The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, 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 ...

that enters the

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

through the superior orbital fissure and innervates

extraocular muscles The extraocular muscles, or extrinsic ocular muscles, are the seven extrinsic muscles of the eye in human eye, humans and other animals. Six of the extraocular muscles, the four recti muscles, and the superior oblique muscle, superior and inferior ...

that enable most movements of the eye and that raise the eyelid. The nerve also contains fibers that innervate the intrinsic eye muscles that enable pupillary constriction and accommodation (ability to focus on near objects as in reading). The oculomotor nerve is derived from the basal plate of the

embryo An embryo ( ) is the initial stage of development for 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 sp ...

nic

midbrain The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

. Cranial nerves IV and VI also participate in control of

eye movement Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e.g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of inte ...

Structure

The oculomotor nerve originates from the third nerve nucleus at the level of the

superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the tectum, roof of the mammalian midbrain. In non-mammalian vertebrates, the Homology (biology), homologous structure is known as the optic tectum or optic lobe. The adjective f ...

in the midbrain. The third nerve nucleus is located ventral to the

cerebral aqueduct The cerebral aqueduct (aqueduct of the midbrain, aqueduct of Sylvius, Sylvian aqueduct, mesencephalic duct) is a small, narrow tube connecting the third and fourth ventricles of the brain. The cerebral aqueduct is a midline structure that passe ...

, on the pre-aqueductal

grey matter Grey matter, or gray matter in American English, is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil ( dendrites and unmyelinated axons), glial cells ( astrocytes and oligodendrocytes), synapses, ...

. The fibers from the two third nerve nuclei located laterally on either side of the cerebral aqueduct then pass through the

red nucleus The red nucleus or nucleus ruber is a structure in the rostral midbrain involved in motor coordination. The red nucleus is pale pink, which is believed to be due to the presence of iron in at least two different forms: hemoglobin and ferritin. ...

. From the red nucleus fibers then pass via the

substantia nigra The substantia nigra (SN) is a basal ganglia structure located in the midbrain that plays an important role in reward and movement. ''Substantia nigra'' is Latin for "black substance", reflecting the fact that parts of the substantia nigra a ...

to emerge from the substance of the brainstem at the oculomotor sulcus (a groove on the lateral wall of the

interpeduncular fossa The interpeduncular fossa is a deep depression of the ventral surface of the midbrain between the two cerebral crus, cerebal crura. It has been found in humans and macaques, but not in rats or mice, showing that this is a relatively new evolutiona ...

). On emerging 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 conti ...

, the nerve is invested with a sheath of

pia mater Pia mater ( or ),Entry "pia mater"
in
arachnoid. It passes between the superior cerebellar (below) and

posterior cerebral The posterior cerebral artery (PCA) is one of a pair of cerebral arteries that supply oxygenated blood to the occipital lobe, as well as the medial and inferior aspects of the temporal lobe of the human brain. The two arteries originate from the ...

arteries (above), and then pierces the dura mater anterior and lateral to the

posterior clinoid process The posterior clinoid processes are the tubercles of the sphenoid bone situated at the superior angles of the dorsum sellae (one on each angle) which represents the posterior boundary of the sella turcica. They vary considerably in size and form. ...

, passing between the free and attached borders of the

tentorium cerebelli The cerebellar tentorium or tentorium cerebelli (Latin for "tent of the cerebellum") is one of four dural folds that separate the cranial cavity into four (incomplete) compartments. The cerebellar tentorium separates the cerebellum from the cere ...

. It traverses 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 ...

, above the other orbital nerves receiving in its course one or two filaments from the

cavernous plexus The cavernous nerve plexus (also called the Walther 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 div ...

of the sympathetic nervous system, and a communicating branch from the

ophthalmic division The ophthalmic nerve (CN V1) is a sensory nerve of the head. It is one of three divisions of the trigeminal nerve (CN V), a cranial nerve. It has three major branches which provide sensory innervation to the eye, and the skin of the upper face an ...

of the trigeminal nerve. As the oculomotor nerve enters the orbit via the superior orbital fissure it then divides into a superior and an inferior branch.

Superior branch

The ''superior branch'' of the oculomotor nerve or the ''superior division'', the smaller, passes medially over the

optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve i ...

. It supplies the

superior rectus The superior rectus muscle is a muscle in the orbit (anatomy), orbit. It is one of the extraocular muscles. It is innervation, innervated by the superior division of the oculomotor nerve (III). In the primary position (looking straight ahead), its ...

and

levator palpebrae superioris The levator palpebrae superioris () is the muscle in the orbit that elevates the upper eyelid. Structure The levator palpebrae superioris originates from inferior surface of the lesser wing of the sphenoid bone, just above the optic foramen. It ...

Inferior branch

The ''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 The inferior rectus muscle is a muscle in the orbit near the eye. It is one of the four recti muscles in the group of extraocular muscles. It originates from the common tendinous ring, and inserts into the anteroinferior surface of the eye. It ...

. * The third and longest runs forward between the inferior recti and lateralis to the

inferior oblique The inferior oblique muscle or obliquus oculi inferior is a thin, narrow muscle placed near the anterior margin of the floor of the orbit. The inferior oblique is one of the extraocular muscles, and is attached to the maxillary bone (origin) an ...

. *From the third one, a short thick branch is given off to the lower part of the

ciliary ganglion The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It is 1–2 mm in diameter and in humans contains approximately 2,500 neurons. The ganglion contains postganglionic parasympathetic neurons. T ...

, and forms its short root. All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the inferior oblique, which enters the muscle at its posterior border.

