Joshua Dudman :: jtd2001@columbia.edu. 0 mV -80 mV

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<ul><li><p>Joshua Dudman :: jtd2001@columbia.edu</p></li><li><p>0 mV</p></li><li><p>-80 mV</p></li><li><p>-80 mV+++++++-------+</p></li><li><p>-80 mV[K+] = 135[Na+] = 7[Cl-] = 11A-[K+] = 2.5[Na+] = 125[Cl-] = 130A-+++++++-------+</p></li><li><p>Resting membrane potential is independent of external Na+ concentration</p></li><li><p>Resting membrane potential strongly depends upon the external K+ concentration</p></li><li><p>Why is the resting potential altered by changes in one ion (K+) but not another (Na+)?Can we quantify how the resting membrane potential depends upon changes in ion concentration?</p></li><li><p>So far</p><p>The resting potential is the result of an unequal distribution of ions across the membrane.</p><p>The resting potential is sensitive to ions in proportion to their ability to permeate the membrane.</p><p>Forget the membrane and consider what factors determine the movement of ions in solution.Aqueous diffusion</p><p>-and-</p><p>Electrophoretic movement</p></li><li><p>++++----+VinVout l</p></li><li><p>A little bit more about the Nernst Equation</p></li><li><p>Some important details:Derives from the Nernst-Planck equation and a few assumptions Uses permeabilities rather than conductancesCl- is flipped to account for a -1 valence</p></li><li><p>There is an important difference between a cell with only potassium flux at rest and one with multiple fluxesNa+K+K+ diffusionK+ electrophoresis</p></li><li><p>A neuron can be modeled with an electrical equivalent circuitWhen to use:To understand the time dependence of changes in ionic concentration, or conductance state, etc.</p><p>To easily separate out the current for a specific sepcies of ion</p></li><li><p>The complete equivalent circuit</p></li><li><p>The passive equivalent circuitSolving for Vrest</p><p>INa + IK = 0</p><p>Vin - Vout = EK + IK / gK</p><p>Vin - Vout = ENa + INa / gNa</p><p>IK = gK(Vm - EK)</p><p>INa = gNa(Vm - ENa)</p></li><li><p>SummaryThe membrane conducts ions very poorly and allows the separation of ionic species. This results is a potential difference between the outside and the inside of the membrane.The magnitude of the resting potential is determined by the selective permeability of the membrane to ionic species.We can quantify the the magnitude of the resting potential by considering both the diffusive and electrophoretic properties.In order to understand the time dependence and individual contributions of ionic species to the membrane potential it is convenient to use an electrical equivalent circuit.</p></li><li><p>What happens to these considerations during the action potential?Why are the ions collected close to the membrane? What does this mean for the polarization of the bulk solution?How many ions are moving across the membrane?Why is the membrane impermeant to ions?If the membrane is impermeant to ions it means that there is a large energetic barrier that must be overcome. How do ion channels overcome this energetic barrier?What is the slope of this line?Some things to think about</p></li></ul>