BISC 421 1st Edition Lecture 35 Outline of Current LectureI. Construction of Neural Circuits ICurrent LectureConstruction of Neural CircuitsConstruction of Neural Circuits!• Axon Guidance Signals"• Growth Cones"• Cell survival and growth factors"• Synapse Formation"How do axons find their targets?!Three main hypotheses:!1. Mechanical guidance (steriotropism)- growing axons follow a track2. Resonance -that growing and connecting axons induce the identity of the postsynaptic cell3. Chemoaffinity – pre and post synaptic cells have ligand/receptor pair•These are old hypotheses•General type: hypothesis of mechanical guidance – the axons were following a physical tract(stereotropism)•Resonance-‐ each cell connects to the next one and they communicate•Chemoafinity-‐ most evidence. Idea that there is a chemical signal on the pre and post synaptic cell (lock and key) such that when a presynaptic cell is moving towards target itis looking for the lock for its keyolfactory sensory neurons for example – convergence. During development you can see the axons moving along like this . What is directing them?Roger Sperry (1940s) and Chemoafnity• Crush optic nerve – nerve regenerates in frogs!• Now rotate the 180 degrees eye !• Result: Frog stick out tongue when fly is 180 degrees in the wrong directionRoger Sperry (1940s) and ChemoafnityWhen the retina is rotated, the temporal and nasal parts are rotated•The former temporal retina is still projecting to the anterior tectum and the formal nasal retina is still projecting to the posterior tectum•They must have some sort of marker on them (nasal and temporal parts)•The fact that the animal has a switched view of the world means that the axons sort of know where to go-‐ cant adaptWhen the retina is rotated, the temporal and nasal parts are rotated•The results•Boil the extracts of the tectal membranes and see no efect of heating the anterior membranes but see heating of posterior membranes disrupts the preference-‐ so active material is in posterior tectumKey molecule is called ephrin-‐ secreted molecule by posterior tectum•It is repelling the axons from the temporal retina•IN this experiment they just put a patch of ephrin – if over expressed see that the temporal axons are avoiding the area with ephrin-‐ but if you get rid of ephrin all together the temporal axons grow all the way through to the posterior tectumWe’ve seen an example of a short range repulsion cue-‐ coming into direct range•Long range repulsion cues that will act at a distanceChick DRG growing onCollagen-coated lanesGrid of collagen•This neuron (DRG) is growing along these growth cones and in the second image can see it turning and staying in the bounds of the collagen coating•Collagen is binding to a certain receptor called an integrin•Membrane of one cell can stick to the membrane of the other (axons that are similarwill grow
View Full Document
Unlocking...