# Homework Help: Action Potenials and Stimulation

1. Feb 8, 2008

### sillucius

Hey guys, I'm an electrical engineer taking a grad course in a biomed signal acquisition course. Its an interesting course, but I feel like a fish out of water in that I'm not used to it.

1. The problem statement, all variables and given/known data

2. Relevant equations

The question is as attached.

3. The attempt at a solution

For the first one, I believe this is the positive side, because it depolarizes the cell.

In the second questions, my natural instinct is to treat this as an EE problem, in that the AP propogates from the positive to negative direction? Sort of like a battery connected to a circuit. Or, like a battery connected to the cable model of the membrane.

The last question I am clueless. I thought the AP propogates down the axon in a lossless manner? At "great distances", would the AP lose some of its strength?

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2. Feb 8, 2008

### Moonbear

Staff Emeritus
I'll start by pointing out that electrophysiology is about my weakest area of physiology, so while I can try helping you with this, if something I say sounds completely confusing, I could be wrong.

The first thing to consider is how an action potential originates and propagates. The origination of an action potential depends on the voltage gradient across a cell membrane...essentially the ratio of positive and negative ions on either side*. So, when talking about positive and negative directions or anything else, you need to keep track of which side of the membrane you're on.

During the propagation of an action potential, the ion gradients are shifting from a negative resting potential (inside relative to outside...more negative ions INSIDE the cell, and more positive, or less negative, ions OUTSIDE the cell). During a depolarization, you have a change in the ion distribution so that there end up being more negative ions OUTSIDE the cell than inside in a localized area (depolarization)...this triggers the same change in adjacent areas while each preceeding area returns to the resting potential (after a brief hyperpolarization).

In the diagram you've provided, the electrodes are applied to the OUTSIDE of the membrane (there are methods that puncture the cell and apply current to the inside too).

I think this should get you started on the first two questions.

As for the third question, I *think* what they are saying when they say "at great distances" is that the opposing electrode (whichever electrode is not stimulating the action potential) is far enough away from the stimulus electrode not to interfere with your measurements...in other words, in the hypothetical example, I think they want you to disregard the other electrode to answer that question.

As an aside, you would have probably loved my old physiology textbook by Berne and Levy...as I'm skimming through the chapters on this stuff to refresh my memory to help you, I'm realizing it has more circuit diagrams than biology! I'm also realizing that NOBODY seems to state whether the electrical "stimulus" applied to a cell is positive or negative when describing such studies...not even in the textbooks. I can reason through the biological principles, but whether an externally applied electrical stimulus, as opposed to an electrochemical gradient, is the same, I'm not 100% sure.

*I can help with the details of this if you need it, but most texts cover the ion gradients in great detail, so I didn't get into it here.

3. Feb 10, 2008

### sillucius

Ahhh... I see. Thanks for clarifying up the depolarization bit. So I guess it propogates from the negative electrode.

Still am clueless about the last part of the question. Got anymore hints/help?

4. Feb 10, 2008

### Moonbear

Staff Emeritus
That's my guess on it.

I think you were already on the right track with what you wrote when you asked the question...that it should propagate in a lossless manner. Is there any reason to think it's directional when an external stimulus is applied?