Questions and Answers about BIOLOGY

by Ken Miller and Joe Levine


QUESTION: I am teaching the chapter on the nervous system presently and am having a problem understanding the propagation of the nerve impulse. Formerly, I taught that a 'gate' opened along a nerve fiber allowing sodium ions to pour in. Later these were pumped out. The new book indicates that electrical changes are brought about by pumping out potassium ions. Wouldn't that mean that sodium ions would accumulate inside the fiber? I thought they were kept out... (From Richard in Michigan)

Richard, I think you are confusing what happens to produce the Resting Potential with the events that cause the Action Potential.

The Resting Potential is explained on page 898 of the Dragonfly Book. The neuron cell membrane contains a sodium/potassium ATPase, a protein that uses the energy of ATP to pump sodium ions out of the cell, and potassium ions into the cell. The pump works all of the time, like a bilge pump in a leaky boat, pumping K+ and Na+ in and out, respectively.

Now, when an action potential is fired, what happens? As described on page 899 (and just like you're used to teaching) "gates" in the membrane open, allowing Na+ ions to pour it. This inrush of positive ions "depolarizes" the membrane. A few milliseconds later, another series of gates opens, allowing potassium ions to rush out, which "repolarizes" the membrane. It's the gates that cause the repolarization, not "pumping out potassium ions." I believe that Figure 35-7 on page 899 shows this pretty well.

There is, however, one thing to keep in mind, and that is the work of the sodium/potassium ATPase pump.... while all of this is happening, just like that pump in the bottom of a leaky boat, it keeps working. Every time an action potential passes along an axon, a little more Na+ rushes in, and a little more K+ rushes out. What the pump does, sort of "in the background" is to pump that Na+ out and the K+ in, to make sure that the resting potential is maintained and the cell is always ready for another Action Potential.

I hope that this explanation (and pages 898-899) are clear on this point. But if they are not, please write again!

Ken Miller (3/7/02)

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