Nuclei

The oculomotor nerve (CN III) arises from the anterior aspect of the

mesencephalon The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

(midbrain). There are two nuclei for the oculomotor nerve: * The

oculomotor nucleus The 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. F ...

originates at the level of the

. The muscles it controls are the striated muscle in levator palpebrae superioris and other

except for the

superior oblique muscle The superior oblique muscle or obliquus oculi superior is a fusiform muscle originating in the upper, medial side of the orbit (anatomy), orbit (i.e. from beside the nose) which abducts, depresses and internally rotates the eye. It is the only e ...

and the

lateral rectus muscle The lateral rectus muscle is a muscle on the lateral side of the eye in the orbit. It is one of six extraocular muscles that control the movements of the eye. The lateral rectus muscle is responsible for lateral movement of the eyeball, specif ...

. * The Edinger-Westphal nucleus supplies

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulat ...

fibers to the eye via the

, and thus controls the

sphincter pupillae The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor ...

muscle (affecting pupil constriction) and the

ciliary muscle The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscleSchachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) . in the eye's middle layer, the uvea ( vascular layer). It controls accommodation for vie ...

(affecting accommodation). 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 superior tarsal (Mueller's) muscle.

Function

The oculomotor nerve includes axons of type GSE, general somatic efferent, which innervate skeletal muscle of the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles. (Innervates all the extrinsic muscles except superior oblique and lateral rectus.) The nerve also includes axons of type GVE, general visceral efferent, which provide preganglionic parasympathetics to the ciliary ganglion. From the ciliary ganglion postganglionic fibers pass through the short ciliary nerve to the constrictor pupillae of the iris and the ciliary muscles.

Clinical significance

Disease

Paralysis of the oculomotor nerve, i.e., oculomotor nerve palsy, can arise due to: *direct trauma, *demyelinating diseases (e.g.,

multiple sclerosis Multiple sclerosis (MS) is an autoimmune disease resulting in damage to myelinthe insulating covers of nerve cellsin the brain and spinal cord. As a demyelinating disease, MS disrupts the nervous system's ability to Action potential, transmit ...

), *increased intracranial pressure (leading to uncal herniation) **due to a space-occupying lesion (e.g.,

brain cancer A brain tumor (sometimes referred to as brain cancer) occurs when a group of cells within the brain turn cancerous and grow out of control, creating a mass. There are two main types of tumors: malignant (cancerous) tumors and benign (non-cance ...

) or a **spontaneous

subarachnoid haemorrhage Subarachnoid hemorrhage (SAH) is bleeding into the subarachnoid space—the area between the arachnoid membrane and the pia mater surrounding the brain. Symptoms may include a severe headache of rapid onset, vomiting, decreased level of c ...

(e.g., berry aneurysm), and *microvascular disease, e.g., diabetes. In people with

diabetes Diabetes mellitus, commonly known as diabetes, is a group of common endocrine diseases characterized by sustained high blood sugar levels. Diabetes is due to either the pancreas not producing enough of the hormone insulin, or the cells of th ...

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 to the anatomical arrangement of the

nerve fiber An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see spelling differences) is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action pot ...

s in the oculomotor nerve; fibers controlling the pupillary function are superficial and spared from

ischemic Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...

injuries typical of diabetes. On the converse, an aneurysm which leads to compression of the oculomotor nerve affects the superficial fibers and manifests as a third nerve palsy with loss of the pupillary reflex (in fact, this third nerve finding is considered to represent an aneurysm—until proven otherwise—and should be investigated).

Examination

Eye muscles

Cranial nerves III, IV, and VI are usually tested together as part of the

cranial nerve examination The cranial nerve exam is a type of neurological examination. It is used to identify problems with the cranial nerves by physical examination. It has nine components. Each test is designed to assess the status of one or more of the twelve cranial ...

. 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

and

eyelid An eyelid ( ) is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. "Palpebral ...

s, the examiner is able to obtain more information about the

, the

levator palpebrae superioris muscle The levator palpebrae superioris () is the muscle in the orbit that elevates the upper eyelid. Structure The levator palpebrae superioris originates from inferior surface of the lesser wing of the sphenoid bone, just above the optic foramen. It ...

, and cranial nerves III, IV, and VI. Loss of function of any of the eye muscles results in ophthalmoparesis. 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 known by its ''down and out'' symptoms, because of the position of the affected eye (lateral, downward deviation of gaze).

Pupillary reflex

The oculomotor nerve also controls the constriction of the

pupil The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black becau ...

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, their pupils should constrict. Shining a light into one eye should result in equal constriction of the other eye. Fibers from the optic nerves cross over in the optic chiasm with some fibers passing to the contralateral optic nerve tract. This is the basis of the " swinging-flashlight test". Loss of accommodation and continued pupillary dilation can indicate the presence of a lesion on the oculomotor nerve.

Additional images

File:Cranial nerve III visceral.svg, Map of the oculomotor nerve. File:Gray720.png, Median sagittal section of brain. File:Gray775.png, Plan of oculomotor nerve. File:Ciliary ganglion pathways.png, Pathways in the Ciliary Ganglion. File:Midbrainsection.svg, Cross-sectional anatomy of the midbrain showing location of the nucleus of the oculomotor nerve and the Edinger-Westphal nucleus

References

External links

* * - "Oculomotor nerve palsy" * *
Animations of extraocular cranial nerve and muscle function and damage (University of Liverpool)
* () {{Authority control Cranial nerves Visual system Otorhinolaryngology Parasympathetic nervous system Autonomic nervous system Human head and neck Nervous system Neurology Ophthalmology Nerves of the head and neck