1. All cells in the body show some degree of electrical behaviour. They show this by having an electrical potential difference between the inside of a cell and outside.
This difference in potential is called the ‘resting potential’. This potential difference is not very big, ranging from -20mV to -90 mV but, as we shall see later, very important!
(mV = millivolt; one thousand of 1 Volt).
For example, the red blood cell (= erythrocyte) has a relatively ‘small’ resting potential; -20 mV while the muscle cell has a much higher resting potential: -90 mV!
By the way, we say ‘-20mV’ because the inside is negative compared to the outside, extracellular, space.
Some cells however, are more excitable than others; these are cells such as the nerve cells and the muscle cells. These cells are capable, when stimulated, of producing an ‘action potential’.
In this and the next page, we will discuss how the resting potential and the action potential are created.
To create a resting potential, the following four components are required:
a) a cell membrane
b) sodium-potassium pumps located in the cell membrane
c) potassium channels
d) potassium ions
The sodium-potassium pumps, continuously, potassium ions into the cell and sodium ions out of the cell. Therefore, the concentration of potassium will become higher inside the cell than outside the cell. This is called the ‘concentration gradient’ for potassium ions.
Then, because the potassium channels, in the resting membrane, are open, the potassium ions will flow (=diffuse) out of the cell (from high concentration inside to low concentration outside).
Every time potassium ions flow out of the cell, small positive charges are also taken out of the cell, leaving behind negative potentials.
This makes the inside of the cell gradually negative (and outside positive). In other words, a potential gradient is being created (= this is the resting potential!).
This potential gradient is opposite to the concentration gradient (since there are many more positive potassium ions inside than outside the cell).
Once the potential gradient is equal (but opposite!) to the concentration gradient, then an equilibrium will have been reached and both gradients remain stable.
The potential reached at this stage is called the ‘equilibrium potential’. Because this relates in this case to potassium, this is called the EK+.
(=Equilibrium potential for potassium)
Students often come up with the following problem; does the concentration gradient not decrease when the potassium ions diffuse outside? The answer is; yes but very little. Charge and concentration are not the same. In fact, very little potassium diffusion is necessary to induce a large potential difference.
There is sometimes confusion about cause and effect! The sequence must be very clear:
a) The pump generates the concentration gradient.
b) The concentration gradient generates
So, this is the correct sequence:
Na-K pump -> concentration gradient for K+ -> electrical gradient (resting potential).
If something would decrease or stop the pump, then of course the concentration gradient, and therefore, the electrical gradient, would decrease and disappear.
As the Na-K pump needs energy (=ATP), anything that affects the energy supply, will affect the resting potential.
D. Some Mind Games to test if you understand the physiology of the resting potential!:
When a cell achieves equilibrium between the concentration gradient and the (opposite) electrical gradient for a particular ion, the cell is then at its ‘equilibrium’ (=’E’).
In the case of potassium ion, of which there many more inside the cell then outside, the equilibrium potential will be negative inside the cell, as discussed above.
For sodium ions, of which many more ions are located outside the cell than inside, the equilibrium potential will be positive! (see Mind Game #D3).
These two equilibrium potentials are very important in order to understand the generation of the action potential (next page).
In this diagram, I have plotted the values of ENa+ (=+35mV) and EK+ (=-90mV).
And, between these two, a shadow of an action potential (to be discussed in the next page!).
Please note that the resting potential is not the same as the Equilibrium potential for potassium. In fact, the resting potential is slightly less negative than the EK+
This is because the cell membrane is not perfect; there are always some sodium channels open and therefore some sodium ions will leak into the cell making the inside less negative than the Equilibrium potential.