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 (biology), 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 Visual perception, vision. I ...

where the optic nerves 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 hypothalamus ...

. 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 ...

, although in cyclostomes (

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 hagfishes), 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 (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 Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...

s such as 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 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 poin ...

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

depth perception Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions. Depth perception happens primarily due to stereopsis ...

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 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 the ...

(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 Stereoscopy (also called stereoscopics, or stereo imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word ''stereoscopy'' derives . Any stereoscopic image is ...

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 In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form '' tectal'' is common ...

) 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 projec ...

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 direct ...

, 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, the crossing of the optic nerves is guided primarily by cues such as netrin, slit, semaphorin and ephrin; and by morphogens such as

sonic hedgehog Sonic hedgehog protein (SHH) is encoded for by the ''SHH'' gene. The protein is named after the character ''Sonic the Hedgehog''. This signaling molecule is key in regulating embryonic morphogenesis in all animals. SHH controls organogenesis a ...

(Shh) and Wnt. This navigation is mediated by the neuronal growth cone, 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 elect ...

receptor Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...

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 co ...

Retinal ganglion cell A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and reti ...

(RGC) axons leaving the eye through the optic nerve are blocked from exiting the developing pathway by Slit2 and

Sema5A Semaphorin-5A is a protein that in humans is encoded by the ''SEMA5A'' gene. Members of the semaphorin protein family, such as SEMA5A, are involved in axonal guidance during neural development. Semaphorin 5A also plays a role in autism, reduc ...

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 Retinotopy (from Greek τόπος, place) is the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream. For clarity, 'retinotopy' can be replaced with 'retinal mapping', and 'retinotopic' with 'r ...

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

diencephalon The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in o ...

and continue to the contralateral

. 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 is expressed at the chiasm midline by radial glia 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 Neuropilin-1 is a protein that in humans is encoded by the ''NRP1'' gene. In humans, the neuropilin 1 gene is located at 10p11.22. This is one of two human neuropilins. Function NRP1 is a membrane-bound coreceptor to a tyrosine kinase recep ...

(NRP1) expressed on RGC axons. 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.

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). According to the axial twist hypothesis the optic chiasm develops as a consequence of a twist in the early embryo. In Siamese cats with certain genotypes 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, also tend to cross their eyes ( strabismus), 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 (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