| Optic chiasm | |
|---|---|
 Brain viewed from below; the front of the brain is above. Visual pathway with optic chiasm (X shape) is labelled "H" (image fromAndreas Vesalius'Fabrica, 1543). | |
 Optic nerves, chiasm, and optic tracts | |
| Details | |
| System | Visual system |
| Function | Transmit visual information from theoptic nerves to the occipital lobes of the brain |
| Identifiers | |
| Latin | chiasma opticum |
| MeSH | D009897 |
| NeuroNames | 459 |
| NeuroLex ID | birnlex_1416 |
| TA98 | A14.1.08.403 |
| TA2 | 5668 |
| FMA | 62045 |
| Anatomical terms of neuroanatomy | |
Inneuroanatomy, theoptic chiasm (/ˈkaɪ.æzəm,ˈkiː-/), oroptic chiasma (from Greek χίασμα (khíasma) 'crossing', from Ancient Greek χιάζω (khiázō) 'to mark with anX'), is the part of thebrain where theoptic nerves cross. It is located at the bottom of the brain immediatelyinferior to thehypothalamus.[1] The optic chiasm is found in allvertebrates, although incyclostomes (lampreys andhagfishes), 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 someinvertebrates, seeChiasm (anatomy)). A midline crossing of nerves inside the brain is called adecussation (seeDefinition of types of crossings).
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 alsoamphibians,reptiles such aschameleons, 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 theoptic tract of the ipsilateral side. By this partial decussation, the part of thevisual field that is covered by both eyes is fused so that the processing of binoculardepth perception bystereopsis is enabled (see Figure 2).
In the case of such partial decussation, the optic nerve fibres on the medial sides of eachretina (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 thevisual cortex to receive the same hemisphericvisual field from both eyes. Superimposing and processing these monocular visual signals allow the visual cortex to generatebinocular andstereoscopic 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 calledoptic tracts. The optic tract inserts on theoptic tectum (inmammals known assuperior colliculus) of themidbrain. In mammals they also branch off to thelateral geniculate body of thethalamus, in turn giving them to the occipital cortex of thecerebrum.[5]
The optic chiasma receives its arterial supply from theanterior cerebral arteries, and from branches of theinternal carotid artery which ascend along thepituitary stalk (the latter supplying the midline portion of the chiasma).[6]
Duringdevelopment, the crossing of the optic nerves is guided primarily by cues such asnetrin,slit,semaphorin andephrin; and bymorphogens such assonic hedgehog (Shh) andWnt.[7] This navigation is mediated by the neuronalgrowth cone, a structure that responds to the cues byligand-receptor signalling systems that activate downstream pathways inducing changes in thecytoskeleton.[8]Retinal ganglion cell (RGC) axons leaving the eye through the optic nerve are blocked from exiting the developing pathway bySlit2 andSema5A inhibition, expressed bordering the optic nerve pathway. Ssh expressed at thecentral nervous system midline inhibits crossing prior to the chiasm, where it is downregulated.[9][10] The organization of RGC axons changes fromretinotopic to a flat sheet-like orientation as they approach the chiasm site.[11]
Most RGCaxons cross the midline at theventraldiencephalon and continue to thecontralateralsuperior colliculus. The number of axons that do not cross the midline and projectipsilaterally depends on the degree of binocular vision of the animal (3% in mice and 45% in humans do not cross).[9]Ephrin-B2 is expressed at the chiasm midline byradial glia and acts as a repulsive signal to axons originating from theventrotemporal retina expressingEphB1 receptor protein, giving rise to the ipsilateral, or uncrossed, projection.[9] RGC axons that do cross at the optic chiasm are guided by the vascularendothelial growth factor, VEGF-A, expressed at the midline, which signals through the receptorNeuropilin-1 (NRP1) expressed on RGC axons.[12] Chiasm crossing is also promoted byNr-CAM (Ng-CAM-relatedcell adhesion molecule) andSemaphorin6D (Sema6D) expressed at the midline, which form a complex that signals to Nr-CAM/Plexin-A1 receptors on crossing RGC axons.[13]
Since all vertebrates, even the earliest fossils[14] and modern jawless ones,[5] possess an optic chiasm, it is not known how it evolved.[15] A number of theories have been proposed for the function of the optic chiasm in vertebrates (seetheories). According to theaxial twist theory the optic chiasm develops as a consequence of a twist in the earlyembryo.[16]
InSiamese cats with certaingenotypes of thealbino gene, the wiring is disrupted, with more of the nerve-crossing than normal.[17] Since Siamese cats, like albinotigers, also tend to cross their eyes (strabismus), it has been proposed that this behavior might compensate the abnormal amount ofdecussation.[18][19]
Incephalopods andinsects the optic tracts do not cross the body midline, so each side of the brain processes theipsilateral eye.
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 beZayn al-Din Gorgani (1042–1137).[20]
