View source for Biology:Optic chiasm

{{Short description|Part of the brain where the optic nerves cross}}
{{Infobox brain
| Name            = Optic chiasm
| Latin           = chiasma opticum
| Image           = 1543,Visalius'OpticChiasma.jpg
| Caption         = Brain viewed from below; the front of the brain is above. Visual pathway with optic chiasm (X shape) is shown in red (image from Andreas Vesalius' ''Fabrica'', 1543).
| Image2          = Gray773.png
| Caption2        = Optic nerves, chiasm, and optic tracts
| IsPartOf        =
| Components      =
| Artery          =
| Vein            =
|System=[[Biology:Visual system|Visual system]]|Function=Transmit visual information from the optic nerves to the occipital lobes of the brain|Location=}}

In [[Biology:Neuroanatomy|neuroanatomy]], the '''optic chiasm''', or '''optic chiasma''' ({{IPAc-en|pron|Q|p|t|I|k|_|k|aI|ae|z|əm}}; {{ety|el|χίασμα|crossing}}, {{ety|grc|χιάζω|to mark with an X}}), is the part of the [[Biology:Brain|brain]] where the optic nerves cross. It is located at the bottom of the brain immediately [[Biology:Anatomical terms of location#Superior and inferior|inferior]] to the [[Biology:Hypothalamus|hypothalamus]].<ref>{{Cite book|last= Colman|first= Andrew M.|edition=2nd|title=Oxford Dictionary of Psychology|publisher=Oxford University Press|page=530 |year=2006 |isbn=978-0-19-861035-9}}</ref> The optic chiasm is found in all vertebrates, although in [[Biology:Cyclostomata|cyclostomes]] ([[Biology:Lamprey|lamprey]]s and [[Biology:Hagfish|hagfish]]es), it is located within the brain.<ref name="Bainbridge2009">{{cite book|last=Bainbridge|first=David|title=Beyond the Zonules of Zinn: A Fantastic Journey Through Your Brain|url=https://books.google.com/books?id=cuCIyLmJkHoC&pg=PA162|access-date=22 November 2015|date=30 June 2009|publisher=Harvard University Press|isbn=978-0-674-02042-9|page=162}}</ref><ref name="de LussanetOsse2012">{{cite journal|last1=de Lussanet|first1=Marc H.E.|last2=Osse|first2=Jan W.M.|title=An ancestral axial twist explains the contralateral forebrain and the optic chiasm in vertebrates|journal=Animal Biology|volume=62|issue=2|year=2012|pages=193–216|arxiv=1003.1872|issn=1570-7555|doi=10.1163/157075611X617102|s2cid=7399128 }}</ref>

This article is about the optic chiasm of vertebrates, which is the best known nerve chiasm, but not every chiasm denotes a crossing of the body midline (e.g., in some invertebrates, see [[Biology:Chiasm (anatomy)|Chiasm (anatomy)]]). A midline crossing of nerves inside the brain is called a [[Biology:Decussation|decussation]] (see [[Biology:Anatomical terms of neuroanatomy#Nerve fibre crossings|Definition of types of crossings]]).

==Structure==

[[File:Optical-transformations.png|300px|thumb|'''Figure 2''' Transformations of the visual field toward the visual map on the primary visual cortex in vertebrates.  U=up; D=down; L=left; R=right; F=fovea]]

In all vertebrates, the optic nerves of the left and the right eye meet in the body midline, ventral to the brain. In many vertebrates the left optic nerve crosses over the right one without fusing with it.<ref name="Polyak1957">{{cite book
|location = Chicago
|first=Polyak|last=Stephen
|publisher = Chicago Univ. Press
|title = The vertebrate visual system
|date=1957}}</ref>

In vertebrates with a large overlap of the visual fields of the two eyes, i.e., most mammals and birds, but also amphibians, [[Biology:Reptile|reptile]]s such as chameleons, the two optic nerves merge in the optic chiasm. In such a merged optic chiasm, part of the nerve fibres do not cross the midline, but continue towards the [[Biology:Optic tract|optic tract]] of the ipsilateral side. By this partial decussation, the part of the [[Biology:Visual field|visual field]] that is covered by both eyes is fused so that the processing of binocular [[Depth perception|depth perception]] by [[Stereopsis|stereopsis]] is enabled (see Figure 2).

