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11 Cards in this Set
- Front
- Back
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To grasp the different hypotheses that have been putforward to explain how connectivity is achieved What are the 2 hypotheses that explain how specific neuronal connectivity is achieved? |
1. Weiss (1928) Resonance theory random neuronal outgrowth to all targets elimination of non-functional connections 2.Sperry (1939) Chemoaffinity hypothesis directed and specific outgrowth axons follow 'ID tags' of embryo cells and fibres. |
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To understand the evidence that indicates that axonsare actively guided to their targets Sperry 1963 |
Cut optic nerve and remove temporal retina allows just nasal axons to grow back axons grow back directly to right place -not elimination Regrowing axon ignored territory normally innervated by 'blue' axons -shows seeking |
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To understand the evidence that indicates that axonsare actively guided to their targets Lance-Jones and Landmesser 1980 |
Motor axons are guided specifically to their targets in chick embryo Cut and reverse segment of neural tube before motor axons grow out. Motor axons still found way to normal muscle targets Suggests axons navigate to targets |
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Pattern of embryo outgrowth in embryo is... |
highly organised reproducible stereotyped |
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To understand what is meant by the term “guidance cue” |
Factors in environment that axons use to find their correct targets. |
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Why were early experiments on guidance cues done on insects? |
simple NS embryos easy to observe/ manipulate in larger insects (grasshopper) individual cells can be ablated using lasers. |
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To be familiar with experiments that suggest growth cones respond to cues in environment. Axons |
Ablation studies G axons stalls in absence of P axon not due to lack of axons on which to extend no due to reduction in no. of axons (not affacted when A ablated) B/c G axongrowth cone is looking for specific cues on P axon |
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Labelled pathway hypothesis |
Axons can selectively fasiculate with other axons Axon surfaces carry labels/ cues Different axon growth cones express different Rs for such cues Pioneers form an axon scaffold on which followers can extend |
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To be familiar with experiments that suggest axon scaffolds are important in vertebrates |
Subplate neurons project from cortex to thalamus prior to innervation of cortex by LGN neurons. If ablate part of subplate early on before axons extend, LGN innervation fails in ablated region |
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To be familiar with experiments that suggest guidancecues are associated with cells Pioneers |
Pioneers also follow stereotyped paths Grasshopper embryo limb Pioneer Ti1 growth cone makes specific turn at limb boundary and as it approaches specific cell, Cx1. No obvious morphological features Implies must be molecular differences in environment. |
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4 Forces of axon guidance |
1. Contact attraction 2. Contact repulsion 3.Chemoattraction 4.Chemorepulsion |
