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Optic chiasm

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Optic chiasm

Optic chiasm
Brain viewed from below; the front of the brain is above. Visual pathway with optic chiasm (X shape) is shown in red (1543 image from Andreas Vesalius' Fabrica)
Optic nerves, chiasm, and optic tracts
Details
Latin chiasma opticum
Identifiers
Gray's p.883
MeSH A08.800.800.120.680.600
NeuroLex ID Optic chiasm
Anatomical terms of neuroanatomy

The optic chiasm or optic chiasma (Greek χίασμα, "crossing", from the Greek χιάζω 'to mark with an X', after the Greek letter 'Χ', chi) is the part of the brain where the optic nerves (CN II) partially cross. The optic chiasm is located at the bottom of the brain immediately below the hypothalamus.[1]

Pathways

The images on the nasal sides of each retina cross over to the opposite side of the brain via the optic nerve at the optic chiasm (decussation of medial fibers). The temporal images, on the other hand, stay on the same side. The inferonasal retina are related to anterior portion of the optic chiasm whereas superonasal retinal fibers are related to the posterior portion of the optic chiasm. This allows the images from either side of the field from both eyes to be transmitted to the appropriate side of the brain, combining the sides together. This allows for parts of both eyes that attend to the right visual field to be processed in the left visual system in the brain, and vice versa. This is linked to skin sensation, which also reaches the opposite side of the body, after reaching the diencephalon (rear forebrain). This decussation (crossing) is an adaptive feature of frontally oriented eyes, rendering binocular vision to animals with this adaptive feature. (Some animals, with laterally positioned eyes, have little binocular vision, so there is a more complete crossover of visual signals.) Beyond the optic chiasm, with crossed and uncrossed fibers, the optic nerves become optic tracts. The signals are passed on to the lateral geniculate body, in turn giving them to the occipital cortex (the outer matter of the rear brain).[2]

Optic chiasm in cats

In Siamese cats with certain genotypes of the albino gene, this wiring is disrupted, with more of the nerve-crossing than is normal, as a number of scholars have reported.[3] To compensate for lack of crossing in their brains, they cross their eyes (strabismus).[4]

This is also seen in albino tigers, as Guillery & Kaas report.[5]

Additional images

See also

References

  1. ^ Colman, Andrew M. (2006). Oxford Dictionary of Psychology (2nd ed.). Oxford University Press. p. 530.  
  2. ^ "eye, human." Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009
  3. ^ "Abnormal retinotopic organization of the dorsal lateral geniculate nucleus of the tyrosinase-negative albino cat.". Retrieved 2014-05-09. 
  4. ^ Guillery, RW; Kaas, JH (June 1973). "Genetic abnormality of the visual pathways in a "white" tiger". Science 180 (4092): 1287–9.  
  5. ^  

External links

  • Anatomy diagram: 13048.000-1 at Roche Lexicon - illustrated navigator, Elsevier
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