In the case of such partial decussation, the optic nerve fibres on the medial sides of each [[Biology:Retina|retina]] (which correspond to the lateral side of each visual hemifield, because the image is inverted) cross over to the opposite side of the body midline. The inferonasal retina are related to the anterior portion of the optic chiasm whereas superonasal retinal fibers are related to the posterior portion of the optic chiasm.

The partial crossing over of optic nerve fibres at the optic chiasm allows the visual cortex to receive the same hemispheric [[Biology:Visual field|visual field]] from both eyes. Superimposing and processing these monocular visual signals allow the visual cortex to generate [[Binocular vision|binocular]] and [[Stereopsis|stereoscopic]] vision. The net result is that the right cerebral hemisphere processes left visual hemifield, and the left cerebral hemisphere processes the right visual hemifield.

Beyond the optic chiasm, with crossed and uncrossed fibers, the optic nerves are called optic tracts. The optic tract inserts on the optic tectum (in mammals known as [[Biology:Superior colliculus|superior colliculus]]) of the [[Biology:Midbrain|midbrain]]. In mammals they also branch off to the lateral geniculate body of the [[Biology:Thalamus|thalamus]], in turn giving them to the occipital cortex of the [[Biology:Cerebrum|cerebrum]].<ref name="Nieuwenhuys1998">{{cite book
|last1=Nieuwenhuys|first1=R.
|last2=Donkelaar|first2=H.J.
|last3=Nicholson|first3=C.
|last4=Smeets|first4=W.J.A.J.
|last5=Wicht|first5=H.
|title=The central nervous system of vertebrates.
|date=1998
|publisher=Springer
|location=New York
|url=https://www.springer.com/us/book/9783540560135
|isbn=9783642621277}}</ref>

==Development in mammals==

During [[Biology:Development of the nervous system|development]], the crossing of the optic nerves is guided primarily by cues such as [[Biology:Netrin|netrin]], [[Biology:Slit (protein)|slit]], [[Biology:Semaphorin|semaphorin]] and [[Biology:Ephrin|ephrin]]; and by morphogens such as [[Biology:Sonic hedgehog|sonic hedgehog]] (Shh) and [[Biology:Wnt signaling pathway|Wnt]].<ref>{{cite journal|last1=Erskine|first1=L.|last2=Herrera|first2=E.|year=2007|title=The retinal ganglion cell axon's journey: Insights into molecular mechanisms of axon guidance.|journal=Developmental Biology|volume=308|issue=1|pages=1–14|doi=10.1016/j.ydbio.2007.05.013|pmid=17560562|doi-access=free|hdl=10261/338550|hdl-access=free}}</ref> This navigation is mediated by the neuronal [[Biology:Growth cone|growth cone]], a structure that responds to the cues by [[Chemistry:Ligand|ligand]]-[[Biology:Receptor (biochemistry)|receptor]] signalling systems that activate downstream pathways inducing changes in the [[Biology:Cytoskeleton|cytoskeleton]].<ref>{{cite book|title=Neuronal Growth Cones|last1=Gordon-Weeks|first1=PR|date=2005|publisher=Cambridge University Press.|isbn=9780511529719}}</ref> [[Biology:Retinal ganglion cell|Retinal ganglion cell]] (RGC) axons leaving the eye through the optic nerve are blocked from exiting the developing pathway by Slit2 and Sema5A inhibition, expressed bordering the optic nerve pathway. Ssh expressed at the [[Biology:Central nervous system|central nervous system]] midline inhibits crossing prior to the chiasm, where it is downregulated.<ref name="Herrera">{{cite journal|last1=Herrera|first1=E|last2=Erskine|first2=L|last3=Morenilla-Palao|first3=C|year=2019|title=Guidance of retinal axons in mammals.|journal=Seminars in Cell & Developmental Biology|volume=85|pages=48–59|doi=10.1016/j.semcdb.2017.11.027|pmid=29174916|s2cid=24381059}}</ref><ref name="Genx">{{cite journal|last1=Rasband|first1=Kendall|last2=Hardy|first2=Melissa|last3=Chien|first3=Chi-Bin|year=2003|title=Generating X, Formation of the Optic Chiasm|journal=Neuron|volume=39|issue=6|pages=885–888|doi=10.1016/S0896-6273(03)00563-4|pmid=12971890|doi-access=free}}</ref> The organization of RGC axons changes from [[Biology:Retinotopy|retinotopic]] to a flat sheet-like orientation as they approach the chiasm site.<ref name="Guillery">{{cite journal|last1=Guillery|first1=RW|last2=Mason|first2=CA|last3=Taylor|first3=JS|year=1995|title=Developmental determinants at the mammalian optic chiasm|journal=The Journal of Neuroscience|volume=15|issue=7|pages=4727–4737|doi=10.1523/JNEUROSCI.15-07-04727.1995|pmid=7623106|pmc=6577905}}</ref>

