I am confused as to what positive/negative currents are when it comes to neurons. I have found 2 conflicting definitions
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Which is correct?
And also, when it comes to Driving force (DF=Vm-Ex) what does a + or - DF mean in regards to current.
Thanks
These definitions are not conflicting, though certainly poorly worded. As someone previously mentioned, "positive" and "negative" current are defined by convention.
Positive current: (1) Positive charges flow out of the cell or (2) negative charges flow into the cell.
Negative current: (1) Positive charges flow into the cell or (2) negative charges flow out of the cell.
This may be easier to understand when looking at a graph of ion current during an action potential (
http://www.felixmay.com/nit/images/na-k_currents.gif). Note that the bottom line on the graph is sodium current and the top line is potassium current.
1. At the beginning of the action potential, you know that sodium rushes into the cell. So by convention: when the positive sodium ions flow into the cells, we call this negative current. This is shown on the graph by the sodium current line moving more negative with time (up to a point).
2. In contrast, potassium ions flow out of the cell during the action potential. Again, convention tells us that the positive ions flowing out of the cell is called positive current. This is shown on the graph by the potassium current line moving more positive with time.
And with regard to driving force, there are 4 key things that help make sense of the relationship between driving force and current:
1. Driving force = (membrane potential, Vm) - (ion (x) equilibrium potential, Ex) = the voltage difference across the membrane (V) ....in other words, driving force is the voltage difference across the membrane.
2. Ohm's Law from physics (V=IR), can be modified to apply to cells in the form I=Vg, where g = conductance and I = ion current.
3. You can then use I=Vg to understand the relationship between driving force (V) and ion current (I).
4. And g is constant.
Main idea: driving force is proportional to current! (Vm - Ex) α I
So for example, if you look at sodium:
When the cell is sitting at a normal resting potential (Vm ~ -65 mv) and the Na ion channels fly open there is a driving force on the ions that will be proportional to the sodium ion current. Remembering that the equilibrium potential for sodium is +55mv, then the sodium current (I) is proportional to driving force = Vm - Ex = (-65 - 55) = -120 (read: large negative current).
And this is what we see in the action potential! When the cell is sitting at normal RMP and the sodium ion channels fly open, there is a large driving force "pulling" positive sodium ions into the cell and we see a really large, negative current.
Hope that helps! Happy to explain anything that wasn't clear.
