A neurone is ‘resting’ when it’s not sending out a signal. However, it’s far from inactive. The membranes of all animal cells contain sodium-potassium pumps which are a form of transport protein. They use energy produced by splitting ATP to pump out three sodium ions and take in two potassium ions at the same time.
First of all, three sodium ions bind to the transport protein. When ATP splits the phosphate group attaches itself to the protein making it change shape and release the sodium ions outside of the cell. As this happened two potassium ions bind to the transport protein. The phosphate is then released which causes the protein to change back to its original shape, releasing the potassium ions inside of the cell.
Potassium and sodium are both positively charged ions. Due to the fact that there’s such a large build-up of positive charge outside the cell, potassium ions don’t move down their concentration gradient.
Sodium and potassium channels ensure that there are always ions to pump from inside and outside the cell. Even though they’re normally shut, they still allow ions to ‘leak’ in and out of the cells, down their concentration gradients.
The following table shows the concentration of potassium, sodium and chloride in a resting neurone.
|
Ion |
Concentration inside the cell (mmol/dm-3) |
Concentration outside the cell (mmol/dm-3) |
|
K+ |
150.0 |
2.5 |
|
Na+ |
15.0 |
145.0 |
|
Cl– |
9.0 |
101.0 |
Despite the difference in positive and negative charges inside and outside the cell, negatively charged chloride ions don’t move into the cell because they’re repelled by the cell membrane.
The sodium-potassium pump and the leaking channels maintain an imbalance of sodium and potassium ions. The potential difference caused by this imbalance is known as the resting membrane potential. This potential is always negative and can range from -20 to -200 mV (millivolts). In humans it’s -70mV.