neuroanatomy Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defi ...

, the optic chiasm, or optic chiasma (; , ), is the part of the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...

where the

optic nerves In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived from ...

cross. It is located at the bottom of the brain immediately inferior to the

hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...

. The optic chiasm is found in all

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

, although in

cyclostomes Cyclostome is a biological term (from the Greek for "round mouth") used in a few different senses: * for the taxon Cyclostomi, which comprises the extant jawless fishes: the hagfish (Myxini) and the lampreys (Petromyzontidae). This was thought fo ...

(

lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

s and

hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...

es), it is located within the brain. 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 Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...

, see

Chiasm (anatomy) In anatomy a chiasm is the spot where two structures cross, forming an X-shape (). This can be: * A ''tendinous chiasm'' is the spot where two tendons cross. For example, the tendon of the flexor digitorum superficialis muscle, and the tendon ...

). A midline crossing of nerves inside the brain is called a

decussation Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. . Similarly, the anatomical term chiasma is named aft ...

(see Definition of types of crossings).

Structure

For the different types of optic chiasm, see 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. In vertebrates with a large overlap of the visual fields of the two eyes, i.e., most mammals and birds, but also amphibians, reptiles such as

chameleons Chameleons or chamaeleons (family Chamaeleonidae) are a distinctive and highly specialized clade of Old World lizards with 202 species described as of June 2015. The members of this family are best known for their distinct range of colors, bein ...

, 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 In neuroanatomy, the optic tract () is a part of the visual system in the brain. It is a continuation of the optic nerve that relays information from the optic chiasm to the ipsilateral lateral geniculate nucleus (LGN), pretectal nuclei, and su ...

of the ipsilateral side. By this partial decussation, the part of the

visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point ...

that is covered by both eyes is fused so that the processing of binocular depth perception by

stereopsis Stereopsis () is the component of depth perception retrieved through binocular vision. Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image becaus ...

is enabled (see Figure 2). In the case of such partial decussation, the optic nerve fibres on the medial sides of each

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

(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

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 optic tracts. The optic tract inserts on the optic tectum (in

mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...

known as superior colliculus) of the

midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...

. In mammals they also branch off to the

lateral geniculate body In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral project ...

of the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...

, in turn giving them to the occipital cortex of the

cerebrum The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb ...

Development in mammals

During

development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...

, the crossing of the optic nerves is guided primarily by cues such as netrin, slit,

semaphorin Semaphorins are a class of secreted and membrane proteins that were originally identified as axonal growth cone guidance molecules. They primarily act as short-range inhibitory signals and signal through multimeric receptor complexes. Semaphorins ...

and ephrin; and by morphogens such as sonic hedgehog (Shh) and Wnt. This navigation is mediated by the neuronal

growth cone A growth cone is a large actin-supported extension of a developing or regenerating neurite seeking its synaptic target. It is the growth cone that drives axon growth. Their existence was originally proposed by Spanish histologist Santiago Ram� ...

, a structure that responds to the cues by

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...

- receptor signalling systems that activate downstream pathways inducing changes in the

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is com ...

. 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 The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

midline inhibits crossing prior to the chiasm, where it is downregulated. The organization of RGC axons changes from retinotopic to a flat sheet-like orientation as they approach the chiasm site. Most RGC

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

s cross the midline at the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

diencephalon and continue to the

contralateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

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).

Ephrin-B2 Ephrin-B2 is a protein that in humans is encoded by the ''EFNB2'' gene. Function This gene encodes a member of the ephrin (EPH) family. The ephrins and EPH-related receptors comprise the largest subfamily of receptor protein-tyrosine kinases a ...

is expressed at the chiasm midline by

radial glia Radial glial cells, or radial glial progenitor cells (RGPs), are bipolar-shaped progenitor cells that are responsible for producing all of the neurons in the cerebral cortex. RGPs also produce certain lineages of glia, including astrocytes and ol ...

and acts as a repulsive signal to axons originating from the ventrotemporal retina expressing EphB1 receptor protein, giving rise to the ipsilateral, or uncrossed, projection. 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. Chiasm crossing is also promoted by Nr-CAM (Ng-CAM-related

cell adhesion molecule Cell adhesion molecules (CAMs) are a subset of cell surface proteins that are involved in the binding of cells with other cells or with the extracellular matrix (ECM), in a process called cell adhesion. In essence, CAMs help cells stick to each ...

) and

Semaphorin Semaphorins are a class of secreted and membrane proteins that were originally identified as axonal growth cone guidance molecules. They primarily act as short-range inhibitory signals and signal through multimeric receptor complexes. Semaphorins ...

6D (Sema6D) expressed at the midline, which form a complex that signals to Nr-CAM/

Plexin A plexin is a protein which acts as a receptor for semaphorin family signaling proteins. It is classically known for its expression on the surface of axon growth cones and involvement in signal transduction to steer axon growth away from the so ...

-A1 receptors on crossing RGC axons.

Other animals

Mammals

Since all vertebrates, even the earliest fossils and modern jawless ones, possess an optic chiasm, it is not known how it evolved. A number of theories have been proposed for the function of the optic chiasm in vertebrates (see

theories A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be ...

). According to the axial twist hypothesis the optic chiasm develops as a consequence of a twist in the early embryo. In

Siamese cat The Siamese cat ( th, แมวไทย, Maeo Thai; แมวสยาม, Maeo Seeaam) is one of the first distinctly recognized breeds of Asian cat. Derived from the Wichianmat landrace, one of several varieties of cat native to Thailand ( ...

s with certain

genotypes The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...

of the

albino Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and pink or blue eyes. Individuals with the condition are referred to as albino. Varied use and interpretation of the term ...

gene, the wiring is disrupted, with more of the nerve-crossing than normal. Since siamese cats, like

tigers The tiger (''Panthera tigris'') is the largest living cat species and a member of the genus ''Panthera''. It is most recognisable for its dark vertical stripes on orange fur with a white underside. An apex predator, it primarily preys on un ...

, also tend to cross their eyes (

strabismus Strabismus is a vision disorder in which the eyes do not properly align with each other when looking at an object. The eye that is focused on an object can alternate. The condition may be present occasionally or constantly. If present during a ...

), it has been proposed that this behavior might compensate the abnormal amount of decussation.

Cephalopods and insects

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 Zayn al-Din Sayyed Isma‘il ibn Husayn Gorgani (c. 1040–1136), also spelled al-Jurjani, was a Persian 12th century royal Islamic physician from Gorgan, Iran. In addition to medical and pharmaceutical sciences, he was also an adept in theolo ...

(1042–1137).

Additional images

File:Gray722.svg, Scheme showing central connections of the

optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived fro ...

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.

References

External links

* {{Authority control Cerebrum Visual system Optic nerve