacteria, or other microorganisms, and less commonly by certain drugs...
), cavernous sinus diseases and various neuropathies. Perhaps the most common overall cause of sixth nerve impairment is diabetic neuropathy
Diabetic neuropathy
Diabetic neuropathies are neuropathic disorders that are associated with diabetes mellitus. These conditions are thought to result from diabetic microvascular injury involving small blood vessels that supply nerves in addition to macrovascular conditions that can culminate in diabetic neuropathy...
.
Rare causes of isolated sixth nerve damage include Wernicke-Korsakoff syndrome
Wernicke-Korsakoff syndrome
Wernicke–Korsakoff syndrome is a manifestation of thiamine deficiency, or beriberi. This is usually secondary to alcohol abuse...
and Tolosa-Hunt syndrome
Tolosa-Hunt syndrome
Tolosa-Hunt syndrome is a rare disorder characterized by severe and unilateral headaches with extraocular palsies, usually involving the third, fourth, fifth, and sixth cranial nerves, and pain around the sides and back of the eye, along with weakness and paralysis of certain eye muscles.In 2004,...
. 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
Brain tumor
A brain tumor is an intracranial solid neoplasm, a tumor within the brain or the central spinal canal.Brain tumors include all tumors inside the cranium or in the central spinal canal...
, hydrocephalus
Hydrocephalus
Hydrocephalus , also known as "water in the brain," is a medical condition in which there is an abnormal accumulation of cerebrospinal fluid in the ventricles, or cavities, of the brain. This may cause increased intracranial pressure inside the skull and progressive enlargement of the head,...
, 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.” 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, 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
Medial longitudinal fasciculus
The medial longitudinal fasciculus is a pair of crossed fiber tracts , one on each side of the brainstem. These bundles of axons are situated near the midline of the brainstem and are composed of both ascending and descending fibers that arise from a number of sources and terminate in different...
(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
Internuclear ophthalmoplegia is a disorder of conjugate lateral gaze in which the affected eye shows impairment of adduction. When an attempt is made to gaze contralaterally , the affected eye adducts minimally, if at all. The contralateral eye abducts, however with nystagmus...
(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, accommodationAccommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....
) involves conjugate gaze, not unilateral eye movement.