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Membrane potential primer (1)

# Membrane potential primer (1) - How to deal with exam...

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How to deal with exam questions on "Membrane Potentials" You may be asked how changing the concentrations of IONS inside or outside of the cell, with or without ion channel openings , will affect the electrical potential of the cell (V m ). These rules and diagrams will help you figure out how to solve those problems. MAJOR RULE: The V m for a cell will be PULLED TOWARD THE EQUILIBRIUM POTENTIAL (E q ) FOR THE ION (or ions) THAT CAN MOVE ACROSS THE MEMBRANE . If more than one ion can move EQUALLY well, the V m will be an average of the E q 's for these ions. How to find the Equilibrium potential for an ion: The Equilibrium potential for an ion can change with temperature but for your problems at 37 degrees Celsius, THE CONCENTRATION OF IONS INSIDE AND OUTSIDE OF THE CELL determines the E q according to a simplified Nernst equation. You must use these to figure out the equilibrium potential for the ion IF IT CAN CROSS THE MEMBRANE : E q K + = 61.54 log [K + ] o /[K + ] in E q Na + = 61.54 log [Na + ] o /[Na + ] in E q Cl - = -61.54 log [Cl - ] o /[Cl - ] in E q Ca 2+ = 30.77 log [Ca ++ ] o /[Ca 2+ ] in Try working these equations with the normal concentrations of ions below. You will see that the E q for K + is about -80 mV; for Na + it is about +62 mV; for Cl - it is -65 mV and for Ca 2+ it is +123 mV. Normal Physiological Concentrations of IONS Inside axon Outside axon Na + 15 mM 150 mM K + 100 mM 5 mM Cl - 13 mM 150 mM

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Ca 2+ 0.0002 mM 2 mM proteins (A - ) 400 mM - - I. If the problem involves CHANGING THE ION CONCENTRATIONS INSIDE OR OUTSIDE OF THE CELL from the normal physiological concentrations to some other value, you MUST figure out how that changes the E q for that ion . To get the new E q , use the simplified Nernst equations above. You will find, by plugging in different numbers for
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Membrane potential primer (1) - How to deal with exam...

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