Optic chiasm

Source: Wikipedia, the free encyclopedia.
Optic chiasm
optic nerves to the occipital lobes of the brain
Identifiers
Latinchiasma opticum
MeSHD009897
NeuroNames459
NeuroLex IDbirnlex_1416
TA98A14.1.08.403
TA25668
FMA62045
Anatomical terms of neuroanatomy]

In

cyclostomes (lampreys and hagfishes), it is located within the brain.[2][3]

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 Chiasm (anatomy)). A midline crossing of nerves inside the brain is called a decussation (see Definition of types of crossings
).

Structure

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
Main article: Chiasm (anatomy) § Structure

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.[4]

In vertebrates with a large overlap of the visual fields of the two eyes, i.e., most mammals and birds, but also

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 optic tract of the ipsilateral side. By this partial decussation, the part of the visual field that is covered by both eyes is fused so that the processing of binocular depth perception by stereopsis
is enabled (see Figure 2).

In the case of such partial decussation, the optic nerve fibres on the medial sides of each 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 visual field from both eyes. Superimposing and processing these monocular visual signals allow the visual cortex to generate binocular and 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

lateral geniculate body of the thalamus, in turn giving them to the occipital cortex of the cerebrum.[5]

Arterial supply

The optic chiasma receives its arterial supply from the anterior cerebral arteries, and from branches of the internal carotid artery which ascend along the pituitary stalk (the latter supplying the midline portion of the chiasma).[6]

Development in mammals

During

sonic hedgehog (Shh) and Wnt.[7] This navigation is mediated by the neuronal growth cone, a structure that responds to the cues by ligand-receptor signalling systems that activate downstream pathways inducing changes in the cytoskeleton.[8] 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 central nervous system midline inhibits crossing prior to the chiasm, where it is downregulated.[9][10] The organization of RGC axons changes from retinotopic to a flat sheet-like orientation as they approach the chiasm site.[11]

Most RGC

Neuropilin-1 (NRP1) expressed on RGC axons.[12] Chiasm crossing is also promoted by Nr-CAM (Ng-CAM-related cell adhesion molecule) and Semaphorin6D (Sema6D) expressed at the midline, which form a complex that signals to Nr-CAM/Plexin-A1 receptors on crossing RGC axons.[13]

Other animals

Mammals

Main article: Contralateral brain
See also: Axial twist theory

Since all vertebrates, even the earliest fossils

Axial Twist theory the optic chiasm develops as a consequence of a twist in the early embryo.[16]

In

tigers, also tend to cross their eyes (strabismus), it has been proposed that this behavior might compensate the abnormal amount of decussation.[18][19]

Cephalopods and insects

Main article: chiasm (anatomy)

In

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).[20]

Additional images

  • Scheme showing central connections of the optic nerves and optic tracts.
    Scheme showing central connections of the optic nerves and optic tracts.
  • Brain seen from below, with the optic chiasm seen in yellow in the centre.
    Brain seen from below, with the optic chiasm seen in yellow in the centre.
  • Transformations of the visual field toward the visual map on the primary visual cortex.
    Transformations of the visual field toward the visual map on the primary visual cortex.
  • Brain and brainstem seen from below
    Brain and brainstem seen from below
  • Left hemisphere of the brain seen in a cadaveric specimen from the side, with the optic chiasm labelled.
    Left hemisphere of the brain seen in a cadaveric specimen from the side, with the optic chiasm labelled.
  • Cerebrum, inferior view, deep dissection.
    Cerebrum, inferior view, deep dissection.
  • Guidance of axon crossing and non-crossing during development.
    Guidance of axon crossing and non-crossing during development.

See also

References

  1. ISBN 978-0-19-861035-9
    .
  2. ISBN 978-0-674-02042-9
    . Retrieved 22 November 2015.
  3. S2CID 7399128
    .
  4. ^ Stephen, Polyak (1957). The vertebrate visual system. Chicago: Chicago Univ. Press.
  5. ^
    ISBN 9783642621277
    .
  6. OCLC 1201341621
    .
  7. PMID 17560562
    .
  8. ISBN 9780511529719
    .
  9. ^
    S2CID 24381059
    .
  10. PMID 12971890
    .
  11. PMID 7623106
    .
  12. PMID 21658587
    .
  13. PMID 22632726
    .
  14. ISBN 978-0198540472
    .
  15. S2CID 7399128
    .
  16. PMID 31211022
    .
  17. S2CID 32536933
    .
  18. S2CID 28568341
    .
  19. PMID 4822986
    .
  20. S2CID 15242391
    .
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