Which will be the path of electric current in a water container?

Discussion in 'Homework Help' started by Felipe, Nov 16, 2010.

  1. Felipe

    Thread Starter New Member

    Nov 16, 2010
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    I work in a laboratory of neurophysiology.
    The experiments goes like this: We want to stimulate neurons electrically, for this we have them in buffer (water + salts) and we have 2 electrodes submerged into the buffer. We applied a train of current (square pulses of 2 ms duration 20 Hz for 30 seconds, with a 100 mA of amplitude). The question is as follows: The current will spread through the whole buffer (≈ 100 µl) afecting all neurons in contact with buffer? or the current will move only between the electrodes and neurons behind the electrodes (not in between) will not "sense" the electric shock?

    Thanks!
     
  2. beenthere

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    Water, as you know, is not a conductor. The current between the electrodes will depend on ions in the water from the various salts. Like charges repel, so the current path tends to spread out from what might look like a direct path.

    But it's all guesswork, as the arrangement of the electrodes, the shape of the container, and the nature of the current is unknown.

    One thing that will cause unwanted effects is using DC.
     
  3. jpanhalt

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    Jan 18, 2008
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    A dimensioned drawing will help. Why do you ask? Are you seeing different behavior from the neurons depending on distance for the electrodes? Can you think of a control experiment to answer your question?

    John
     
  4. LVTech

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    Nov 16, 2010
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    The answer is #2. The static current flow will be highest between the electrodes. You can get a lot more information concerning this if you look up electroplating where the intent is to get the most even flow over the whole object in question.
    Also: just because an object is within the buffer does not mean it will be submitted to the electrical flow...if the object itself has a higher resistance to the current flow than the buffer surrounding it, it may not be affected at all.
    Less is more when adding you salts (ions) to the soup.
     
  5. jpanhalt

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    What does that mean?

    John
     
  6. beenthere

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    Not quite:
    The buffer solution has to be correct, or cell death results.

    The intent is stimulation, not plating. As the process continues, pores in the cells will open and may do things like pump calcium ions in an attempt to balance the surrounding voltage field.
     
  7. jpanhalt

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    DELETED

    John

    Edit: The original comment here was in reference to a post that appears now to have been deleted by a moderator. Without that context, there is no reason to retain it.
     
    Last edited: Nov 17, 2010
  8. Felipe

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    Nov 16, 2010
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    The neurons are siting on a round glas coverslip of 18 mm of diameter and 2 platinum wires run in paralell on top of it. Wires and cells are covered with 100 microlitres of buffer. And now maybe you can imaging that the parallel wires on top of a round surface will not cover it all!, which means that some neurons will be behind the electrodes. And indead, our preliminary data suggest that the neurons behind the electrodes react different thatn the ones that are in between the electrodes...

    Thanks for interest!
     
  9. Kermit2

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    to get a portion of that current to 'flow' in the neuron will require more than immersion in a conductive soup. The cells MUST have a resistance inside that is in close proportion to the resistance of the 'soup'. Otherwise the electrons will not flow through the cell, but around it.

    The electric field probably will have more effect on the neurons than the electron current flow in the soup, up until a certain amperage is reached, then the effect will be life threatening for the cell.

    You need to arrange for the cells to 'be' a low resistance path to your electric current source, not just floating in the low resistance path.
     
  10. jpanhalt

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    Are you studying evoked potentials in nerves or just passive electrical conduction? Based on that statement, I assumed you are studying evoked potentials in live nerves.

    Some of the posters here seem to assume you are studying passive electrical conduction. In fact, the word picture you gave for the test apparatus makes me wonder whether that is what you are doing too.

    Again, it is very difficult to understand what you are trying to do and why. Why be so vague? Your question is, after all, posted in the homework section.

    John
     
  11. beenthere

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    These cuvettes for electroporation are not cheap, but they will be much more likely to give a better voltage and current distribution in your sample - http://www.usascientific.com/electroporation-cuvettes.aspx

    Note that you can use wire and foil to make a reasonable facsimile. Sterility is probably not a requirement.
     
  12. Felipe

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    Nov 16, 2010
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    Thank again John,
    Yes it is evoke potential (by fiel stimulation, not voltage or current clamp), as you seems to know, neurons have channels that detect a change in voltage and they will make the neuron to fire and action potential. However, I do not think it is needed to explain you the whole scope of the investigation I am doing. I am loking for the answer to a rather simple question: Will the neurons that are not in between the electrodes "feel" the electric shock!

    Felipe
     
  13. Felipe

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    Nov 16, 2010
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    My sample are mature rat neurons, I can not just put them in suspension in the electroporation cuvetes.... Additionally, you will need an extra device to operate them (even more expensive) and third, this cuvetes will fried the neurons... these guys are made to make wholes on cells (and bacterias).

    Thanks a lot for taking the time!

    Felipe
     
  14. jpanhalt

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    It is still unclear to me what you mean by "shock" the neurons.

    Here is a discussion from All About Circuits e-book about electrical shock: http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

    However, in your experiment, I suspect the charge carriers are your buffer, not electrons as occurs in metals. For a discussion of the difference, see:
    If a neuron were touching both electrodes, then shocking like described in the AAC book might apply. However, your description indicates that is not the case, so let's consider the charge carriers are ions that move in the field between the electrodes. For simplicity, assume the neurons are not touching either electrode. The medium is a liquid, so there will be diffusion around each electrode of the charge carriers that move in the field. Thus, a neuron not directly between the electrodes, still would be subject to the same ionic changes as one between the electrodes, but to a lesser extent. If you are trying for electroporation (as mentioned by beenthere), then the lesser might well include insufficient membrane potential changes to reach that endpoint.

    John
     
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