Optic nerve
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| Nerve: Optic nerve | |
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| The left optic nerve and the optic tracts. | |
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| Optic nerve leaving the back of a calf eye (from dissection). | |
| Latin | nervus opticus |
| Gray's | subject #197 882 |
| MeSH | Optic+Nerve |
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Anatomy
The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during embryonic development. Consequently, the fibers are covered with myelin produced by oligodendrocytes rather than the Schwann cells of the peripheral nervous system. Similarly, the optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in peripheral nerves. This is an important issue, as fiber tracks of the mammalian central nervous system (as opposed to the peripheral nervous system) are incapable of regeneration and hence optic nerve damage produces irreversible blindness. The fibers from the retina run along the optic nerve to nine primary visual nuclei in the brain, from whence a major relay inputs into the primary visual cortex.
The optic nerve is composed of retinal ganglion cell axons and support cells. It leaves the orbit (eye) via the optic canal, running postero-medially towards the optic chiasm where there is a partial decussation (crossing) of fibers from the temporal visual fields of both eyes. Most of the axons of the optic nerve terminate in the lateral geniculate nucleus from where information is relayed to the visual cortex. Its diameter increases from about 1.6 mm within the eye, to 3.5 mm in the orbit to 4.5 mm within the cranial space. The optic nerve component lengths are 1 mm in the globe, 25 mm in the orbit, 9 mm in the optic canal and 16 mm in the cranial space before joining the optic chiasm. There, partial decussation occurs and about 53% of the fibers cross to form the optic tracts. Most of these fibers terminate in the lateral geniculate body.
From the lateral geniculate body, fibers of the optic radiation pass to the visual cortex in the occipital lobe of the brain. More specifically, fibers carrying information from the contralateral superior visual field traverse Meyer's loop to terminate in the lingual gyrus below the calcarine fissure in the occipital lobe, and fibers carrying information from the contralateral inferior visual field terminate more superiorly.
Physiology
The optic nerve contains 1.2 million nerve fibers. This number is low compared to the roughly 100 million photoreceptors in the retina,[1] and implies that substantial pre-processing takes place in the retina before the signals are sent to the brain through the optic nerve.
The eye's blind spot is a result of the absence of retina where the optic nerve leaves the eye. This is because there are no photoreceptors in this area.
Role in disease
Damage to the optic nerve typically causes permanent and potentially severe loss of vision, as well as an abnormal pupillary reflex, which is diagnostically important. The type of visual field loss will depend on which portions of the optic nerve were damaged. Generally speaking:
- Damage before the optic chiasm causes loss of vision in the visual field of the same side only.
- Damage in the chiasm causes loss of vision laterally in both visual fields (bitemporal hemianopia). It may occur in large pituitary adenomata.
- Damage after the chiasm causes loss of vision on one side but affecting both visual fields: the visual field affected is located on the opposite side of the lesion.
Injury to the optic nerve can be the result of congenital or inheritable problems like Leber's Hereditary Optic Neuropathy, glaucoma, trauma, toxicity, inflammation, ischemia, infection (very rarely), or compression from tumors or aneurysms. By far, the three most common injuries to the optic nerve are from glaucoma, optic neuritis (especially in those younger than 50 years of age) and anterior ischemic optic neuropathy (usually in those older than 50).
Glaucoma is a group of diseases involving loss of retinal ganglion cells causing optic neuropathy in a pattern of peripheral vision loss, initially sparing central vision.
Optic neuritis is inflammation of the optic nerve. It is associated with a number of diseases, most notably multiple sclerosis.
Anterior Ischemic Optic Neuropathy is a particular type of infarct that affects patients with an anatomical predisposition and cardiovascular risk factors.
Ophthalmologists, particularly those sub specialists who are neuro-ophthalmologists, are often best suited to diagnose and treat diseases of the optic nerve.
The International Foundation for Optic Nerve Diseases IFOND sponsors research and information on a variety of optic nerve disorders and may provide general direction.
Additional images
 MRI scan of human eye showing optic nerve. |
 The ophthalmic artery and its branches. (optic nerve is yellow) |
 Dura mater and its processes exposed by removing part of the right half of the skull, and the brain. |
 Tentorium cerebelli from above. |
 Superficial dissection of brain-stem. Lateral view. |
 Dissection of brain-stem. Lateral view. |
 Mesal aspect of a brain sectioned in the median sagittal plane. |
 Scheme showing central connections of the optic nerves and optic tracts. |
 The fornix and corpus callosum from below. |
 Nerves of the orbit. Seen from above. |
 Nerves of the orbit, and the ciliary ganglion. Side view. |
 The arteries of the choroid and iris. The greater part of the sclera has been removed. |
 The veins of the choroid. |
 The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section. |
References
External links
- The optic nerve on MRI
- BrainMaps at UCDavis optic%20nerve
- IFOND
- online case history - Optic nerve analysis with both scanning laser polarimetry with variable corneal compensation (GDx VCC) and confocal scanning laser ophthalmoscopy (HRT II - Heidelberg Retina Tomograph). Also includes actual fundus photos.
- Norman/Georgetown lesson3 (orbit4)
- Norman/Georgetown cranialnerves (II)
Sensory system - Visual system |
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| Eye • Optic nerve • Optic chiasm • Optic tract • Lateral geniculate nucleus • Optic radiation • Visual cortex • Blobs |
da:Synsnerve de:Sehnerveu:Nerbio optiko fr:Nerf optique it:Nervo ottico lt:Regos nervas nl:Nervus opticus ja:視神経 no:Nervus opticusfi:Näköhermo sv:Synnerv
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Acknowledgement and Attribution Regarding Sources of Content
Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .
