Editing Optic nerve

{{Short description|Second cranial nerve, which connects the eyes to the brain}}
{{other uses|Optic Nerve (disambiguation){{!}}Optic Nerve}}

{{More footnotes needed|date=June 2018}}
{{Infobox nerve
| Name        = Optic nerve
| Latin       = nervus opticus
| Image       = Gray773.png
| Caption     = The left optic nerve and the [[optic tract]]s.
| Innervates  = [[visual system|Vision]]
| BranchFrom  = 
| BranchTo    = 
}}
{{Cranial nerves short}}

In [[neuroanatomy]], the '''optic nerve''', also known as the '''second cranial nerve''', '''cranial nerve II''', or simply '''CN II''',<!--Each CN article, CN 0 plus CN I to CN XII, should not fail to have the standard, established synonyms at outset, nor be styled substantially differently from the others; do not change this aspect of any particular one of them without first discussing the same change for all of them.--> is a paired [[cranial nerve]] that transmits [[visual system|visual]] information from the [[retina]] to the [[brain]]. In humans, the optic nerve is derived from [[optic stalk]]s during the seventh week of development and is composed of [[retinal ganglion cell]] axons and [[glial cell]]s; it extends from the [[optic disc]] to the [[optic chiasma]] and continues as the [[optic tract]] to the [[lateral geniculate nucleus]], [[Pretectal area|pretectal nuclei]], and [[superior colliculus]].<ref name="Vilensky">{{cite book|last1= Vilensky|first1= Joel| last2=Robertson| first2 = Wendy| last3 = Suarez-Quian| first3 = Carlos| title=The Clinical Anatomy of the Cranial Nerves: The Nerves of "On Olympus Towering Top" |year=2015|publisher=Wiley-Blackwell|location=Ames, Iowa|isbn=978-1118492017}}</ref><ref name="Selhorst">{{Cite journal |last1=Selhorst |first1=John |last2=Chen |first2=Yanjun |date=February 2009 |title=The Optic Nerve |journal=Seminars in Neurology |language=en |volume=29 |issue=1 |pages=029–035 |doi=10.1055/s-0028-1124020 |pmid=19214930 |issn=0271-8235}}</ref>

==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>

==Function==
The optic nerve transmits all visual information including [[brightness]] perception, [[Color vision|color perception]] and [[Contrast (vision)|contrast]] ([[visual acuity]]). It also conducts the visual impulses that are responsible for two important neurological reflexes: the [[light reflex]] and the [[accommodation reflex]]. The light reflex refers to the constriction of both [[pupil]]s that occurs when light is shone into either eye. The accommodation reflex refers to the swelling of the [[Lens (anatomy)|lens of the eye]] that occurs when one looks at a near object (for example: when reading, the lens adjusts to near vision).<ref name = Vilensky />

The eye's [[Blind spot (vision)|blind spot]] is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.<ref name = Vilensky />

==Clinical significance==

===Disease===
Damage to the optic nerve typically causes permanent and potentially severe loss of [[Visual perception|vision]], as well as an abnormal pupillary reflex, which is important for the diagnosis of nerve damage.

The type of [[visual field]] loss will depend on which portions of the optic nerve were damaged. In general, the location of the damage in relation to the optic chiasm (see diagram above) will affect the areas of vision loss. Damage to the optic nerve that is [[anterior]], or in front of the optic chiasm (toward the face) causes loss of vision in the eye on the same side as the damage. Damage at the optic chiasm itself typically causes loss of vision laterally in both visual fields or [[bitemporal hemianopsia]] (see image to the right). Such damage may occur with large [[pituitary]] tumors, such as [[pituitary adenoma]]. Finally, damage to the [[optic tract]], which is posterior to, or behind the chiasm, causes loss of the entire visual field from the side opposite the damage, e.g. if the left optic tract were cut, there would be a loss of vision from the entire right visual field.

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 [[tumor]]s 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 atrophy|optic neuropathy]] in a pattern of [[peripheral vision]] loss, initially sparing central vision.  Glaucoma is frequently associated with increased intraocular pressure that damages the optic nerve as it exits the eyeball. The [[trabecular meshwork]] assists the drainage of [[Aqueous humour|aqueous humor]] fluid. The presence of excess aqueous humor, increases IOP, yielding the diagnosis and symptoms of glaucoma.<ref>{{Cite web|title=The Eye's Drainage System, the Trabecular Meshwork & Glaucoma {{!}} BrightFocus Foundation|url=https://www.brightfocus.org/glaucoma/article/glaucoma-and-importance-eyes-drainage-system|access-date=2022-01-10|website=www.brightfocus.org}}</ref>

[[Optic neuritis]] is inflammation of the optic nerve. It is associated with a number of diseases, the most notable one being [[multiple sclerosis]].  The patient will likely experience varying vision loss and eye pain.  The condition tends to be episodic.

