Prob Set 2 Q&A - Problem Set 2 Cellular...

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Problem Set 2 Cellular Neurobiology Fall 2010 1. Bill the scientist wants to use the patch clamp method to record the effects of acetylcholine on the current of a single channel. Which patch clamp configuration is most suitable for this goal? Explain how it works. A: Outside-out recording is the most suitable. If the pipette is retracted while in the whole-cell configuration, a membrane patch is produced with the extracellular surface exposed after the ends of the membrane anneal. Once he has isolated a channel, Bill wants to study its ion selectivity and creates the following conditions: Ion [ion]in [ion]out Na+ 150 mM 15 mM K+ 5 mM 150 mM Cl- 120 mM 10 mM Determine E ion for each of the above ions. A: E Na = -58 mV , E K = +85.7 mV , E Cl = -62.6 mV Bill also finds that the conductances are 0.2 for Na+, 0.8 for K+, and 0.3 for Cl-. With the conditions above, he studies the current properties at various holding potentials and finds that the current changes direction at +57 mV. Given this information, to which ion(s) is this channel likely to be selectively permeable? A: Na+ and K+ using Erev = GNa x ENa + GK x EK = +57 mV. Other GNa+GK combinations of conductances yield different Erev’s. 2. What are three characteristics of myelin that function to increase conduction velocity? A: Acts as resistors in series (increase resistance) and leak is decreased. Acts as capacitances added in parallel (decrease capacitance) and less current needed to charge membrane, so more available at the nodes. Makes lambda ( λ , the length constant) longer. The result is saltatory conduction: the action potential jumps from node to node. Describe how myelination increases conduction velocity in terms of passive current flow. A: When depolarization elicits an action potential at a node of Ranvier, current flows passively inside the axon to the next node of Ranvier and the localized current flow generates the next action potential without having leaked 1
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out before then due to the insulation provided by myelin. Passive ion flow is the
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