Oculomotor nerve
Encyclopedia
The oculomotor nerve is the 3rd of 12 paired cranial nerves
. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle
. The optic nerve is derived from the basal plate
of the embryo
nic midbrain. Cranial nerves IV and VI also participate in control of eye movement.
(midbrain). There are two nuclei for the oculomotor nerve:
Sympathetic postganglionic fibres also join the nerve from the plexus on the internal carotid artery in the wall of the cavernous sinus and are distributed through the nerve, e.g., to the smooth muscle of levator palpebrae superioris.
, and enclosed in a prolongation from the arachnoid
.
It passes between the superior cerebellar (below) and posterior cerebral arteries (above), and then pierces the dura mater anterior and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli
.
It runs along the lateral wall of the cavernous sinus
, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus
of the sympathetic, and a communicating branch from the ophthalmic division of the trigeminal.
Here the nerve is placed below the trochlear nerve
and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve
is placed between its two rami:
s, the examiner is able to obtain more information about the extraocular muscles
, the levator palpebrae superioris muscle
, and cranial nerves III, IV, and VI.
Since the oculomotor nerve controls most of the eye muscles, it may be easier to detect damage to it. Damage to this nerve, termed oculomotor nerve palsy
is also known by the down n' out symptoms, because of the position of the affected eye.
s and thickening of the lens of the eye. This can be tested in two main ways. By moving a finger toward a person's face to induce accommodation
, as well as his going cross-eyed, his pupils should constrict.
Shining a light into one eye should result in equal constriction of the other eye. The neurons in the optic nerve decussate in the optic chiasm with some crossing to the contralateral optic nerve tract. This is the basis of the "swinging-flashlight test
".
, is a rare condition. It can arise due to:
In people with diabetes
and older than 50 years of age, an oculomotor nerve palsy, in the classical sense, occurs with sparing (or preservation) of the pupillary reflex. This is thought to arise due the anatomical arrangement of the nerve fiber
s in the oculomotor nerve; fibers controlling the pupillary function are superficial and spared from ischemic
injuries typical of diabetes. On the converse, a subarachnoid haemorrhage, which leads to compression of the oculomotor nerve, usually affects the superficial fibers and manifests as a palsy with loss of the pupillary reflex.
Cranial nerves
Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...
. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle
Levator palpebrae superioris muscle
The levator palpebrae superioris is the muscle in the orbit that elevates the superior eyelid.-Structure:...
. The optic nerve 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 midbrain. Cranial nerves IV and VI also participate in control of eye movement.
Nuclei
The oculomotor nerve (CN III) arises from the anterior aspect of mesencephalonMesencephalon
The midbrain or mesencephalon is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal , and temperature regulation....
(midbrain). There are two nuclei for the oculomotor nerve:
- The oculomotor nucleusOculomotor nucleusThe fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle...
originates at the level of the superior colliculusSuperior colliculusThe optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
. The muscles it controls are the striated muscle in levator palpebrae superioris and all extraocular musclesExtraocular musclesThe extraocular muscles are the six muscles that control the movements of the eye . The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction.-List of muscles:-Importance:...
except for the superior oblique muscleSuperior oblique muscleFor the abdominal muscle see: Abdominal external oblique muscleThe superior oblique muscle, or obliquus oculi superior, is a fusiform muscle originating in the upper, medial side of the orbit which abducts, depresses and internally rotates the eye...
and the lateral rectus muscleLateral rectus muscleThe 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....
.
- The Edinger-Westphal nucleusEdinger-Westphal nucleusThe Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...
supplies parasympathetic fibres to the eye via the ciliary ganglionCiliary ganglionThe ciliary ganglion is a parasympathetic ganglion located in the posterior orbit. It measures 1–2 millimeters in diameter and contains approximately 2,500 neurons. Preganglionic axons from the Edinger-Westphal nucleus travel along the oculomotor nerve and form synapses with these cells...
, and thus controls the sphincter pupillae muscle (affecting pupil constriction) and the ciliary muscleCiliary muscleThe ciliary muscle is a ring of striated smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye not the size of the pupil which is...
(affecting accommodationAccommodation reflexThe accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object , comprising coordinated changes in vergence, lens shape and pupil size...
).
Sympathetic postganglionic fibres also join the nerve from the plexus on the internal carotid artery in the wall of the cavernous sinus and are distributed through the nerve, e.g., to the smooth muscle of levator palpebrae superioris.
Emergence from brain
On emerging from the brain, the nerve is invested with a sheath of pia materPia mater
Pia mater often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord. The word finds its roots in Latin, meaning literally "tender mother." The other two meningeal membranes are the dura mater and the arachnoid mater....
, and enclosed in a prolongation from the arachnoid
Arachnoid mater
The arachnoid mater, literally from Latin "spider -like mother", is one of the three meninges, the membranes that cover the brain and spinal cord...
.
It passes between the superior cerebellar (below) and posterior cerebral arteries (above), and then pierces the dura mater anterior and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli
Tentorium cerebelli
The tentorium cerebelli or cerebellar tentorium is an extension of the dura mater that separates the cerebellum from the inferior portion of the occipital lobes.-Anatomy:...
.
It runs along the lateral wall of 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:...
, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus
Cavernous plexus
The cavernous nerve plexus is situated below and medial to that part of the internal carotid artery which is placed by the side of the sella turcica in the cavernous sinus, and is formed chiefly by the medial division of the internal carotid nerve....
of the sympathetic, and a communicating branch from the ophthalmic division of the trigeminal.
Superior and inferior rami
It then divides into two branches, which enter the orbit through the superior orbital fissure, between the two heads of the lateral rectus.Here the nerve is placed below the trochlear nerve
Trochlear nerve
The trochlear nerve is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye....
and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve
Nasociliary nerve
The nasociliary nerve is a branch of the ophthalmic nerve. It is intermediate in size between the two other main branches of the ophthalmic nerve, the frontal nerve and the lacrimal nerve, and is more deeply placed.-Path:...
is placed between its two rami:
- superior branch of oculomotor nerveSuperior branch of oculomotor nerveThe superior branch of the oculomotor nerve or the superior division, the smaller, passes medialward over the optic nerve.It supplies the Superior rectus and Levator palpebrae superioris....
- inferior branch of oculomotor nerveInferior branch of oculomotor nerveThe inferior branch of the oculomotor nerve or the inferior division, the larger, divides into three branches.* One passes beneath the optic nerve to the medial rectus.* Another, to the inferior rectus....
Eye muscles
Cranial nerves III, IV, and VI are usually tested together. The examiner typically instructs the patient to hold his head still and follow only with the eyes a finger or penlight that circumscribes a large "H" in front of the patient. By observing the eye movement and eyelidEyelid
An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...
s, the examiner is able to obtain more information about the extraocular muscles
Extraocular muscles
The extraocular muscles are the six muscles that control the movements of the eye . The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction.-List of muscles:-Importance:...
, the levator palpebrae superioris muscle
Levator palpebrae superioris muscle
The levator palpebrae superioris is the muscle in the orbit that elevates the superior eyelid.-Structure:...
, and cranial nerves III, IV, and VI.
Since the oculomotor nerve controls most of the eye muscles, it may be easier to detect damage to it. Damage to this nerve, termed oculomotor nerve palsy
Oculomotor nerve palsy
Oculomotor nerve palsy is an eye condition resulting from damage to the third cranial nerve or a branch thereof. As the name suggests, the oculomotor nerve supplies the majority of the muscles controlling eye movements. Thus, damage to this nerve will result in the affected individual being unable...
is also known by the down n' out symptoms, because of the position of the affected eye.
Pupillary reflex
The oculomotor nerve also controls the constriction of the pupilPupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...
s and thickening of the lens of the eye. This can be tested in two main ways. By moving a finger toward a person's face to induce accommodation
Accommodation reflex
The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object , comprising coordinated changes in vergence, lens shape and pupil size...
, as well as his going cross-eyed, his pupils should constrict.
Shining a light into one eye should result in equal constriction of the other eye. The neurons in the optic nerve decussate in the optic chiasm with some crossing to the contralateral optic nerve tract. This is the basis of the "swinging-flashlight test
Swinging-flashlight test
The swinging-flashlight test is used to help a practitioner decide whether reduced vision is due to ocular disease.-Process:For an adequate test, vision must not be entirely lost. In dim room light, the examiner notes the size of the pupils...
".
Pathology
Paralysis of the oculomotor nerve, i.e., oculomotor nerve palsyOculomotor nerve palsy
Oculomotor nerve palsy is an eye condition resulting from damage to the third cranial nerve or a branch thereof. As the name suggests, the oculomotor nerve supplies the majority of the muscles controlling eye movements. Thus, damage to this nerve will result in the affected individual being unable...
, is a rare condition. It can arise due to:
- direct traumaPhysical traumaTrauma refers to "a body wound or shock produced by sudden physical injury, as from violence or accident." It can also be described as "a physical wound or injury, such as a fracture or blow." Major trauma can result in secondary complications such as circulatory shock, respiratory failure and death...
, - demyelinating diseases (e.g., multiple sclerosisMultiple sclerosisMultiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...
), - increased intracranial pressure (leading to uncal herniation)
- due to a space-occupying lesion (e.g., brain cancer) or a
- spontaneous subarachnoid haemorrhage (e.g., berry aneurysm), and
- microvascular disease, e.g., diabetes.
In people with diabetes
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
and older than 50 years of age, an oculomotor nerve palsy, in the classical sense, occurs with sparing (or preservation) of the pupillary reflex. This is thought to arise due the anatomical arrangement of the nerve fiber
Nerve fiber
A nerve fiber is a threadlike extension of a nerve cell and consists of an axon and myelin sheath in the nervous system. There are nerve fibers in the central nervous system and peripheral nervous system. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system , myelin...
s in the oculomotor nerve; fibers controlling the pupillary function are superficial and spared from ischemic
Ischemia
In medicine, ischemia is a restriction in blood supply, generally due to factors in the blood vessels, with resultant damage or dysfunction of tissue. It may also be spelled ischaemia or ischæmia...
injuries typical of diabetes. On the converse, a subarachnoid haemorrhage, which leads to compression of the oculomotor nerve, usually affects the superficial fibers and manifests as a palsy with loss of the pupillary reflex.