Decussation

'''Decussation''' is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' ({{lang|la|decussis}}), {{ety|la|decem|ten||as|as}}). In Latin anatomical terms, the form '''{{lang|la|decussatio}}''' is used, e.g. {{lang|la|decussatio pyramidum}}.

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, various kinds of crossings in the peripheral nervous system are called chiasma.

[[File:Gray688.png|200px|thumb|Section of the medulla oblongata at the level of the decussation of the pyramids]] [[File:Crassula rupestris-PICT3087.jpg|200px|thumb|Decussate phyllotaxis of {{lang|la|Crassula rupestris}}]] [[File:Dysdercus decussatus (13476425975).jpg|200px|thumb|{{center|In this "true bug", ''Dysdercus decussatus'', the specific epithet refers to the bandolier-like markings on the back.}}]]

Examples include: * In the brain, 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 and sensory decussation. In neuroanatomy, the term ''chiasma'' is reserved for crossing of- or within nerves such as in the optic chiasm. * In botanical 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, 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 {{lang|la|decussatus}} or {{lang|la|decussata}} or otherwise in part containing "decuss..." are common, especially in the specific epithet.<ref name="Jaeger">{{cite book |author=Jaeger, Edmund C.|author-link=Edmund Jaeger |title=A source-book of biological names and terms |url=https://archive.org/details/sourcebookofbiol0000jaeg |url-access=registration |publisher=Thomas |location=Springfield, Ill |orig-date=1st Pub. 1959 |isbn=0-398-06179-3 }}</ref>

== Evolutionary significance == The origin of the contralateral organization, the optic chiasm and the major decussations on the nervous system of vertebrates has been a long standing puzzle to scientists.<ref name="Vulliemoz">{{cite journal | first1=S. | last1=Vulliemoz | first2=O. | last2=Raineteau | first3=D. | last3=Jabaudon | title=Reaching beyond the midline: why are human brains cross wired? | journal=The Lancet Neurology | volume=4 | issue=2 | date=2005 | pages=87–99 | doi=10.1016/S1474-4422(05)00990-7 | pmid=15664541 | s2cid=16367031}}</ref> The visual map theory of Ramón y Cajal has long been popular<ref name="Cajal">{{cite journal | last=Ramón y Cajal | first=Santiago | title=Estructura del quiasma óptico y teoría general de los entrecruzamientos de las vías nerviosas. (Structure of the Chiasma opticum and general theory of the crossing of nerve tracks) | journal=Rev. Trim. Micrográfica | year=1898 | volume=3 | pages=15–65 | trans-title= Die Structur des Chiasma opticum nebst einer allgemeine Theorie der Kreuzung der Nervenbahnen (German, 1899, Verlag Joh. A. Barth) | language=es}}</ref><ref>{{cite journal | first1=R.R. | last1=Llinás | title=The contribution of Santiago Ramón y Cajal to functional neuroscience. | journal=Nat. Rev. Neurosci. | volume=4 | issue=1 | date=2003 | pages=77–80 | doi=10.1038/nrn1011 | pmid=12511864 | s2cid=30442863}}</ref> but has been criticized for its logical inconsistence.<ref name="lussanet2015">{{cite journal | last1=de Lussanet | first1=M.H.E. | last2=Osse | first2=J.W.M. | title=Decussation as an axial twist: A comment on Kinsbourne (2013) | journal=Neuropsychology | volume=29 | issue=5 | year=2015 | pages=713–14 | doi=10.1037/neu0000163 | pmid=25528610 | url=https://peerj.com/preprints/432v3.pdf | access-date=2020-01-01 | archive-date=2021-07-14 | archive-url=https://web.archive.org/web/20210714184518/https://peerj.com/preprints/432v3.pdf | url-status=dead }}</ref> More recently, it has been proposed that the decussations are caused by an axial twist by which the anterior head, along with the forebrain, is turned by 180° with respect to the rest of the body.<ref name="Lussanet2012">{{cite journal | first1=M.H.E. | last1=de Lussanet | first2=J.W.M. | last2=Osse | year=2012 | title=An ancestral axial twist explains the contralateral forebain and the optic chiasm in vertebrates | journal=Animal Biology | volume=62 | issue=2 | pages=193–216 | doi=10.1163/157075611X617102 | arxiv=1003.1872 | s2cid=7399128}}</ref><ref>{{cite journal |last1=Kinsbourne |first1=M |title=Somatic twist: a model for the evolution of decussation. |journal=Neuropsychology|date=Sep 2013 |volume=27 |issue=5 |pages=511–15 |doi=10.1037/a0033662 |pmid=24040928}}</ref>

== See also == * Chiasm * Commissure * Contralateral brain * Definition of types of crossings * Fissure (anatomy) * Palpebral commissure (of the eye)

== References == {{Reflist}}

== Further reading == * [https://neuroscience.stanford.edu/news/ask-neuroscientist-why-does-nervous-system-decussate Why does the nervous system decussate?]: Stanford Neuroblog * {{Cite web |last=Fields |first=R. Douglas |date=2023-04-19 |title=Why the Brain's Connections to the Body Are Crisscrossed |url=https://www.quantamagazine.org/why-the-brains-connections-to-the-body-are-crisscrossed-20230419/ |website=Quanta Magazine}}

== External links == * {{Commons category-inline}}

Category:Anatomical terminology