## resistance

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biology_06er
Coral
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Joined: Tue Mar 21, 2006 9:33 am

### resistance

Hi there,

just wondering when you measure the RMP in a cell with via impailment (thats a word right?) why does the resistance have to be about 7Mohms..it says in my notes that if its less then 3Mohms then the micropipette will be too large to penetrate the cell wall soo just wondering what this exactly means? By resistance is it meaning the diameter of the pipette thus if resistance to to big (i.e. easier for things to get through like dust/debri) or just as it state the fact that if resistance is to high it cant penetrate the wall...hope i am making sense...

help would be greaaatly appreciated..(also if someone feels the need to help me again can they answer my other post-done about 2 days ago (General Discussion topic) can they answer that as well)...its about RMP and current direction flow...

Cheers,
b_06er

blcr11
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### Re: resistance

You really need to have this answered by an electorphysiologist, probably not me, but you're talking about electrical, not mechanical resistance. For a given applied voltage (here millivoltage I would guess) and resistance, you will have a certain amount of current flowing. In this case—and here I am really just guessing—that the larger the “hole” represented by the pipet, the greater will be the conductance of the path through it. That is, I expect the resistance of the pipet to be lower if the apature is larger, but if you pull it too far and make the apature too small, you get a higher resistance (lower conductance) than you want. I’m not sure, but I think the measurement goes someting like a null potentiometer; you apply a voltage across the membrane that opposes the normal voltage gradient until the current becomes zero. The potential at that point has to be equal to the potential of the cell membrane, but in the opposite direction. You have to know the resistance of your pipet in order to know what the actual potential is, and that resistance varies with the way you pull your electrode/pipet. I don’t know what I’m talking about here, so you should take my explanation with a pretty large grain so salt, but at least it makes physical sense to me.

mith
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oh you're right blcr, the resistivity of the pipe is inversely proportional to cross-sectional area(diameter) and a large diameter indicates lower resistivity so a resistivity less than 3 represents too large of a pipe. it is also proportional to the length of the pipe so a longer pipe has higher resistance but I'm assuming you have that normalized somehow.
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blcr11
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### Re: resistance

As I understand it, it's an art to pull your own electrodes--and get them to work, anyway. I think there are machines now that can make them somewhat automatically.

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