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It is still uncertain whether the interthalamic adhesion contains fibers that cross the midline – and for this reason, it is inappropriate to call it a commissure. The interthalamic adhesion is notably enlarged in patients with the type II Arnold–Chiari malformation. [3]
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).
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 ]
Comm expression turns off after the growth cone has crossed the midline; this permits Robo/Slit repulsion and prevents the growth cone from crossing the midline again. Vertebrates, on the other hand, do not possess a comm homolog ; instead they facilitate midline crossing through alternative splicing of Robo3 (aka.Rig-1).
The anterior commissure (also known as the precommissure) is a tract that connects the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix.
The first-order neurons from the trigeminal ganglion enter the pons and synapse in the principal (chief sensory) nucleus or spinal trigeminal nucleus.Axons of the second-order neurons cross the midline and terminate in the ventral posteromedial nucleus of the contralateral thalamus (as opposed to the ventral posterolateral nucleus, as in the dorsal column medial lemniscus (DCML) system).
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 ...