Oculomotor nerve

Nerve: Oculomotor nerve
Nerves of the orbit. Seen from above.
Inferior view of the human brain, with the cranial nerves labelled.
Latin nervus oculomotorius
Gray's subject #198 884
Innervates Superior rectus, Inferior rectus, Medial rectus, Inferior oblique, Levator palpebrae, sphincter pupillae (parasympathetics), ciliaris muscle (parasympathetics)
From oculomotor nucleus, Edinger-Westphal nucleus
To superior branch, inferior branch
MeSH Oculomotor+Nerve
Cranial Nerves
CN I – Olfactory
CN II – Optic
CN III – Oculomotor
CN IV – Trochlear
CN V – Trigeminal
CN VI – Abducens
CN VII – Facial
CN VIII – Vestibulocochlear
CN IX – Glossopharyngeal
CN X – Vagus
CN XI – Spinal Accessory
CN XII – Hypoglossal
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The oculomotor nerve is the 3rd of 12 paired cranial nerves. It controls most of the eye's movement and constriction of the pupil, and maintains an open eyelid. The optic nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in control of eye movement.

Contents

Path

Nuclei

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

Emergence from brain

On emerging from the brain, the nerve is invested with a sheath of pia mater, 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.

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 and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami:

Testing the oculomotor nerve

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 eyelids, 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.

Pupillary reflex

The oculomotor nerve also controls the constriction of the pupils 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".

Pathology

Paralysis of the oculomotor nerve, i.e., oculomotor nerve palsy, is a rare condition. It can arise due to:

  • direct trauma,
  • demyelinating diseases (e.g., multiple sclerosis),
  • 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 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 fibers 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.[1]

Additional images

References

  1. ^ Goodwin J. Oculomotor Nerve Palsy. eMedicine.com. URL:http://emedicine.medscape.com/article/1198462-overview. Accessed on: January 16, 2009.

See also

External links


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Look at other dictionaries:

  • Oculomotor nerve — The oculomotor nerve is the third cranial nerve. (The cranial nerves emerge from or enter the cranium, the skull, as opposed to the spinal nerves which emerge from the vertebral column). The oculomotor nerve is responsible for the nerve supply to …   Medical dictionary

  • oculomotor nerve — noun supplies extrinsic muscles of the eye • Syn: ↑oculomotor, ↑nervus oculomotorius, ↑third cranial nerve • Hypernyms: ↑cranial nerve …   Useful english dictionary

  • oculomotor nerve — the third cranial nerve (III), which is composed of motor fibres distributed to muscles in and around the eye. Fibres of the parasympathetic system are responsible for altering the size of the pupil and the lens of the eye. Fibres outside the eye …   The new mediacal dictionary

  • oculomotor nerve — oculomo′tor nerve n. anat. either one of the third pair of cranial nerves, which innervate most of the muscles of the eyeball • Etymology: 1880–85 …   From formal English to slang

  • oculomotor nerve — noun Date: 1881 either of the pair of chiefly motor nerves that comprise the third pair of cranial nerves, arise from the midbrain, and supply four muscles of the eye …   New Collegiate Dictionary

  • oculomotor nerve — Anat. either one of the third pair of cranial nerves, consisting chiefly of motor fibers that innervate most of the muscles of the eyeball. See diag. under brain. [1880 85] * * * …   Universalium

  • oculomotor nerve — noun The third of twelve paired cranial nerves, which controls most of the eyes movements and constriction of the pupil and maintains an open eyelid …   Wiktionary

  • oculomotor nerve — /ɒkjəloʊˈmoʊtə nɜv/ (say okyuhloh mohtuh nerv) noun either of the two cranial nerves which supply most of the muscles of the eyeball …   Australian English dictionary

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