EXCITABLE TISSUES: Resting Membrane Potential

The resting membrane potential is the membrane potential that you observe in any excitable cell that is simply sitting around, minding its own business and not being influenced in any way by other cells (i.e. it is at rest).

We can demonstrate the resting membrane potential by taking a neurone from the nervous system and maintaining it in culture, where it receives all the gases and nutrients it requires to keep it alive, but is not influenced by other cells.

If we take a voltmeter with very fine leads (known as microelectrodes) and begin with them both outside the cell then of course we would record a potential difference of 0 mV (see opposite).

However if we carefully place one of them into the cell then we record the potential difference between the inside and outside of the cell, we find that the inside of the cell is negative compared to the outside of the cell.

The magnitude of this membrane potential varies from cell to cell but is usually around about -80 mV.

So because we are measuring the membrane potential of a cell at rest, it is known as the resting membrane potential (RMP).

Plot of potential difference against time

Although there is nothing very complex about the RMP it is important because the other membrane potentials that we will be talking about originate from it. The RMP also forms the basis for some of the terminology used to describe changes in membrane potential.
Let us have a look at some of this terminology now.

Because the inside of an excitable cell is negatively charged it is clearly different from the outside, so the cell is said to be polarised (i.e. the inside is different from the outside). This term polarised is used as the root for a number of terms to describe how the membrane potential changes relative to the RMP:

When the membrane potential decreases in magnitude from the RMP (i.e. the inside of the cell becomes less negative) then the membrane becomes less polarised and we refer to this type of movement as depolarisation.

If the membrane potential increases in magnitude back towards the RMP (i.e. the inside of the cell becomes more negative) then we refer to this type of movement as repolarisation.

When the membrane potential increases in magnitude from the RMP (i.e. the inside of the cell becomes more negative) then the membrane becomes more polarised and we refer to this type of movement as hyperpolarisation.

Image illustraing how changes in membrane potential can be described

These terms are important because they enable us to describe how membrane potentials change and form the basis for much of what will follow. Make sure you understand exactly what these words mean before moving on.