# Are muscle cells electric dipoles?

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).

• this is kinda related: physics.stackexchange.com/questions/201394/… although you might have seen it already
– user81619
Aug 22, 2015 at 14:21
• 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. Aug 22, 2015 at 14:31
• 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).
– user81619
Aug 22, 2015 at 15:29