Modifica di Electromyography (sezione)

===Electrodes===
Everyone knows that the recording of biological electrical signals starts with electrodes, but very few realize the real "necessity" of these. Electrodes seem like something inherent in the recording process, and no one really questions their role.

In reality, the problem is quite simple. The electronic circuits for amplifying and recording sEMG signals are essentially made of electrical wires. These wires are obviously metallic (copper), and common electrical charges of a single type flow through them: electrons. Surely everyone knows that electrons flow in electrical wires. However, few people question whether electrons can also flow in the human body. Certainly, cellular potentials, which are the basis for the potential differences detectable on the skin, cause electric currents, i.e., flows of electric charges. But these charges in the body's tissues cannot be electrons. In fact, it is difficult to find free-moving electrons in the human body, as happens in the metallic lattice of a wire. In our body, we have other carriers of electric charge, which are ions. Ions are "pieces" of molecules with a net electric charge different from zero. They are very different from electrons: they can weigh tens or hundreds of thousands of times more and may have multiple charges compared to an electron and even of opposite sign. Unfortunately, they can only flow in an aqueous environment and certainly not in a wire due to their size. So, the situation is as follows: we have an electric current in the metallic wires of the amplifier, just as we have an electric current in the body's tissues, where the carriers are ions. How can we ensure that the electric charge flows in such a "mixed" circuit? How can we ensure that the carriers exchange electric charge? This is precisely the important role of the electrode. Here, a chemical reaction exchanges electric charges between electrons and ions. The only chemical reaction that does this is the one known as redox (oxidation-reduction). Thus, the purpose of the electrodes is to provide a site for a redox reaction that "closes the circuit" and allows electric charges to flow continuously from the body's tissues to the amplifier and vice versa, thus enabling the biopotentials on the skin to be detected and amplified. It all works as if the electric charge travels on one type of transport (electrons) in one environment and another type of transport (ions) in a different environment. We need a sort of "interchange" where the electric charge can be transferred from one medium to another.

This is why electrodes are so important and not just simple and trivial pieces of wire to connect to the skin.

If no one had yet invented an electrode, one could think of making it as follows. It would seem appropriate to make it in two parts: a metallic part to connect to the wire going to the amplifier and a saline part, attached to the former, capable of participating in the redox reaction. Furthermore, it would be important that the electrode's resistance is as low as possible to avoid excessive voltage drop at the electrode, which would result in a smaller value being measured on the trace. Therefore, a low-resistivity metal (and dermatologically suitable) such as silver should be chosen (not gold, as it is too expensive). For the saline part, a silver salt would obviously be chosen. Which one? Since the electrode is placed on the skin, which is in direct communication with the extracellular fluids of the tissues rich in chloride, silver chloride would be chosen. So the electrode would be made as follows: a small metallic silver plate covered with a layer of silver chloride in the area that comes into contact with the skin. To conclude, a sponge soaked in a silver chloride solution in water could be used to ensure the appropriate mobility of ions. It would be wise to keep the entire setup protected from light since light decomposes silver salts, as you might recall from film photography, which has now disappeared. And so we have "invented" a nice electrode. But how does it work?

The redox reaction that occurs between the electrode and the skin is the following:

<math>AgCl + e^-\Leftrightarrow Ag + Cl^-</math>

and everything seems to work well. In particular, since the reaction is reversible, there is the possibility of current flowing in both directions with the same redox reaction. The electrode is said to be reversible. But what happens if the current flows in only one direction, as in long-duration electromyographic measurements? In this case, the electrode could "wear out," meaning that the chloride layer could dissolve entirely, and the metallic silver would come into direct contact with the skin. Thus, the electrode is said to be consumable. A silver/silver-chloride electrode is both reversible and consumable. The depletion of the electrode is not a positive outcome. To make a measurement with the amplifier, at least two electrodes are needed. Each of them will probably "see" a different concentration of chloride ions in the area where it is placed. This will cause each electrode to generate its own half-cell potential (Nernst equation) different from the other. This potential is also known as the liquid junction potential. Since the two potentials are different, they will not cancel each other out, and thus the measured value will be the muscle potential added to the difference in the half-cell potentials of the electrodes. The muscle electrical potential has values well below a millivolt, while the liquid junction potential has values on the order of volts. This fact makes the measurement somewhat complicated, but it is still possible to manage this phenomenon and obtain good recordings. At least until the electrode is in good condition! Once the chloride is completely depleted, the half-cell potential becomes unpredictable and erratic, depending on other ions present in the area, as well as impurities in the silver. It will be very difficult for the electromyographic amplifier to compensate and overcome this effect. At this point, it is said that the electrode has become polarized and can be discarded without regret.

<gallery mode="slideshow">
File:SnapShot 241014 165659.jpg|'''Figure 1:''' Schematic diagram of the transformation of an ion current (negatively charged) into an electron current (negatively charged) through the exploitation of the redox reaction made possible by the presence of the electrode. 
</gallery>

It would be nice, then, to invent an inexhaustible electrode. One could be made with a plate of metallic platinum. Platinum catalyzes the electrolysis of water (we are obviously in an aqueous environment), and we have the following reaction:

<math>2e^-+2H_2O\rightarrow 2OH^- + H_2</math>

However, this time it is a non-reversible reaction, so if the current direction is reversed, a different reaction occurs:

<math>2H_2O\rightarrow 4H^++O_2+4e^-</math>

Thus, we have an inexhaustible electrode (platinum catalyzes the reaction but does not chemically participate in it, so it does not wear out), but it is irreversible. The production of gas (gaseous hydrogen or gaseous oxygen) during the electrolysis reaction is quite inconvenient because the gas tends to insulate the electrode from the skin, making this type of electrode not particularly useful.

Although there are at least two or three other types of electrodes for electromyography, the Ag/AgCl electrode is the most commonly used and is now sold for just a few dozen cents each.

Historically, an interesting electrode is worth mentioning: the "spray-on" electrode, developed by NASA for monitoring the electrocardiograms of the first astronauts. The spray-on electrode was made by spraying colloidal graphite (carbon powder) onto the skin, effectively painting it. The conductive graphite created an intimate contact with the skin, and a normal metal wire could simply be placed on the "black patch." Today, the spray-on electrode is almost no longer used.