Decussation is used in biological contexts to describe a crossing (due to the shape of the
Roman numeral for ten, an uppercase 'X' (decussis), from
Latin decem 'ten', and as '
as'). In Latin
anatomical terms, the form decussatio is used, e.g. 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.
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
botanicalleaf 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 decussatus or decussata or otherwise in part containing "decuss..." are common, especially in the
specific epithet.[1]
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.[2] The visual map theory of
Ramón y Cajal has long been popular[3][4] but has been criticized for its logical inconsistence.[5] 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.[6][7]
^Ramón y Cajal, Santiago (1898). "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)" [Die Structur des Chiasma opticum nebst einer allgemeine Theorie der Kreuzung der Nervenbahnen (German, 1899, Verlag Joh. A. Barth)]. Rev. Trim. Micrográfica (in Spanish). 3: 15–65.
^Llinás, R.R. (2003). "The contribution of Santiago Ramón y Cajal to functional neuroscience". Nat. Rev. Neurosci. 4 (1): 77–80.
doi:
10.1038/nrn1011.
PMID12511864.
S2CID30442863.
^de Lussanet, M.H.E.; Osse, J.W.M. (2012). "An ancestral axial twist explains the contralateral forebain and the optic chiasm in vertebrates". Animal Biology. 62 (2): 193–216.
arXiv:1003.1872.
doi:
10.1163/157075611X617102.
S2CID7399128.
^Kinsbourne, M (Sep 2013). "Somatic twist: a model for the evolution of decussation". Neuropsychology. 27 (5): 511–15.
doi:
10.1037/a0033662.
PMID24040928.