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When stimulating a group of neurons with an electrode, let's say we put it in a region that contains axons, what is the mechanism by which the axons are stimulated?

I've been told electrodes (usually silver or gold) must be coated with ions, usually chloride, to allow the passing of the stimulus. So I'm not sure if the stimulus is based on changing the extracellular concentration of ions by injecting chloride, therefore changing the membrane potential with a depolarization (elevating the potential, possibly generating an action potential). Let's call this idea A.

The other idea, B, that seems possible would be having the ions in the extracellular fluid conduct the current to the nearby axons, thus changing the potential but not the intra or extracellular concentration of ions.

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    $\begingroup$ The electrodes just produce an electric field that will change the membrane potential. They need not be coated with anything. Electrodes are not for conducting current $\endgroup$ – WYSIWYG Oct 10 '14 at 13:04
  • $\begingroup$ I get that they change the membrane potential. The question is how. Do they generate an ion flow, or a an electron flow? Obviously, the can and do generate an ion flow if the depolarization of the membrane leads to an opening of Na channels, for example, but that'd be the response mechanism, not the stimulation. $\endgroup$ – facuq Oct 10 '14 at 13:35
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    $\begingroup$ Electric field is created between two electrodes separated by a dielectric (non-conductor). It is a fundamental nature of a charge. When the charge moves it creates current but your charged plate of electrode is static and no current can flow between the electrodes- therefore a potential is generated. $\endgroup$ – WYSIWYG Oct 10 '14 at 13:39
  • $\begingroup$ Great, sorry if I seem a bit thick around basic stuff but i've had no formal training in either physics, chemistry or biology. So there is no current, the electrode just polarizes the extracellular solution? So if the membrane potential is -65mV, by introducing extracellularly a negative electrode, the potential of the membrane is depolarized? $\endgroup$ – facuq Oct 10 '14 at 14:13
  • $\begingroup$ @WYSIWYG I think you should write a longer answer if you have time. :) $\endgroup$ – Memming Oct 12 '14 at 15:06
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Good question. Just to set some stuff straight: In contrast to a comment placed earlier, there is definitely a current flow between electrodes in neural tissue, as long as the impedance is not too high. The potential difference between the electrodes and impedance determines how much current flows, basically following Ohm's law: I = U/R.

As to your hypothesis (A) - I confirm WYSIWYG's answer - electrodes indeed do not have to be coated, as long as they are conductive. For instance, Ag/AgCl coatings are used to electrically stabilize the electrode, mainly for sensitive reference electrodes used for recording purposes (see a commercial link here). For stimulation purposes chloride salts are a big no no, as AgCl or comparable chloride salts will be converted into metal (e.g. silver) + Cl2, obviously a toxic gas.

As to your hypothesis (B) - It is the electrical field that activates neurons (Basser & Roth 2000) by means of inducing current flow. For example, imagine a cathode (negative electrode) placed close to a neuron. Normally in rest, a neuron's membrane is hyperpolarized (more negative ions inside the cell, the extracellular fluid can be considered to be neutral). The negative electrode will now cause the extracellular fluid to be negatively charged. Subsequently, positive current will flow out from the cell close to the electrode and into the electrode. According to Ohm's law, the membrane voltage is thus positive (note Ohm's law: U = IxR, with I being positive and R being always positive, means U is positive). Hence, the membrane is depolarized, as the resting state is a negatively charged state (hyperpolarized). With an anodic electrode neurons can also be depolarized but due to a slightly different mechanism. Basically, the flanking regions of the now negative current flow to the positive electrode become subject to positive current, as it counterbalances the negative flow to the electrode. The chapter of two veterans in electrical stimulation, Abbas and Miller, in (Cochlear implants) is particularly helpful, as is (Basser & Roth 2000). I can't link a pdf to Abbas and Miller, excuse me for that. I bet your university library will have access to it. Hope this helps.

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