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.