A.3.2. The Resting Potential



A. Introduction to Cellular Potentials
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.

2.
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).
an erythrocyte
resting potential in a muscle cell
3.
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!
4.
By the way, we say ‘-20mV’ because the inside is negative compared to the outside, extracellular, space.

5.
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’.
6.
In this and the next page, we will discuss how the resting potential and the action potential are created.



B. Creation of the Resting Potential:

top?

1.
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
resting potential in a muscle cell
2.
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.
resting potential in a muscle cell
3.
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).
resting potential in a muscle cell
4.
Every time potassium ions flow out of the cell, small positive charges are also taken out of the cell, leaving behind negative potentials.
resting potential in a muscle cell
5.
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!).
resting potential in a muscle cell
6.
This potential gradient is opposite to the concentration gradient (since there are many more positive potassium ions inside than outside the cell).
resting potential in a muscle cell
7.
Once the potential gradient is equal (but opposite!) to the concentration gradient, then an equilibrium will have been reached and both gradients remain stable.

8.
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)

C. Some important Notes:

top?

1.
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.
2.
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
3.
So, this is the correct sequence:
Na-K pump -> concentration gradient for K+ -> electrical gradient (resting potential).
4.
If something would decrease or stop the pump, then of course the concentration gradient, and therefore, the electrical gradient, would decrease and disappear.
5.
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!:

top?

1.
As a mind experiment, you could figure out, in this example, what would be the potential gradient if the (potassium) ion were not a positive but a negative ion?
Answer? The resting potential then becomes positive inside the cell
Q1: resting potential
2.
Another (simple) mind experiment: The Na-K pump also pumps sodium ions out of the cell. What then is the concentration gradient for sodium?
Answer? High sodium concentration outside and low inside the cell.
Q2: resting potential
3.
Following on that experiment; if the channels for potassium were closed and the channels for sodium ions were open, what would the sodium ions do and what would happen to the potential gradient?
Answer? Sodium ions will flow into the cell taking positive charges with them; this will make the inside positive (Hint: this is actually how the action potential starts!)
Q3: resting potential

(E. Equilibrium Potentials; F. Nernst Equation)

E. More in depth: Equilibrium Potentials

top?

1.
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’).
2.
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.
3.
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).


equilibriumPotentials
4.
These two equilibrium potentials are very important in order to understand the generation of the action potential (next page).
5.
In this diagram, I have plotted the values of ENa+ (=+35mV) and EK+ (=-90mV).

6.
And, between these two, a shadow of an action potential (to be discussed in the next page!).
7.
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+

8.
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.

F. More in depth: The Nernst Equation:

top?

1.
In fact, if you know the intra- and extracellular concentrations of a particular ion, you can calculate its Equilibrium potential!
Nernst Potentials
2.
This was first developed by Walter Nernst, a German chemist who got a Nobel prize in 1920 for his work!


3.
The R, T and F are physical constants that don’t change in normal physiology.
(R, T and F?) R = ideal gas constant;
T = temperature in Kelvin;
F = Faraday’s constant


4.
So, in the case of potassium ions, and given the usual concentration difference in a muscle cell, the Nernst formula would look like this:
5.
And, in the case of sodium ions, the Nernst potential would look like this:

Nernst Potassium
Nernst Sodium


Page Menu:

Gallery

Page PDF

Page Glossary

MCQ's

(hover to peek!)

A.3.2. The Resting Potential

© BasicPhysiology.com 2017-2019
mail to: info@BasicPhysiology.com