Most RGC [[Biology:Axon|axon]]s cross the midline at the ventral [[Biology:Diencephalon|diencephalon]] and continue to the contralateral [[Biology:Superior colliculus|superior colliculus]]. The number of axons that do not cross the midline and project ipsilaterally depends on the degree of binocular vision of the animal (3% in mice and 45% in humans do not cross).<ref name="Herrera" /> Ephrin-B2 is expressed at the chiasm midline by radial glia and acts as a repulsive signal to axons originating from the [[Biology:Anatomical terms of location|ventrotemporal]] retina expressing EphB1 receptor protein, giving rise to the ipsilateral, or uncrossed, projection.<ref name="Herrera" /> RGC axons that do cross at the optic chiasm are guided by the vascular endothelial growth factor, VEGF-A, expressed at the midline, which signals through the receptor Neuropilin-1 (NRP1) expressed on RGC axons.<ref>{{cite journal|last1=Erskine|first1=L|last2=Reijntjes|first2=S|last3=Pratt|first3=T|year=2011|title=VEGF signaling through neuropilin 1 guides commissural axon crossing at the optic chiasm|journal=Neuron|volume=70|issue=5|pages=951–965|doi=10.1016/j.neuron.2011.02.052|pmc=3114076|pmid=21658587}}</ref> Chiasm crossing is also promoted by Nr-CAM (Ng-CAM-related [[Biology:Cell adhesion molecule|cell adhesion molecule]]) and [[Biology:Semaphorin|Semaphorin]]6D (Sema6D) expressed at the midline, which form a complex that signals to Nr-CAM/[[Biology:Plexin|Plexin]]-A1 receptors on crossing RGC axons.<ref>{{cite journal|last1=Kuwajima|first1=T|last2=Yoshida|first2=Y|last3=Pratt|first3=T|year=2012|title=Optic chiasm presentation of Semaphorin6D in the context of Plexin-A1 and Nr-CAM promotes retinal axon midline crossing|journal=Neuron|volume=74|issue=4|pages=676–690|doi=10.1016/j.neuron.2012.03.025|pmc=3361695|pmid=22632726}}</ref>

==Other animals==

=== Mammals ===

{{Main|Biology:Contralateral brain}}

Since all vertebrates, even the earliest fossils<ref name="Janvier1996">{{cite book|title=Early vertebrates|last=Janvier|first=P.|publisher=Clarendon Press, Oxford University Press|year=1996|isbn=978-0198540472|location=New York}}</ref> and modern jawless ones,<ref name="Nieuwenhuys1998" /> possess an optic chiasm, it is not known how it evolved.<ref name="Lussanet2012">{{cite journal|last1=de Lussanet|first1=M.H.E.|last2=Osse|first2=J.W.M.|year=2012|title=An ancestral axial twist explains the contralateral forebain and the optic chiasm in vertebrates|journal=Animal Biology|volume=62|issue=2|pages=193–216|arxiv=1003.1872|doi=10.1163/157075611X617102|s2cid=7399128 }}</ref> A number of theories have been proposed for the function of the optic chiasm in vertebrates (see [[Biology:Contralateral brain#Theories|theories]]). According to the Axial Twist theory the optic chiasm develops as a consequence of a twist in the early [[Biology:Embryo|embryo]].<ref name="lussanet2019">{{cite journal|last=de Lussanet|first=M.H.E.|year=2019|title=Opposite asymmetries of face and trunk and of kissing and hugging, as predicted by the axial twist hypothesis|journal=PeerJ|volume=7|pages=e7096|doi=10.7717/peerj.7096|pmid=31211022|pmc=6557252 |doi-access=free }}</ref>

