Editing Optic nerve (section)

==Structure==
The optic nerve has been classified as the second of twelve paired [[cranial nerves]], but it is technically a myelinated tract of the [[central nervous system]], rather than a classical nerve of the [[peripheral nervous system]] because it is derived from an out-pouching of the [[diencephalon]] ([[optic stalk]]s) during embryonic development. As a consequence, the fibers of the optic nerve are covered with [[myelin]] produced by [[oligodendrocyte]]s, rather than [[Schwann cell]]s of the peripheral nervous system, and are encased within the [[meninges]].<ref>{{cite book |last1=Smith |first1=Austen M. |last2=Czyz |first2=Craig N. |title=StatPearls |date=2021 |publisher=StatPearls Publishing |location=Treasure Island (FL) |url=https://www.ncbi.nlm.nih.gov/books/NBK507907/ |access-date=14 June 2021 |chapter=Neuroanatomy, Cranial Nerve 2 (Optic)|pmid=29939684 }}</ref> [[Peripheral neuropathy|Peripheral neuropathies]] like [[Guillain–Barré syndrome]] do not affect the optic nerve. However, most typically, the optic nerve is grouped with the other eleven cranial nerves and is considered to be part of the peripheral nervous system.

The optic nerve is ensheathed in all three meningeal layers ([[dura mater|dura]], [[arachnoid mater|arachnoid]], and [[pia mater]]) rather than the [[epineurium]], [[perineurium]], and [[endoneurium]] found in peripheral nerves. [[Nerve tract|Fiber tracts]] of the mammalian central nervous system have only limited regenerative capabilities compared to the peripheral nervous system.<ref>{{Cite journal|last1=Benowitz|first1=Larry|last2=Yin|first2=Yuqin|date=August 2010|title=Optic Nerve Regeneration|journal=Archives of Ophthalmology|volume=128|issue=8|pages=1059–1064|doi=10.1001/archophthalmol.2010.152|issn=0003-9950|pmc=3072887|pmid=20697009}}</ref> Therefore, in most mammals, optic [[Nerve injury|nerve damage]] results in irreversible [[Visual impairment|blindness]]. The fibers from the [[retina]] run along the optic nerve to nine primary visual nuclei in the brain, from which a major relay inputs into the [[primary visual cortex]].

[[File:Fundus photograph of normal left eye.jpg|left|thumb|A [[fundus photograph]] showing the back of the retina. The white circle is the beginning of the optic nerve.]]
The optic nerve is composed of [[retinal ganglion cell]] axons and [[glia]]. Each human optic nerve contains between 770,000 and 1.7 million nerve fibers,<ref>{{cite journal|title=Human optic nerve fiber count and optic disc size| journal=Investigative Ophthalmology & Visual Science|date=May 1992|first=Jost B.|last=Jonas|volume=33|issue=6|pages=2012–8|display-authors=etal|pmid=1582806}}</ref> which are axons of the retinal ganglion cells of one retina. In the [[Fovea centralis|fovea]], which has high acuity, these ganglion cells connect to as few as 5 [[photoreceptor cell]]s; in other areas of the retina, they connect to thousands of photoreceptors.

The optic nerve leaves the [[Orbit (anatomy)|orbit]] (eye socket) via the [[optic canal]], running postero-medially towards the [[optic chiasm]], where there is a partial decussation (crossing) of fibers from the temporal [[visual field]]s (the nasal hemi-retina) of both eyes. The proportion of decussating fibers varies between species, and is correlated with the degree of [[binocular vision]] enjoyed by a species.<ref>Textbook of Veterinary Anatomy, 4th Edition. Dyce, Sack and Wensing</ref> Most of the [[axon]]s of the optic nerve terminate in the [[lateral geniculate nucleus]] from where information is relayed to the [[visual cortex]], while other axons terminate in the [[pretectal area]]<ref>{{Cite journal|last1=Belknap|first1=Dianne B.|last2=McCrea|first2=Robert A.|date=1988-02-01|title=Anatomical connections of the prepositus and abducens nuclei in the squirrel monkey|journal=The Journal of Comparative Neurology |volume=268|issue=1|pages=13–28|doi=10.1002/cne.902680103|pmid=3346381|s2cid=21565504|issn=0021-9967}}</ref> and are involved in reflexive [[eye movement]]s. Other axons terminate in the [[suprachiasmatic nucleus]] and are involved in regulating the [[Circadian rhythm|sleep-wake cycle]]. Its diameter increases from about 1.6&nbsp;mm within the eye to 3.5&nbsp;mm in the orbit to 4.5&nbsp;mm within the cranial space. The optic nerve component lengths are 1&nbsp;mm in the globe, 24&nbsp;mm in the orbit, 9&nbsp;mm in the optic canal, and 16&nbsp;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.<ref name =Vilensky />

Based on this anatomy, the optic nerve may be divided into four parts as indicated in the image at the top of this section (this view is from above as if you were looking into the orbit after the top of the skull had been removed): 1. the optic head (which is where it begins in the eyeball (globe) with fibers from the retina); 2. orbital part (which is the part within the orbit); 3. intracanicular part (which is the part within a bony canal known as the optic canal); and, 4. cranial part (the part within the cranial cavity, which ends at the optic chiasm).<ref name="Selhorst" />

From the lateral geniculate body, fibers of the [[optic radiation]] pass to the [[visual cortex]] in the [[occipital lobe]] of the brain. In more specific terms, fibers carrying information from the contralateral superior visual field traverse Meyer's loop to terminate in the [[lingual gyrus]] below the [[Calcarine sulcus|calcarine fissure]] in the occipital lobe, and fibers carrying information from the contralateral inferior visual field terminate more superiorly, to the [[cuneus]].<ref>{{Cite web|url=http://casemed.case.edu/clerkships/neurology/NeurLrngObjectives/Vision.htm|title=Vision|website=casemed.case.edu|access-date=2020-01-23|archive-date=2020-01-26|archive-url=https://web.archive.org/web/20200126165029/http://casemed.case.edu/clerkships/neurology/NeurLrngObjectives/Vision.htm|url-status=dead}}</ref>