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I'm wondering how we can detect the electrical activity of a muscle cell. My initial thought on this question was that the membrane potential, around $\Delta V_{\text{membrane}} = -80mV$ across the cellular membrane, would create some sort of dipole. The contraction of a muscle cell happens when calcium ions come pouring inside the cell, negating the $\Delta V_{\text{membrane}}$. Since a dipole has an associated electric field all around it, then we would be able to detect the disappearance of the dipole field and say "this muscle cell just contracted!".

The problem is that most cell membranes form something like a concentric cylinder shape. Wouldn't this mean that there wouldn't have an electric dipole in the first place? If it is the case, can somebody tell me what we actually detect with electrical lead during e.g. a heartbeat or muscle contraction?

Here is a pertinent link by Acid Jazz. This is not the first time that I have found this explanation that is basically saying: "The heart acts as a big dipole". This seems strange to me as it would imply there is a biological mechanism for storing differently-charged ions across the heart. It seems much more intuitive to me that what we detect at the surface of the skin is a sum of multiple small contributions coming from each muscle cells.

I'll make an analogy: the dielectric in a capacitor has fixed atoms that get polarized by the electric field. The contribution of each of those tiny dipole form a big equivalent dipole. In a heart cell, however, it looks like there would be no equivalent dipole, so multiple heart cells cannot contribute together to form a big equivalent dipole (see figure).

Equivalent dipoles

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  • $\begingroup$ this is kinda related: physics.stackexchange.com/questions/201394/… although you might have seen it already $\endgroup$ – user81619 Aug 22 '15 at 14:21
  • $\begingroup$ I actually did not find this particular post, but I have been reading Webster's "Medical Instrumentation" which says pretty much the same thing. The problem with this explanation, I find, is that it basically says that the heart is one big dipole, which doesn't make sense to me. I really think the detected field is the contribution of multiple small signals coming from individual cells, although I have not found anything to support this idea. $\endgroup$ – victorbg Aug 22 '15 at 14:31
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    $\begingroup$ Maybe rephrase your question with the link in it and the question in your comment included? Or ask it as a seperate question? I know you are dealing with muscle cells, but I seem to remember something about squid neurons being individually measured (as they are so large). $\endgroup$ – user81619 Aug 22 '15 at 15:29
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The heart only acts as a big dipole, when it is electrically active. ECG measures the potential difference between different places on the skin. These potential differences are created when different parts of the heart muscle are in dfferent stages of their action potential. For example, when the septum and the subendocardial myocytes are depolarized, but the apex and the subepicardial myocytes are not. I will try to demonstrate this using your drawing (by the power of MS paint and bad editing :) ):badly drawn heart

As you can see, at the myocardial surface, your individual cells will create an evenly deposited positive charge (circled with red). Image "A" shows that, when all de myocardium has repolarized, there is no "big dipole moment". There are still small dipole moments however, because the myocardial surface is more positive than the inside of the myocardium, but these dipoles cancel out just like they cance out in the case of a single cell in your drawing.

When one part of the myocardium depolarizes, (image "B") the outside of the cells becomes negative, and the inside becomes positive in that region. So there is an evenly distributed negative charge on the subendocardial surface. The apex of the heart and the subepicardial still hasn't depolarized, so there will be a potential difference between the inside and outside of the heart. This potential difference can be measured and a big dipole moment vector can be determined.

When the whole heart muscle is contracted, every region is depolarized, an evenly distributed negative charge will be on the surface. That is the ST segment on the ECG. Remember, that in physiological conditions, the ST segment is isoelectric, so there is no big dipole vector present in that case, just like when the heart is fully repolarized. Elevations or depressions in the ST segment means that some regions of the heart is still not depolarized entirely, and might indicate conduction problems (blocks) or ischaemic damage to the myocytes.

I hope this explains your questions. In the case of electromyography, the same thing is measured. Sorry for the bad drawing.

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  • $\begingroup$ -I have the original images, which are basically the same, but are copyright protected, that's why I chose to recreate them quickly in paint. The originals are in a Hungarian medical physiology textbook: "Fonyó A. Az orvosi élettan tankönyve" 2011 edition. $\endgroup$ – David Nov 1 '15 at 16:41

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