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Crossing of commissural axons across the midline in vertebrates is mediated by signaling in the floor plate of the neural tube. On the left panel an axon initiates its projection within the tube. On the right panel, the neuron initially receives chemoattractive signaling from netrin ligands and chemorepellents from slit ligands (1).
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, see Chiasm (anatomy)). A midline crossing of nerves inside the brain is called a decussation (see Definition of types of crossings).
The posterior commissure (also known as the epithalamic commissure) is a rounded nerve tract crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct. It is important in the bilateral pupillary light reflex.
The inability of these axons to cross the midline results in anomalous axonal guidance and front-to-back projections within each hemisphere, rather than connecting between the hemispheres in the normal corpus callosum. These longitudinal callosal fascicles were originally described by Moriz Probst in 1901 by gross anatomical observation. [1]
In mammals and birds and other vertebrates with frontal eyes, the optic nerves do blend in the optic chiasm, and only part of the nerve fibres cross the midline. [5] The drawings of Cajal suggest that the axons of the optic nerve may branch in the optic chiasm, and thus give off a branch both in the ipsi- and contralateral optic tract. [ 5 ]
The Robo proteins are critical regulators of midline crossing across species. In Drosophila embryos, Robo1 and Robo2 are required to keep ipsilaterally projecting axons from inappropriately crossing the midline, and to prevent contralateral axons from remaining stuck at the midline. Robo3, while it also binds Slit, does not appear to play a ...
Slit genes, as well as their roundabout receptors, act as chemorepellents by helping prevent the wrong types of axons from crossing the midline of the central nervous system during establishment or remodeling of the neural circuits. The binding of Slit to any member of the Roundabout receptor family results in axon repelling through changes in ...
The lateral corticospinal tract neurons cross the midline at the level of the medulla oblongata, and controls the limbs and digits. [1] [3] The lateral tract forms about 90% of connections in the corticospinal tract; [2] the vast majority cross over in the medulla, while the rest (about 2-3%) remain ipsilateral. The anterior corticospinal tract ...
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