EXCITABLE TISSUES: Ionic Basis of the RMP

In an earlier part of this lesson we saw that excitable tissue cells have a resting membrane potential of around about -80 mV. In this section we will consider the physiology that is responsible for generating this potential. Because we have already dealt with all the important concepts in the membrane potentials lesson, understanding the resting membrane potential should be fairly straightforward.

At rest, the membrane of an excitable tissues cell is freely permeable to K⁺ because of the presence of resting channels that are selectively permeable to K⁺. These channels are not gated and therefore are open all the time. Armed with this vital piece of information we should know be in a position to see why the resting membrane potential is negative.

As is shown in the schematic diagram on the right, if we have a high concentration of K⁺ inside the cell, a low concentration outside the cell and the membrane is freely permeable to K⁺ then we would expect that K⁺ will flow out of the cell down its concentration gradient (red arrow).

As a result of the K⁺ ions leaving the cell, there is a net flow of positive charge out of the cell and so the inside of the cell (and therefore the membrane potential) will become negative.

As more and more K⁺ ions leave the cell the inside of the cell become more and more negative.

As the inside of the cell becomes more and more negatively charged an electrical gradient (purple arrow) forms that attracts the positively charged K⁺ ions back into the cell.

Eventually a point of equilibrium is reached where the concentration gradient is exactly balanced by the electrical gradient. Therefore no net movement of ions occurs and consequently the membrane potential stabilises. Note that the ion concentrations don't become equal but that the electrical gradient is sufficiently large to effectively balance the existing concentration gradient.

So the negative resting membrane potential is simply a consequence of fact that (at rest) the membrane is permeable to K⁺ ions and the concentration of K⁺ is much higher inside the cell than outside.

Animation showing how the balance between the concenration and electrical gradients is responsible for the resting membrane potential.

Image showing how leakage of sodium into the cell modifies the resting memberane potential.

From your consideration of the equilibrium potentials in the previous lesson on membrane potentials you will recall that if we know that the membrane is selectively permeable to one ion species and we know the intracellular and extracellular concentrations of that ion then we should be able to calculate the membrane potential at which equilibrium is reached using the Nernst equation.

When we do this for K⁺ using the standard intracellular and extracellular ion concentrations (see table in the previous module of this lesson) we obtain an equilibrium potential of -96.9 mV. Whilst some neuroglia have a resting membrane potential that is exactly the same as the equilibrium potential for K⁺, most excitable tissues cells have a resting membrane potential around -80 mV. This is slightly less negative than the equilibrium potential for K⁺. How do we explain the 16 mV difference?

The answer is that we do not live in a perfect world (but you won't be surprised to hear that) and at rest the membrane is not only permeable to K⁺ but is slightly leaky with small numbers of Na⁺ ions also able to move across the membrane. This means that in the real world the membrane is freely permeable to K⁺ and very slightly permeable to Na⁺ at rest.

Now you will recall that the concentration gradient for Na⁺ favours its movement into the cell (see table in previous module). So in addition to the K⁺ leaving the cell (making the inside negative) we have a small amount of Na⁺ entering the cell and making it less negative (thin green arrow in diagram opposite).

So the net effect of the slightly leaky membrane is that the resting membrane potential is slightly less negative than the equilibrium potential for K⁺ because of the contamination by Na⁺.

Another way of looking at this is to think of the resting membrane potential as trying to reach the equilibrium potential for K⁺ but being prevented from doing so by Na⁺ leaking into the cell.

So in summary, there is nothing particularly complex about the resting membrane potential. It is simply the direct consequence of the fact the there is a concentration gradient for K⁺ and that the membrane is almost exclusively permeable to K⁺ in cells at rest.