Potential, or potential difference, occurs when there is a difference in electrical charge between two points. This difference in charge is usually due to a concentration of oppositely charged ions at each point. Action potential occurs when there is a sudden and sharp change in the potential difference across the membrane of a nerve cell that is propagated along the length of the cell.
When a nerve impulse is not being transmitted, the inside of the nerve cell has a negative charge and the outside a positive one. It is said to be in its resting state, so the potential difference at this time is the resting potential. The difference in charges is due to amounts of ions being found inside and surrounding the cell. In the case of nerve cells, the potential difference is due to sodium and potassium ions.
All nerve impulses are ionic in nature. When the nerve cell is at rest, there are different concentrations of the potassium and sodium ions on either side of the membrane. This difference is maintained by sodium-potassium pumps in the membrane. This pump pumps sodium ions out of the cell and potassium ions in.
Potassium and sodium ions diffuse across the membrane due to the difference in concentration on either side. Potassium ions can easily diffuse out of the cell, but the membrane is relatively impermeable to sodium ions diffusing in. The overall result is that the inside of the nerve cell has a negative charge relative to the outside of the cell.
When the nerve cell is stimulated and an impulse is initiated, the situation is momentarily reversed. The inside of the cell becomes positive and the outside negative. This sudden reversal of the resting potential that accompanies the impulse is the action potential. An action potential is extremely short-lived, so an impulse is actually a wave of depolarization, or action potentials, that passes along the cell.
During an impulse, the cell membrane becomes permeable to sodium ions. The sodium ions have a very high concentration outside the membrane, so they quickly diffuse into the cell. This takes place very rapidly and reverses the resting potential. With so many positive ions now found within the cell, the inside has a positive charge relative to the outside.
Sodium ions are able to enter the cell through ion channels. When the cell is resting, the ion channels remain closed and keep the sodium ions from entering the cell. When they are stimulated by an impulse, they open up and allow the inrush of the sodium ions. In this way, action potentials and impulses are self-propagating. The action potential in one area of the membrane stimulates the following area causing the ion channels to open. This in turn begins an action potential, which then stimulates the following area and so on.
As the sodium ions enter the cell, the potassium ions leave. This is the beginning of the recovery process where the inside of the cell begins to regain its negative charge. After the action potential has passed and moved along the cell membrane, the ion channels close and the membrane becomes impermeable to sodium ions. The sodium-potassium pump once again pumps the sodium ions out and potassium ions in, resulting in the resting potential being restored.