In Siamese cats with certain genotypes of the albino gene, the wiring is disrupted, with more of the nerve-crossing than normal.<ref>{{cite journal|vauthors=Schmolesky MT, Wang Y, Creel DJ, Leventhal AG|year=2000|title=Abnormal retinotopic organization of the dorsal lateral geniculate nucleus of the tyrosinase-negative albino cat.|journal=J Comp Neurol|volume=427|issue=2|pages=209–19|doi=10.1002/1096-9861(20001113)427:2<209::aid-cne4>3.0.co;2-3|pmid=11054689|s2cid=32536933 |doi-access=free}}</ref> Since siamese cats, like albino tigers, also tend to cross their eyes ([[Medicine:Strabismus|strabismus]]), it has been proposed that this behavior might compensate the abnormal amount of [[Biology:Decussation|decussation]].<ref>{{cite journal|last1=Guillery|first1=RW|last2=Kaas|first2=JH|date=June 1973|title=Genetic abnormality of the visual pathways in a "white" tiger|journal=Science|volume=180|issue=4092|pages=1287–9|bibcode=1973Sci...180.1287G|doi=10.1126/science.180.4092.1287|pmid=4707916|s2cid=28568341 }}</ref><ref>{{cite journal|author=Guillery RW|date=May 1974|title=Visual pathways in albinos|journal=Sci. Am.|volume=230|issue=5|pages=44–54|bibcode=1974SciAm.230e..44G|doi=10.1038/scientificamerican0574-44|pmid=4822986}}</ref>

=== Cephalopods and insects ===

{{Main|Biology:Chiasm (anatomy)}}

In cephalopods and insects the optic tracts do not cross the body midline, so each side of the brain processes the ipsilateral eye.

== History ==
The crossing of nerve fibres, and the impact on vision that this had, was probably first identified by Persian physician "Esmail Jorjani", who appears to be Zayn al-Din Gorgani (1042–1137).<ref>{{Cite journal|last1=Davis|first1=Matthew C.|last2=Griessenauer|first2=Christoph J.|last3=Bosmia|first3=Anand N.|last4=Tubbs|first4=R. Shane|last5=Shoja|first5=Mohammadali M.|date=2014-01-01|title=The naming of the cranial nerves: A historical review|journal=Clinical Anatomy|language=en|volume=27|issue=1|pages=14–19|doi=10.1002/ca.22345|issn=1098-2353|pmid=24323823|s2cid=15242391 }}</ref>

==Additional images==
<gallery>
File:Gray722.svg|Scheme showing central connections of the [[Biology:Optic nerve|optic nerve]]s and optic tracts.
File:Gray724.png|Brain seen from below, with the optic chiasm seen in yellow in the centre.
File:Optical-transformations.png|Transformations of the visual field toward the visual map on the primary visual cortex.
File:Human brainstem anterior view 2 description.JPG|Brain and brainstem seen from below
File:Slide13qq.JPG|Left hemisphere of the brain seen in a cadaveric specimen from the side, with the optic chiasm labelled.
File:Slide2Dsa.JPG|Cerebrum, inferior view, deep dissection.
File:Optic chiasm development.jpg|Guidance of axon crossing and non-crossing during development.
</gallery>

==See also==
* [[Medicine:Chiasmal syndrome|Chiasmal syndrome]]
* [[Biology:Chiasm (anatomy)|Chiasm (anatomy)]]
* [[Biology:Anatomical terms of neuroanatomy#Nerve fibre crossings|Definition of types of crossings]]
* [[Biology:Contralateral brain|Contralateral brain]]

==References==
{{Reflist}}
* {{cite journal |author=Jeffery G |title=Architecture of the optic chiasm and the mechanisms that sculpt its development |journal=Physiol. Rev. |volume=81 |issue=4 |pages=1393–414 |date=October 2001 |pmid=11581492 |doi= 10.1152/physrev.2001.81.4.1393|s2cid=203231 }}

==External links==
* {{cite web|url=http://www.tk.de/rochelexikon/pics/s13048.000-1.html|title=Anatomy diagram: 13048.000-1|work= Roche Lexicon - illustrated navigator|publisher= Elsevier|archive-url=https://web.archive.org/web/20141107120933/http://www.tk.de/rochelexikon/pics/s13048.000-1.html|archive-date=2014-11-07}}


[[Category:Cerebrum]]
[[Category:Visual system]]

{{Sourceattribution|Optic chiasm}}