Bi150 PS2_Answers_2011

Bi150 PS2_Answers_2011 - PS2 Answer Key Problem 1. Action...

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PS2 Answer Key Problem 1. Action potentials (1.5 points) A. (0.5 point) Generation V 1 a. In a typical neuron’s cell body, the experimenter gives a series of current pulses (top trace). The smallest pulse gives an axon potential, as indicated (bottom trace). Draw the remaining part of the bottom trace, showing the qualitative and approximate trend in number and shape of action potentials. APs are all-or-none electrical events, and will therefore, always generate spikes of the same amplitdue. As the depolarizing stimulus increases, the number of APs generated will also increase, and the frequency with which they fire should have a relatively stable period. b. The generation of action potentials is a regenerative or positive-feedback process. Explain the mechanism of positive feedback during action potential generation. When voltage-gated sodium channels open, current will flow into neuron, causing membrane potential depolarization. Membrane depolarization further increases the channel opening of sodium channels and induces membrane potential depolarization. This process will repeat and result in positive feedback. It will last until sodium channel inactivation. B. (1.0 point) Propagation
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PS2 Answer Key This question deals with the influence of an axon’s diameter on its basic properties. Use the simulation program on the website: http://nerve.bsd.uchicago.edu/nerve1.html (You may need to restart your browser occasionally) Use the “V vs x” option. Click on “axon parameters” and make “Max gNa” and “Max gK” zeros, to examine passive properties of membrane. Open “Stimulus” and make the duration of current injection long enough (e.g. 20 msec). You may also need to change “amplitude” of the current pulse and “Total Time” of simulation accordingly. Start the simulation, then you will see spatial voltage change in the axon induced by tonic current injection. a. Describe the change in the spatial decay of membrane potentials: When you increase or decrease “Radius” in “axon parameters”. The spatial decay of a membrane potential traveling through a cell is inversely proportional to the radial area of the segment within which it is traveling. As this area increases, the spatial decay decreases because there is a reduction in the resistance of the volume within which the potential is traveling. When you increase or decrease “IntR” in “axon parameters”. Spatial decay of a membrane potential traveling through a cell is proportional to the IntR (internal resistance) of a cell. As the IntR decreases, so too will the spatial decay. b. (Without simulation) When the axon’s diameter is doubled (length is infinite), how much will the g m (membrane conductance per length) change? How much do c m (membrane capacitance per length. The axon is **not a coaxial cable), and r a (axial resistance per length) change? What about the length constant? Explain. Ra = ρ/π(a)^2
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This note was uploaded on 01/03/2012 for the course BI 150 taught by Professor Kennedy,m during the Fall '08 term at Caltech.

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Bi150 PS2_Answers_2011 - PS2 Answer Key Problem 1. Action...

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