Beginning Neurobiology: The issue of teaching voltage, current, and membrane potentials

A common problem I’ve noted while teaching this course across multiple semesters, is to make certain students have sufficient grounding in understanding the relationship between voltage, current, and membrane potentials. Furthermore, this ties in with understanding how equilibrium potential is determined for each ion.

Major issues to address:

  1. What is voltage, what is a potential difference?
  2. What is current, and what is positive vs. negative current?
  3. What is the relationship between ion flow and current flow?
  4. What is equilibrium potential?
  5. What is the relationship between membrane potential and equilibrium potential?

How to solve:

  1. What is voltage, what is a potential difference?
    • In describing this, make sure they understand that it is between 2 points. You require a reference point, and in our case, it is typically outside the cell. What we are concerned with then, in layman’s terms, is whether the inside of the cell is more negative or positive relative to outside the cell.
  2. What is current, and what is positive vs. negative current?
    • In describing current… be sure they understand that when we talk about current, we are talking about the flow of a (+) ion either into or out of a cell. Then, let them know, that, per convention, we have determined that when + ions flow out of a cell, this is positive current. Conversely, when you have + ions flowing into a cell, this is negative current.
  3. What is the relationship between ion flow and current flow?
    • This is where you make the distinction between + vs. – ions. Clarify the point that if you are talking about a + ion, its movement is in sync with current direction. However, if you are talking about a – ion, its direction is opposite that of current flow. Remember, current talks about the flow of a + ion.
  4. What is equilibrium potential?
    • To make them understand equilibrium potential, I like to ask them about the 2 forces which act on an ion, say Na+. They are usually able to come up with diffusive and electrical force, which make up the electrochemical gradient. Students understand the idea of diffusive force, but get confused on how electrical force pushes back against the diffusive force in some cases. A nice way to visualize this for them, is to draw 2 Na+(An-) on the inside of a cell, and ~10 Na+(An-) ions on the right side. Then, say, the membrane is perfectly permeable to ONLY Na+. Now in this case, I move a couple of Na+ ions over, maybe 3 or so, at which point we have 2 Na+(An-) and 3 Na+ ions on the left, and 7 Na+(An-) & 3 An- on the right. I add up the valences, which adds up to +3 on the inside of the cell, and -3 on the outside of the cell. This helps them visualize that there becomes this potential difference! Then, also show that you still have that concentration gradient, and thus a diffusive force acting to push the ions into the cell, but that this is counterbalanced by the positivity of charge on the inside. (see images below for reference)
  5. What is the relationship between membrane potential and equilibrium potential?
    • This brings all of the concepts together. As long as they understand that equilibrium potential determines membrane potential… and furthermore, that the equilibrium potential that most influences membrane potential depends on the permeability/conductance of the membrane. So, if the membrane is most conducive to K+, Ek (equilibrium potential for potassium will be the primary determinant of the membrane potential). Alternatively, if membrane is most conducive to Na+, membrane potential will be near ENa. If its a mix between them, then it depends on the relative conductances between them!
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