Action Potential

Action Potential: An "action potential" is a rapid temporary change in the electrical charge of a cell membrane. This occurs when a stimulus reaches a threshold that results in sodium channels in the cell membrane to open. A rapid influx of Na⁺ ions rushes into the cell and changes the polarity of the cell membrane from positive to negative (depolarization). Note that this "all or none" process has many implications on the function of motor nerves and muscle cells, including that a nerve cell or muscle cannot be partially stimulated to create a smaller response. This is an "all-or-none " process.

• Note: Action potentials are the electrical events that enable communication between nerves and muscles, playing a central role in processes such as nerve transmission and muscle contraction (Excitation-Contraction Coupling ). They are typically 50–100 mV in amplitude.

For more on muscle cells, check out these courses:

• Muscle Cell Structure and Function
• Muscle Fiber Types

Action Potential Process:

1. Resting State: In a resting cell, the inside and outside of the cell membrane have different charges, with the outside being more positive and the inside being more negative. This is called the resting membrane potential.
2. Depolarization: When a stimulus is strong enough to reach the excitation threshold, sodium (Na⁺) channels open, resulting in a rush of Na⁺ ions into the cell. This changes the inside of the cell to be more positively charged. This process is known as depolarization (the inside and outside of the cell become less different or "polarized").
3. Repolarization: After the cell becomes depolarized, potassium (K⁺) channels open, allowing K⁺ ions to flow out of the cell. This causes the cell to return to a more negative state, restoring the resting membrane potential.
4. Propagation: The action potential moves along the cell membrane (e.g., along a nerve axon or the sarcolemma of a muscle cell) as one section of the membrane triggers the next section, creating a wave-like signal.
5. All-or-Nothing Priniciple : Action potentials follow an "all-or-nothing" principle. Once the threshold is reached, the action potential will occur fully; if the threshold is not met, no action potential occurs.