NPB 100 Lecture 14

NPB 100 Lecture 14 - Lecture 14-Growth of Axons and Dendri...

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Lecture 14 -Growth of Axons and Dendrites -Problem of how they are guided -Axonal pathfinding -Synapse formation -Cell death and control of numbers of neurons -When neurons differentiate, they begin to migrate in the nervous system, and extend processes -Have an amoeboid cell that is moving around, it beings to extend a process in one direction and another in another. When these processes grow, their tips are called growth cones. -Growth cones have elaborate structures. They might consist of a sequence of very elaborate long thing filaments extending out. Thin processes are called filipodia. Flattened areas are lamellipodia. The growing axon might be only .5-1 micrometer but the filipodia might be as long as 50 micron and the lamellipodium can be 10 micron across. The growth cones act as active sensors. They are constantly moving all the time. They can move forward and grow or if they meet something they do not like, they will retract. New membrane is being synthesized all the time. -slide 2: time lapse photographs of axons growing from the retina of tadpoles and it’s growing into a structure of the animal’s brain called the optic tectum (primary visual area for reptiles and fish). The cells are expressing a modified surface protein that has linked to it a green fluorescent protein. Thus, we can see the surface of the cell really well. -Growth cones are growing in the central nervous system and try to go to the optic tectum -these things are extremely plastic, grow on surfaces they like and avoiding ones that they don’t
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-mobility in the growth cones are created by actin and myosin -actin around edge of lamellipodia, tubulin main part of the axon -direction of growth in part has to do with the substrate -substrate in the forms of cell surfaces, extracellular matrix (complex array of protein and carbohydrates many cells secrete which can have locally very different composition; growth cones will have sensory for that can detect these different components and find some attractive and some repulsive) -what are the axons that lay down the initial pathway that others can follow? -sensory neurons differentiate out in the ectoderm in grasshoppers -look at developing limb of the grasshopper, starts out as a small protrusion of the ectoderm -2 cells in each limb early differentiate out of the ectoderm, and falls inside, and starts to send out axons, they grow in a very stereotyped pattern. They go toward the CNS and make contact with particular landmark cells. -They are called pioneer neurons. Lay down a pathway that other neurons will follow -These 2 cells grow along, they contact a landmark cell, stop, make gap junctions, and then they break off, and they continue to the CNS -if knockout landmark cells, pattern of axonal pathfinding is disrupted, it slows
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NPB 100 Lecture 14 - Lecture 14-Growth of Axons and Dendri...

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