| Axon Guidance |
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| developmental neuroscience | |
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MECHANISMS Growing axons have a highly motile structure at the growing tip called the Growth Cone , which "sniffs out" the extracellular environment for signals that instruct the axon which way to grow. These signals, called guidance cues, can be fixed in place or diffusible; they can attract or repel axons. Growth cones contain Receptors that recognize these guidance cues and interpret the signal into a Chemotropic response. The general theoretical framework is that when a growth cone "senses" a guidance cue, the receptors activate various signaling molecules in the growth cone that eventually affect the Cytoskeleton . If the growth cone senses a gradient of guidance cue, the intracellular signaling in the growth cone happens asymmetrically, so that cytoskeletal changes happen asymmetrically and the growth cone turns toward or away from the guidance cue. STUDYING AXON GUIDANCE Scientists use various methods to work out how axons find their way. In genetic Model Organisms like Mice , Nematodes , and Fruit Flies , scientists can generate Mutation s and see whether and how they cause axons to make errors in navigation. In Vitro experiments can be useful for direct manipulation of growing axons. A popular method is to grow neurons in culture and expose growth cones to purified guidance cues to see whether these cause the growing axons to turn. These experiments are often done using cells from the frog Xenopus Laevis because ''Xenopus'' embryos are easy to obtain and ''Xenopus'' cells are relatively hardy and grow well at Room Temperature . Axon guidance model systems Several axon "paths" have been extensively studied and are used as model systems to further understand the mechanisms of axon guidance. These include:
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