Décussation

decussation

The decussation is a medical term which more particularly indicates a crossing in the shape of X., this term indicates the passage through the Median plane of the nervous fibers connecting a cerebral Hémisphère to body half contralatérale.

The decussation of the nervous ways

The Cerveau sends orders and receives information of the opposite half of the body. For example, the driving Cortex left order muscles of the hémicorps right while the visual Cortex right processes the visual informations which are in the left half of our field of view. Consequently, the nervous fibers must necessarily cross the Median plane to connect the nerve center and the area concerned.

Driving ways

For the way of the Motricity voluntary, known as pyramidal Way, the decussation of the side Faisceau corticospinal (which orders the distal muscles of the members) takes place right in lower part of the medulla. In the ventral Faisceau corticospinal (which orders the axial muscles of the trunk for the Posture) the decussation takes place at the exit of the Spinal-cord.

For involuntary motricity, the Ways extrapyramidales Réticulospinal E and Vestibulospinal E did not disappoint.

Visual ways

The visual decussation of the ways takes place on the level of the optical Chiasma but relates to only fibers originating in the nasal part of each of both Rétine S as well as half of fibers (of each one) of the foveae. Consequently, information coming from each half of the field of vision which they are collected by the left œoil or the right eye is projected with the hemisphere contralatéral.

See the optical article Chiasma .

Why the principal nervous ways disappoint did?

The decussation of nervous fibers is known since the Antiquité because already Hippocrates had noted that a wound of the left side of the head generated convulsions of the right part of the body. But the explanation of this phenomenon which seems, at first sight, unnecessarily to complicate the nervous system is one of the most difficult questions of the neurosciences. Thus Ramon Cajal said there that it was one of the most obscure questions of biology .

The explanation which with the favor of the contemporary neurosciences rest on an adaptive explanation proposed by Ramon there Cajal: the presence of a danger in the right field of vision must be accompanied by an escape in the opposite direction. The reflex of escape would be thus faster if the visual information comes from to the nervous structures which order the muscles which allow the escape. However at the Vertebrate S primitive, like the Poisson S, such an escape is done by an inflection of the contralatéraux axial muscles to the danger which are under the order of the ways Réticulospinal E and Vestibulospinal E which did not disappoint. At the Vertebrate S later, provided with members, this reflex of escape concerned the ipsilatéraux distal muscles at the danger (as the leg of frog on which it rests to leap in the direction opposed to the danger). Consequently, the driving ways Corticospinal E and Rubrospinal E which orders these distal muscles owed décusser to ensure connection with the driving centers where the reflex of escape implying was initiated the axial muscles. Finally, at the Mammalian S, and the Primate S in particular, where voluntary motricity implies the pyramidal way mainly, the decusation would be a phylogenetic vestige of the preponderance of the driving ways extrapyramiales of vertebrate primitive. Recent work in neurosciences of the development seems to confirm this theory.

Sources

  • Serge Vulliemoz, Olivier Raineteau and Denis Jabaudon, “Reaching beyond the midline: why are human brains cross-country race wired? ”, The Lancet Neurology 4 (2) 2005,87-99. Comment in Washington Post, 6/13/2005.

See too

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