Optic chiasm

Brain: 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
Latin	chiasma opticum
Gray's	p.883
MeSH	Optic+chiasm
NeuroLex ID	birnlex_1416

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. 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 less 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

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  Mesal aspect of a brain sectioned in the median sagittal plane.

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  Median sagittal section of brain.

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  Scheme showing central connections of the optic nerves and optic tracts.

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  Base of brain.

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  Coronal section of brain through anterior commissure.

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  The fornix and corpus callosum from below.

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  Diagram showing the positions of the three principal subarachnoid cisternæ.

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  The hypophysis cerebri in position. Shown in sagittal section.

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  Median sagittal through the hypophysis of an adult monkey. Semidiagrammatic.

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  3D schematic representation of optic tracts

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  Human brainstem anterior view

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  Optic chiasm

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  Optic chiasma

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  Cerebrum. Deep dissection. Inferior dissection.

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  Cerebrum.Inferior view.Deep dissection

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  Spinal cord. Brachial plexus. Cerebrum.Inferior view.Deep dissection.

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  Cerebral peduncle, optic chasm, cerebral aqueduct. Inferior view. Deep dissection.

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  Cerebral peduncle, optic chasm, cerebral aqueduct. Inferior view. Deep dissection.

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  Cerebrum.Inferior view.Deep dissection.

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  Cerebrum.Inferior view.Deep dissection.

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  Cerebrum. Optic and olfactory nerves.Inferior view. Deep dissection.

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  Cerebrum.Inferior view. Deep dissection.

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  Cerebrum. Inferior view.Deep dissection

See also

Wikimedia Commons has media related to Optic chiasm.

• Chiasmal syndrome

References

• Jeffery G (October 2001). "Architecture of the optic chiasm and the mechanisms that sculpt its development". Physiol. Rev. 81 (4): 1393–414. PMID 11581492. 

External links

• Roche Lexicon - illustrated navigator, at Elsevier 13048.000-1