The action potential curve is a graphical representation of the electrical activity that occurs in a nerve cell when it is stimulated. This curve typically consists of several distinct phases: resting potential, depolarization, repolarization, and hyperpolarization.
1. **Resting Potential**: At the start of the curve, the nerve cell is at rest with a stable membrane potential of about -70 mV. During this phase, the neuron is not transmitting signals, and the inside of the cell is negative relative to the outside due to the distribution of ions across the membrane.
2. **Depolarization**: When a stimulus occurs, sodium channels open, allowing Na+ ions to rush into the cell. This influx of positive ions causes the membrane potential to become less negative, eventually reaching a threshold (typically around -55 mV). Once the threshold is crossed, more sodium channels open, leading to a rapid rise in membrane potential (up to +30 mV). This phase is characterized by a steep upward slope on the action potential curve.
3. **Repolarization**: After the peak of the action potential, sodium channels close and potassium channels open, allowing K+ ions to flow out of the cell. This outflow of positive ions causes the membrane potential to become more negative again, descending back toward the resting membrane potential. This is shown as a sharp downward slope on the curve.
4. **Hyperpolarization**: The membrane potential can briefly become more negative than the resting potential due to the continued outflow of K+ ions. At this point, the neuron is in a state of hyperpolarization, which makes it less likely to fire another action potential immediately. This phase is often represented as a dip below the resting potential on the graph before returning to -70 mV.
In summary, the action potential curve effectively illustrates the rapid changes in voltage across the neuronal membrane that facilitates communication within the nervous system.