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Medical How Electrodes in salt solution can increase the electric potential

  1. Oct 23, 2012 #1

    I'm not sure if this is the right place for this question, but I am trying to understand how placing electrodes in the brain elicits action potentials.

    In a neuron (brain cell), there exists a solution of sodium and potassium ions (among other things). When the brain wants to produce an action potential (electrical spike), the neuron brings in a lot of sodium ions from the outside to make the neuron more positive. It is this positive depolarization that starts an action potential. To artificially simulate this process, we place electrodes on the neuron. My question is how is an electrode able to increase the potential? From what I understand, electrodes can only supply a current of electrons (negatively charged).

    EDIT: A lot of papers refer to "injecting current in a cell." I don't understand what that means as you would need an acceptor of electorons within the cell (or a donor)

    Thank you
    Last edited: Oct 23, 2012
  2. jcsd
  3. Oct 24, 2012 #2
    An electrode can also be positively charged with respect to some other thing, meaning it would remove electrons from it.

    Regardless, I also don't know exactly how conventional electricity causes neurons to fire, but I surmise it is really only acting on the cell membrane, itself, pushing it over the threshold where the ion channels open and allow the sodium ions into the cell.

    The ion channels are "voltage gated":


    so I surmise this outside source of voltage artificially opens them.
  4. Oct 24, 2012 #3
    Yeah, the only way I can think of right now is that it creates a local depolarization. So if the electrode were charged positive, it would attract many negatively charged ions, thus leaving the surroundings positively polarized.

    But I have a feeling that's wrong. I know that electrodes are commonly placed within the cytoplasm, so it's not just acting on the membrane. I suspect that electrons from the electrode are transferred to positive ions in the cytoplasm, thus leaving a net negative charge. But I don't know.
  5. Oct 24, 2012 #4
    It's not a matter of "acting on the membrane". The membrane is just an insulator (an almost perfect one) between the inside of the cell and the outside, with a bunch of proteins embedded in the surface; it's the potential difference across the membrane that determines the electrical behaviour of the neuron. If you make the inside of the cell positive enough (in the right circumstances), then you'll provoke the opening of the sodium channels, and the electrical gradient will take care of the rest.
  6. Oct 24, 2012 #5
    Yes and how does sticking an electrode within the neuron achieve this. I understand the concept of depolarization, but I am more interested in the particular details leading to the state of depolarization.

    The neuron contains a salt solution of sodium ions, potassium ions, calcium ions, chlorine ions, etc. Placing an electrode into this salt solution should allow for the transfer of electrons from one substance to another. I am interested in this mechanism.
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