There are few concepts in physiology that cause as much initial bewilderment as how membrane potentials work.
Imagine that you're teaching a class filled with budding young minds. It's time to introduce them to a conceptual cornerstone in physiology. You explain that biologically-relevant ions carry electrical charge and that they're distributed in different concentrations inside and outside the cell. You show carefully constructed animations of ions moving across the membrane (or not) depending on the permeability of the membrane to those ions. You connect these ideas to the basis of the action potential, highlighting that these electrical events are intimately coupled to subsequent chemical and mechanical events.
These are phenomena that underlie every thought, every emotion, every action... indeed, the most important processes in our lives.
Then, you show one of the things that many students dread the most. The stuff of nightmares... equations. First the Nernst equation. Then the Goldman-Hodgkin-Katz equation. The students are reeling. You decide to throw them a softball question on the quiz/exam:
For a typical cell at rest, what happens to the membrane potential if the extracellular potassium concentration is increased?
Half the class correctly answers the question. A success?
One of the issues in trying to explain membrane potential is that we often don't have a cell/tissue/organ system available for students to actually manipulate the parameters that matter most when determining membrane potential and its physiological effects. Oh, how wonderful it would be to have a cannulated blood vessel for students to try bathing in different extracellular ion concentrations! Or, to have a neuron preparation that could be exposed to different neurotransmitters while recording membrane potential. Even for a modestly sized class, these demonstrations would be difficult to set up or impractical for other reasons.
But, we'd still like students to at least come to grips with the quantitative nature of physiology and the power equations afford us to predict changes.
One of AZPS's very own members, Dr. Stephen Wright, endeavored to make these equations describing membrane potential easier to grasp. Working with Michael Branch and Biomedical Communications at the University of Arizona, an educational app was created for the University of Arizona College of Medicine.
Enter the Nernst/Goldman Simulator
This digital tool allows you to play with sliders that control different elements of the cellular system: temperature, ion concentrations, relative membrane permeability.
A "live" readout of membrane potential is displayed and reacts in real-time to the changes that you make. The math is done behind the scenes, but the relevant equations are displayed along with the calculated equilibrium potentials based on where the sliders are adjusted. Presets for various cell types can even be selected. It's rather slick not just for its functionality, but also for a very nice interface design.
Two versions are currently available. An iOS version that can be downloaded from the iTunes store as well as a (deprecated) Flash version hosted on the University of Arizona's servers. Both are provided for free. You can access them with the buttons below.
Ideas for student exercises
1. Pose the question above (or one similar to it) about raising extracellular potassium. Have students conceptually or mathematically reason through the question. Poll the students to see how many correctly answered. Follow up the question by introducing the Nernst-Goldman simulator and have them download it on their phones or laptops. Re-poll the class after giving them a chance to play with the app.
2. Create a table that describes changes to various parameters of the Nernst and/or Goldman-Hodgkin-Katz equation. For boxes that students need to fill in, have them predict changes to equilibrium/reversal potential for the given parameters in the table. Introduce the Nernst-Goldman simulator. Give students the opportunity to input parameters into the app and check to see if their answers match conceptually and/or mathematically.
If you give it a go, we'd love to hear your experiences using the app and the exercises you come up with! Please share them in the comments below. Hope you found this teaching spotlight helpful!
Dr. Kanady is a lecturer for the Department of Physiology at the University of Arizona. You can connect with him on Twitter @JDKPhD.