Most of my recordings were performed with 0.3.mm Teflon-coated silver wires. The Teflon coating is to isolate the silver wire from the bathing fluid. The silver is, in my experience, the best material as it quickly (in minutes) forms an Ag/AgCl bridge with the perfusing fluid, thereby reducing junction voltages at the tip. I have tried tungsten and platinum but I prefer silver. There are many companies that sell Teflon-coated silver wires (W3, Cooner wires, Science Products Gmbh, Capable, to name a few). Once chlorided, electrode tips usually do not have to be chlorided again.
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I use a diameter of 0.3 mm that, as it turns out, is quite adequate. Making the diameter larger is counter-productive. This is because the propagation of the slow wave is so slow that the time the depolarization passes under the electrode surface becomes bigger with larger electrode tips, thereby cancelling earlier and later signals collected at that same tip. The smaller the tip the better. However, too small a tip makes current flow (higher resistance) more difficult. I am happy in the range of 0.03 to 0.3 mm (not scientifically tested!).
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It is possible to measure smooth muscle action potentials using intracellular microelectrodes or even patch electrodes. But it becomes impossible to do this with more than a few electrodes. If you want to record from larger number of electrodes, extracellular (=external) electrodes is the only way.
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I have constructed needle electrodes and tried them a few times in (canine) small intestine. It did not work. I don’t know why. The electrodes recorded well enough when placed on the serosal surface of the small intestine (therefore, it was not an electrode malfunction) but, inside the muscle, it did not record anything. Same thing applies for implanted needle electrodes. Some groups have used pin electrodes. I suspect the signals are actually obtained from the outer part of the shaft while the part that is stuck in the muscle helps in the mechanical anchoring but does not record a potential.
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If the tissue to be recorded from is perfused by its own circulation, and you want to record from the serosal surface (as in an open-abdomen model), then an electrode array with the electrode tips flush with the base of the assembly is fine. This is because the tissue does not depend on the serosal fluid for its viability. But in isolated pieces of intestine, isolated from the circulation of the animal, and kept alive by superfusion in an organ bath, any resistance or hindrance of access of tissue fluid to the tissue and resistance to diffusion into the tissue will make the preparation less viable. Then, it is imperative that the contact between the electrodes and the tissue is minimal and restricted to the electrode tips. This will allow the tissue fluid to access the tissue through the electrodes. A distance of 3-5 mm from the base of the assembly to the tip of the electrodes is sufficient. However, the thickness of the tissue itself must also be as thin as possible. In practice, intestines from small animals, up to rabbits and cats, is possible. Larger animals (dogs etc.) just have a too thick muscle coat to be superfused in a tissue bath.
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| 5a. Examples of flush electrodes:
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5b. Examples of sticking out electrodes:
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With a tissue superfused in an organ bath, it is not necessary for the electrode tip to actually touch the tissue to be able to record a signal. As the signal conducts in the ion-rich surrounding fluid, it can still be picked up a few mm distant from the surface of the tissue although the amplitudes quickly diminish with distance. I used this possibility in a study of pendular contractions in the rabbit small intestine while recording from the same segment with a row of 32 electrodes that did not touch the serosal surface (Am J Physiol Gastrointest Liver 289:G898-G903, 2005). |
I have been able to record from several types of smooth muscle tissues but not from all. The notable exceptions are:
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If the muscle is too thick. For example, canine small intestine or stomach fine when recorded in the anaesthetized open-abdominal preparation but not isolated in an organ bath. You will then need to slice thin preparation to keep the tissue alive.
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The colon. I have never been able to record sensible electrical signals from the colon (and I tried!). Nor in an organ bath, nor in an open-abdomen model, nor with implanted electrodes in a conscious animal (small or large). I don't know why.
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Many signals recorded from smooth muscles can mimic real electrical signals, such as slow waves or spikes. Especially if the preparation is mechanically active, waves can be generated at a silver electrode that can look like slow waves. How do I determine the difference? Mechanical artefacts tend to be very large, to occur simultaneously over a set of electrodes and never have a sharp down stroke.
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There you are very much on your own. Most software’s have been written for the analysis of cardiac or brain recordings. Unfortunately, smooth muscle signals are quite different and need their own software for analysis. Please read my literature or contact me if you did get that far and are still interested.
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That is as important as the recording electrodes. I have used mainly the system that was developed at the department of Physiology, Maastricht, the Netherlands (a 240-electrode recording system). There are also other systems available on the market such as the Biosemi system and others. The important characteristics of these systems must be
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the number of simultaneously recorded channels (the more the better)
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the sampling frequency (I have always used 1 Khz but this could be higher or lower)
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the bandwidth (I have used 2-400 Hz; but 1-40 Hz is probably fine for smooth muscles).
Because I had a 2-400 Hz, I had to get rid of the 50 Hz noises, which I did with a (primitive) 20-point moving average. Others may wish to implement much more sophisticated band filters.
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