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Unformatted text preview: BioNB222 Spring 2008 Cornell University Joseph Fetcho 1 Lecture 5. Action Potentials and Propagation II Lecture Outline 1. What is responsible for threshold? It is the point at which influx of sodium exceeds the efflux of potassium. At this point a positive feedback develops because the extra positivity inside from the sodium ions depolarizes the neuron more, which opens more sodium channels, which depolarizes the neuron more and opens more channels 2. The positive feedback loop also explains the all or nothing behavior of the action potential and the overshoot to positive potentials. The feedback drives the membrane potential to near the sodium equilibrium potential by a massive opening of sodium channels. 3. Some other important bits about action potentials. A. Not many sodium ions need to move into the cell to depolarize it. Still, after very many action potentials, the concentration of sodium would rise inside the cell and the ability to generate action potentials would go away. inside the cell and the ability to generate action potentials would go away....
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- Spring '08