[[Anterior ischemic optic neuropathy]] is commonly known as a "stroke of the optic nerve" and affects the optic nerve head (where the nerve exits the eyeball). There is usually a sudden loss of blood supply and nutrients to the optic nerve head. Vision loss is typically sudden and most commonly occurs upon waking up in the morning. This condition is most common in diabetic patients 40–70 years old.

Other optic nerve problems are less common. [[Optic nerve hypoplasia]] is the underdevelopment of the optic nerve resulting in little to no vision in the affected eye. Tumors, especially those of the pituitary gland, can put pressure on the optic nerve causing various forms of visual loss. Similarly, [[Intracranial aneurysm|cerebral aneurysms]], a swelling of [[Blood vessel|blood vessel(s)]], can also affect the nerve. Trauma can cause serious injury to the nerve. Direct optic nerve injury can occur from a penetrating injury to the orbit, but the nerve can also be injured by indirect trauma in which severe head impact or movement stretches or even tears the nerve.<ref name = Vilensky />

[[Ophthalmologist]]s and [[optometrist]]s can detect and diagnose some optic nerve diseases but [[neuro-ophthalmologist]]s are often best suited to diagnose and treat diseases of the optic nerve. The International Foundation for Optic Nerve Diseases (IFOND) sponsors research and provides information on a variety of optic nerve disorders.

==Additional images==
<gallery>
 File:MRI of human eye.jpg|MRI scan of human eye showing optic nerve.
 File:Gray514.png|The [[ophthalmic artery]] derived from [[internal carotid artery]] and its branches. (optic nerve is yellow)
 File:Gray682.png|Superficial dissection of brain-stem. Lateral view.
 File:Gray683.png|Dissection of brain-stem. Lateral view.
 File:Gray722.svg|Scheme showing central connections of the optic nerves and optic tracts.
 File:Gray776.png|Nerves of the orbit. Seen from above.
 File:Gray777.png|Nerves of the orbit, and the ciliary ganglion. Side view.
 File:Gray880.png|The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section.
File:Three Main Layers of the Eye.png|Structures of the eye labeled
File:Three Internal chambers of the Eye.svg|This image shows another labeled view of the structures of the eye
File:Slide17ior.JPG|Optic nerve. Deep dissection. Inferior view.
File:Slide18ior.JPG|Optic nerve. Deep dissection. Inferior view.
 File:Optic nerve.jpg|Optic nerve
 File:Cranial endobasis of a 19-20 weeks foetus.JPG|Optic nerve
 File:Human brain dura mater (reflections) description.JPG|Human brain dura mater (reflections)
 File:Slide1dd.JPG|Optic nerve
 File:Slide5ee.JPG|Optic nerve
 File:Slide6uu.JPG|Optic nerve
 File:Slide3PIT.JPG|Cerebrum. Inferior view. Deep dissection.
 File:Slide2HOM.JPG|Cerebral peduncle, optic chasm, cerebral aqueduct. Inferior view. Deep dissection.
</gallery>

==See also==
{{Commons}}
{{Anatomy-terms}}
* [[Ophthalmic nerve]] (CN V<sub>1</sub>)
* [[Bistratified cell]]

== References ==
<references />

==External links==
* [http://www.med.harvard.edu/AANLIB/cases/caseM/mr1_t/023.html The optic nerve on MRI]
* {{BrainMaps|optic%20nerve}}
* [http://www.ifond.org/projects.php3 IFOND]
* [http://fiteyes.com/blogs/dave/ 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.
* [http://www.liv.ac.uk/~rbj/RBJ/rbjteaching/Oculomotors.htm Animations of extraocular cranial nerve and muscle function and damage (University of Liverpool)]
* {{NormanAnatomy|lesson3}} ({{NormanAnatomyFig|orbit4}})
* {{NormanAnatomy|cranialnerves}} ({{NormanAnatomyFig|II}})

{{Cranial nerves}}
{{Visual system}}
{{Nervous system}}
{{Authority control}}

[[Category:Optic nerve| ]]
[[Category:Cranial nerves]]
[[Category:Human head and neck]]
[[Category:Nervous system]]
[[Category:Neurology]]
[[Category:Nerves of the head and neck]]
[[Category:Ophthalmology]]
[[Category:Otorhinolaryngology]]
[[Category:Visual system]]