An action potential is a rapid and transient change in the membrane potential of a cell, particularly in excitable cells like neurons and muscle cells. This phenomenon is a key mechanism for transmitting signals within the nervous system and initiating muscle contractions. The process of an action potential involves a series of electrical events that occur along the cell membrane.
Here is a detailed description of the action potential process:
1. Resting Membrane Potential
– Neurons have a resting membrane potential, typically around -70 millivolts (mV), maintained by the unequal distribution of ions across the cell membrane.
– At rest, there is a higher concentration of sodium ions (Na+) outside the cell and a higher concentration of potassium ions (K+) inside the cell.
2. Depolarization
– The action potential begins with a stimulus that causes the membrane to become more permeable to sodium ions.
– When the cell reaches a threshold stimulus, voltage-gated sodium channels open, permitting the rush of sodium ions into the cell.
– This influx of positive ions causes depolarization, and the membrane potential becomes less negative.
3. Rising Phase
– As more sodium channels open, the positive feedback loop accelerates the depolarization, rapidly increasing membrane potential.
– This phase is known as the rising phase of the action potential.
4. Overshoot
– The membrane potential can briefly exceed 0 mV, reaching a positive value. This is known as the overshoot.
5. Repolarization
– After reaching its peak, voltage-gated sodium channels close, and voltage-gated potassium channels open.
– Potassium ions exit the cell, causing repolarization and a return to a negative membrane potential.
6. Falling Phase
– The falling phase corresponds to the repolarization process, where the membrane potential decreases.
7. Undershoot/Hyperpolarization
– The repolarization may temporarily overshoot, leading to a hyperpolarization or undershoot of the membrane potential.
Key Points
– The action potential is an all-or-nothing event. Once the threshold is reached, it will propagate along the entire axon length without a decrease in strength.
The refractory period, ensuring the unidirectional propagation of the action potential by preventing the initiation of a second action potential, is in effect.
