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Neuron Electrical Model, time constant

  1. Oct 23, 2012 #1
    Hi all,
    I'm trying to understand the electrical model of a neuron, and I have 2 questions.

    I'm a little confused about the time constant in this electrical model of neuron recording: Udhvu.png

    The time constant shown is tau = C*(Rm*Rs)/(Rm+Rs), which implies that the resistors are in parallel. However, to me, it looks like the resistors are in series. I feel like I'm missing something here.

    First question: Why is the time constant C*(Rm*Rs)/(Rm+Rs) rather than C*(Rm+Rs)?

    To make matters worse, earlier in the lecture this circuit was shown, implying the resistor represented membrane resistance: (ignore the values)
    TZ69g.png

    There is a current source here.
    In the first picture, the diagram is using a voltage source. The second diagram is the same thing as a series RC circuit with a voltage source, just using a source transformation (I think). I looked up parallel RC circuits (with a voltage source) and found they had very different properties than series RC circuits. Note: The first picture seems to show a voltage clamp, while the second was just a general model. Might or might not change things.

    Second Question: How can I reconcile these two diagrams?

    What's a correct neuron model?

    Thanks.

    Edit: Some of the information I was given could be erroneous; i.e. I don't trust the lecture slides.
     
    Last edited: Oct 23, 2012
  2. jcsd
  3. Oct 26, 2012 #2
    I'm guessing I might have been unclear. So I'll just ask one question:

    What's the time constant of this circuit?

    iZ31Z.png
     
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