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1. Introduction
1. What is the membrane potential (V_m), and why important?
1. Make glass microelectrode, stick in into a cell, membrane seals around the electrode compare the electrical potential on the inside of the cell with that on the outside. (p132-133)
2. measure small voltage across membrane (ca. 0.070 V = 70mV)
1. The convention is to express inside relative to outside, inside is lower, hence V_m is negative
3. Differs in different cells and in different species of animals (-20 - -100 mV)
4. It changes when nerves conduct impulses and when muscles are activated
2. What it the basis for it and what causes it to change
1. Answer - ion gradients across the membrane & membrane has different permeabilities to different ions
2. Changes in membrane potential produced by changes in membrane permeability to particular ions
3. Membrane permeability is produced by membrane channels
4. Changes in permeability are produced by opening and closing of membrane channels
2. Nernst Equation
1. Concept of Balancing Forces
1. Muscle vs gravity holding a mass said to be in EQUILIBIRUM
2. Two opposing forces can balance each other - even if different kinds of force
1. Thought question - Osmotic Pressure set-up - bag composed of semi-permeable membrane in a jar, sugar dissolved in water inside the bag, pure water outside, tube emerging from bag.
1. What happens - water flows into bag, does it ever stop?
2. Yes, when difference in concentration balances the hydrostatic pressure
3. For ions, two forces that can act on them are electrical field & concentration gradient
4. Thought question: membrane permeable to K⁺ but not lactate (salt of lactic acid)
1. What will happen?
2. K⁺ diffuses out, making inside negative until forces balance
3. Combined, the two forces constitute the ELECTROCHEMICAL GRADIENT
4. When forces balance, the voltage equals the "Equilibrium Potential"
2. Quantifying the Equilibrium potential - the Nernst Equation (p141)
1. The Nernst equation:



1. R & F= constants
2. T = absolute temperature
3. z = valence of the ion
4. [X] concentrations Outside and Inside the cell of the ion in question
3. Goldman Equation
1. Membrane has finite permeability to ions other than K⁺ -
4. Membrane Potential

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