Membrane Potential

The membrane potential is the separation of opposite charges across the plasma membrane (i.e. a difference of the relative number of cations and anions in the ICF and ECF).

It should be noted that the majority of charged ions in the ECF and ICF are not involved in setting up the membrane potential.

IMPORTANT: the measure of membrane potential (mV) is NOT the ratio of positive:negative charges, but the number of charges involved.  I.e. the more charges separated, the greater the potential.

Ions and the resting membrane potential

The main ions responsible for setting up the resting membrane potential are Na(+), K(+) and large, intracellular protein (-).

From the above table, we know that the flow of sodium will be towards the cell, while the flow of potassium will be away from the cell (by concentration gradient).  We can also tell that it will be a lot easier for potassium to move out of the cell than sodium (25-30 times), and that large IC proteins will remain in the cell.

So what effect do these ions actually have?

The Na/K Pump

Transports 3 Na(+) out of the cell for every 2 K(+) into the cell.  Because both ions have equal charge- this generates a membrane potential- although this is very small (1-3mV) and insignificant compared to the mechanisms described below.  What it does, however, do is set up the concentration gradient needed to drive the below mechanisms.

Other ion movement

If we look at sodium and potassium separately:

  • Potassium (and IC proteins)
    1. The concentration gradient will tend to move the ion out of the cell
    2. The ECF becomes more positive as K(+) moves down its concentration gradient
    3. Because the membrane is impermeable to IC proteins (-), the ICF becomes more negative
    4. This tends to move the ion back into the cell
    5. The process equilibrates, at which point the resting equilibrium potention is -90mV (the +/- denotes the polarity of excess charge on the inside of the membrane i.e. -90 means magnitude of 90 with the inside being negative relative to the outside).

The membrane potential is calculated using the Nernst equation:

 where z is the ion’s valency (no. of excess charge i.e. 1 for Na/K).

  • Sodium (and extracellular chloride ions)- a similar process would occur, resulting in an E(Na)= 61mV.

However, when you look at them together, the greater the permeability of the plasma membrane for a given ion, the greater the tendency for that ion to drive the membrane potential to its equilibrium potential.

  1. The Na/K pump actively transports Na out of the cell and K into the cell, maintaining the concentration of Na in the ECF high and K low (and vice versa).
  2. Given this gradient, movement of the ions down this gradient by diffusion occurs.  However, the plasma membrane is more permeable to K than to Na, so the membrane potential that is eventually produced is closer to the equilibrium potential of K.
  3. Due to the impermeable negative large IC proteins, the inside of the cell is always negative compared to the outside.

The resting membrane potential can then be calculated using the GHK equation:

 where P is the permeability of the ion.

  • This produces the final resting membrane potential of -69mV.

A note about Chloride

The equilibrium potential of Chloride is -70mV (i.e. the same as the resting membrane potential) and there is no active transport (only diffusion) of this ion across the plasma membrane.  Therefore, it does not play an active role in maintaining the resting membrane potential.  It does, however, play a role in depolarisation/hyperpolarisation.

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