|MadSci Network: General Biology|
Dear Karen, I hope my answer helps. We'll start with the basics. As you've said it's all about depolarization. First of all, we need to say that the cells responsible for the heartbeat, the cardiac muscle cells or myocytes have a resting membrane potential which is caused by the distribution of ions inside and outside the cell- high potassium, low sodium inside and high sodium, low potassium outside. In a cardiac action potential, the upswing or depolarization is caused by an influx of sodium ions into the cell carried along its diffusion gradient by a protein called the sodium channel. Shortly after the sodium channels open, they close and another set of channels (permeable to potassium) opens and lets potassium out, thus repolarizing the cell. In addition there are a number of pumps and other channels which help maintain the resting potential between beats. So, why does increased external potassium cause the heart to beat faster? Well, if you increase the external potassium concentration you change the resting potential and thus you change the threshold for firing an action potential. Basically, you make it easier for the cell to fire an action potential (it requires less of a depolarization) and thus it can beat faster. The second thing this change in external potassium concentration does is shorten the time between action potentials. It does this by effecting the driving force (i.e. the difference between external and internal potassium concentration) for potassium and thus many of the potassium channels which are opened upon repolarization act differently than normal thus causing the cell to repolarize faster than normal. I've simplified a very complicated process here but I think you get the general idea. Good luck, Terry
Try the links in the MadSci Library for more information on General Biology.