Mechanisms of Axon Guidance

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Axons are sensitive and their respond depend on the local environment. When the tectum is removed, retinal axons grow toward the missing tissue, demonstrating that optic axons utilize these neighbourhood signals as opposed to a long-run diffusible attractant from the tectum as they grow along the optic tract (Taylor, 1990). In the event of a small piece of the optic tract neuroepithelium is turned 90° preceding the axons enter it, then they become misoriented when they enter the pivoted transplant (Harris, 1989) and correct their course of growth once they exit. These results confirm that the neuroepithelium contains local information to which growing axons respond and that they are not simply following gradients of attractants released by their targets (Sanes et al., 2012). Growth cone filopodia are long, motile, and covered with receptors, also have a very high surface to volume ratio, which can help convert membrane signals into large changes in intracellular messengers such as calcium. Filopodia can show localized transient elevations of intracellular calcium and these transients reduce filopodial motility (Sanes et al., 2012). Calcium transients can be directly artificially activated in growth cone filopodia by loading neurons with a calcium caging agent that releases calcium upon stimulation with a pulse of light (Gomez et al., 2001). Experiments in which calcium is uncaged on one side of a growth cone generally cause the growth cone to turn toward the side that has
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