In
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, defin ...
, 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 v ...
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, ...
, 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, a ...
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 chordate ...
, 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,
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 rhynchocephalians ( ...
s 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
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 an ...
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
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 ...
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
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 sup ...
. The optic tract inserts on the
optic tectum
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 commonl ...
(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 or ...
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 commonly ...
) 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. In ...
.
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
Netrins are a class of proteins involved in axon guidance. They are named after the Sanskrit word "netr", which means "one who guides". Netrins are genetically conserved across nematode worms, fruit flies, frogs, mice, and humans. Structurall ...
,
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
Ephrins (also known as ephrin ligands or Eph family receptor interacting proteins) are a family of proteins that serve as the ligands of the Eph receptor. Eph receptors in turn compose the largest known subfamily of receptor protein-tyrosine kinas ...
; and by
morphogens
A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the various ...
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 electr ...
-
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 ...
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 compos ...
.
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 human eye, eye. It receives visual information from photoreceptor cell, photoreceptors via two intermediate neuron typ ...
(RGC) axons leaving the eye through the optic nerve are blocked from exiting the developing pathway by
Slit2
Slit homolog 2 protein is a protein that in humans is encoded by the ''SLIT2'' gene.
Interactions
SLIT2 has been shown to interact
Advocates for Informed Choice, dba interACT or interACT Advocates for Intersex Youth, is a 501(c)(3) nonprofit ...
and
Sema5A
Semaphorin-5A is a protein that in humans is encoded by the ''SEMA5A'' gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth' ...
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 par ...
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 po ...
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 language, Greek roots, describe something in its standard anatomical position. Th ...
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
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 commonly ...
. 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 (f ...
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
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 ...
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.
See also
*
Chiasmal syndrome
*
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 ...
*
Definition of types of crossings
*
Contralateral brain
The contralateral organization of the forebrain (Latin: contra‚ against; latus‚ side; lateral‚ sided) is the property that the hemispheres of the cerebrum and the thalamus represent mainly the contralateral side of the body. Consequently, th ...
References
*
External links
*
{{Authority control
Cerebrum
Visual system
Optic nerve