Potential difference occurs due to a difference in electric charge between two points, and an action potential occurs when there is a sudden change in potential difference in a nerve cell. Sodium and potassium ions maintain the potential difference, and during an impulse, sodium ions enter the cell through ion channels, causing a reversal of the resting potential. Recovery occurs when the ion channels close and the sodium-potassium pump restores the resting potential.
Potential, or potential difference, occurs when there is a difference in electric charge between two points. This charge difference is usually due to an oppositely charged ion concentration at each point. An action potential occurs when there is a sudden, sharp change in the potential difference across the membrane of a nerve cell that propagates along the length of the cell.
When a nerve impulse is not transmitted, the inside of the nerve cell has a negative charge and the outside a positive charge. It is said to be in its resting state, so the potential difference at this moment is the resting potential. The difference in charge is due to the amount of ions that are in and around 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 potassium and sodium ions on both sides 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 both sides. Potassium ions can diffuse out of the cell easily, 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 a pulse is actually a wave of depolarization, or action potentials, that passes through the cell.
During a pulse, the cell membrane becomes permeable to sodium ions. Sodium ions have a very high concentration outside the membrane, so they diffuse rapidly into the cell. This happens very quickly and reverses the resting potential. With so many positive ions now found inside 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 at rest, the ion channels remain closed and prevent sodium ions from entering the cell. When stimulated by a pulse, they open and allow the influx of sodium ions. In this way, action potentials and impulses are self-propagating. The action potential in one area of the membrane stimulates the next area causing the ion channels to open. This in turn initiates an action potential, which then stimulates the next area and so on.
When sodium ions enter the cell, potassium ions leave. This is the beginning of the recovery process where the cell’s interior begins to recover 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 the potassium ions in, resulting in the restoration of the resting potential.
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