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Decussation is used in biological contexts to describe a crossing (due to the shape of the
Roman numeral Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, ea ...
for ten, an uppercase 'X' (), ). In Latin
anatomical Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...
terms, the form is used, e.g. . Similarly, the anatomical term chiasma is named after the Greek uppercase 'Χ' ( chi). Whereas a decussation refers to a crossing within 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 ...
, various kinds of crossings in the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brai ...
are called chiasma. Examples include: * In 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 nerve fibers obliquely cross from one lateral side of the brain to the other, that is to say they cross at a level other than their origin. See for examples
Decussation of pyramids In neuroanatomy, the medullary pyramids are paired white matter structures of the brainstem's medulla oblongata that contain motor fibers of the corticospinal and corticobulbar tracts – known together as the pyramidal tracts. The lower limi ...
and
sensory decussation In neuroanatomy, the sensory decussation or decussation of the lemnisci is a decussation (i.e. crossover) of axons from the gracile nucleus and cuneate nucleus, which are responsible for fine touch, vibration, proprioception and two-point discr ...
. 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, defi ...
, the term ''chiasma'' is reserved for crossing of- or within nerves such as in the
optic chiasm In neuroanatomy, the optic chiasm, or optic chiasma (; , ), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrat ...
. * In
botanical Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek w ...
leaf taxology, the word ''decussate'' describes an opposite pattern of leaves which has successive pairs at right angles to each other (i.e. rotated 90 degrees along the stem when viewed from above). In effect, successive pairs of leaves cross each other. Basil is a classic example of a decussate leaf pattern. * In
tooth enamel Tooth enamel is one of the four major tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, ...
, where bundles of rods cross each other as they travel from the enamel-dentine junction to the outer enamel surface, or near to it. * In taxonomic description where decussate markings or structures occur, names such as or or otherwise in part containing "decuss..." are common, especially in the
specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bo ...
.


Evolutionary significance

The origin of the contralateral organization, the optic chiasm and the major decussations on the nervous system of
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 ...
has been a long standing puzzle to scientists. The visual map theory of Ramón y Cajal has long been popular but has been criticized for its logical inconsistence. More recently, it has been proposed that the decussations are caused by an axial twist by which the anterior head, along with the
forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three primary ...
, is turned by 180° with respect to the rest of the body.


See also

* Definition of types of crossings * Palpebral commissure (of the eye) * Commissure * Chiasm *
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 ...
*
Fissure (anatomy) In biological morphology and anatomy, a sulcus (pl. ''sulci'') is a furrow or fissure (Latin ''fissura'', plural ''fissurae''). It may be a groove, natural division, deep furrow, elongated cleft, or tear in the surface of a limb or an organ, ...


References


Further reading


Why does the nervous system decussate?
Stanford Neuroblog


External links

* {{Authority control Biology terminology Anatomical terminology