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| 1. What do I need to know? | |
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This area of cell physiology is sometimes called electrochemical theory. This is not a welcoming term but its does illustrate that it is the chemical (concentration) gradients that exist across the membranes of living cells that gives rise to the electrical potential (or voltage) which you recorded in artificial membrane l practical. I see that you're still skeptical about this. OK, ask again: The materials here will help you with the vocabulary (to understand meaning and recognise equivalent terms) By following the links (click on any blue text) you will find descriptions of the Nernst and Goldman equations that should help you to:
In addition to all this there are some questions that you can answer. These will challenge you to think critically about what you think you know. It can be a surprise how quickly a topic that you think you understand, suddenly becomes as clear as mud when you try to use your knowledge to answer questions or solve problems. There is an additional resource (Goldman simulator) available to you via the Internet. These can be a useful extention of lectures, practicals and your text books, especially if you find this area:
The Goldman simulator is designed to illustrate the Goldman equation. You will recall that the resting membrane potential of neurones is around -70 mV (range -85 to -55 mV). You should also understand that the 'resting' membrane is essentially permeable to K+ which explains why the resting potential is close to the equilibrium potential for K+ (EK = -90 mV). Equilibrium potentials are calculated by the Nernst equation. If this is all foreign to you, you need to go back over your notes from element 1 and the first 2 lectures from element 3. You will definitely need to master the Nernst equation and selective permeability to survive much of physiology and pharmacology. You may also recall that during an action potential the membrane briefly becomes very much more permeable to Na+ (about 10 times more permeable to Na+ than to K+ ) and the membrane potential approaches the equilibrium potential for Na+ (ENa = +65 mV), this explains the overshoot of the action potential. You may have worked out by now that this principal also explains why the resting membrane potential can be -70 rather than -85 mV i.e. the membrane has a small permeability to Na ions. So, the Nernst equation can calculate the equilibrium potential for a single ion. There are notes on the Nernst equation available to help you revise this topic. The Nernst equation cannot calculate the membrane potential due to the permeabilities of more than one type of ion. By modifying the Nernst equation, however, we can do just this. The modified equation is called the Goldman equation (or sometimes the Goldman-Hodgkin-Katz equation, and sometimes the Goldman Constant Field equation - further examples of equivalent terms). You have been given the form of the Goldman equation in elements 1 and 3. Essentially, the Goldman equation calculates the membrane potential based on the electrochemical gradient of all permeant ions (usually Na+, K+, Cl- and sometimes Ca2+ ) and the permeability of the membrane to each ion. Permeability is expressed either in 'absolute' terms (units of cm.s-1) or 'relative' to other permeabilities; i.e. total permeability of membrane at rest is 1.0; K+ being 0.8, Na+ being 0.05 and Cl- being 0.15 (0.8 + 0.05 + 0.15 = 1.0). The links to the simulators and some supplementary notes are to found in the 'Resources' section.
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| Copyright © 1999 University of Bristol. All rights reserved.
Author: Phil Langton |
Last modified: 29 Nov 1999 15:55 Authored in CALnet |