An action potential is primarily generated when the membrane potential of a neuron reaches a specific critical level known as the threshold potential.
Understanding Action Potential Generation
The generation of an action potential is a fundamental process in nerve and muscle cells, allowing for rapid, long-distance communication. It's essentially a brief, rapid, large change in the membrane potential that spreads along the cell membrane.
Key Trigger: Reaching Threshold
Based on the provided information, an action potential is generated when a stimulus changes the membrane potential to the values of threshold potential. This initial stimulus could be a signal from another neuron, a sensory input, or even an artificial electrical pulse.
- Resting Potential: Before an action potential occurs, the neuron is typically at a resting potential, usually around -70 mV (inside is negative relative to the outside).
- Stimulus: A stimulus causes a change in the membrane potential, making it less negative (depolarization).
- Reaching Threshold: If this depolarization is strong enough to reach the threshold potential, an action potential is triggered.
The reference states that the threshold potential is usually around -50 to -55 mV. This is the point of no return – once the membrane potential crosses this value, the full action potential sequence is initiated automatically.
The All-or-None Law
It is important to know that the action potential behaves upon the all-or-none law. This means:
- If the stimulus is too weak and doesn't reach the threshold potential, no action potential is generated (the "none" part). There might be a small local depolarization, but it fades away.
- If the stimulus reaches or exceeds the threshold potential, a full-blown action potential of a standard size and shape is generated (the "all" part). Increasing the stimulus strength beyond the threshold doesn't make the action potential larger; it either happens fully or not at all.
Steps in Action Potential Generation (Simplified)
While the reference focuses on the trigger, the generation involves a sequence of events driven by voltage-gated ion channels:
- Depolarization to Threshold: A stimulus causes the membrane potential to rise towards the threshold (e.g., from -70 mV to -55 mV).
- Rapid Depolarization (Rising Phase): Once the threshold is reached, voltage-gated sodium (Na⁺) channels open rapidly, causing a large influx of Na⁺ ions into the cell. This makes the inside of the membrane positive, peaking around +30 mV.
- Repolarization (Falling Phase): Voltage-gated potassium (K⁺) channels open more slowly, and Na⁺ channels inactivate. K⁺ ions flow out of the cell, making the inside negative again.
- Hyperpolarization (Undershoot): K⁺ channels close slowly, leading to a brief period where the membrane potential is more negative than the resting potential.
- Return to Resting Potential: The membrane potential returns to the resting level due to the action of the sodium-potassium pump and leak channels.
Summary Table
| Event | Description | Membrane Potential Change |
|---|---|---|
| Stimulus | Causes initial depolarization. | Towards threshold |
| Reaching Threshold | Membrane potential reaches -50 to -55 mV. Triggers action potential. | Critical point |
| Rising Phase | Rapid Na⁺ influx through voltage-gated channels. | Becomes positive |
| Falling Phase | K⁺ efflux through voltage-gated channels, Na⁺ channels inactivate. | Becomes negative |
| Hyperpolarization | Brief period below resting potential. | More negative than rest |
| Return to Rest | Ion pumps restore resting potential. | Back to resting potential |
Understanding how a stimulus pushes the membrane potential to the threshold is key to grasping action potential generation, as highlighted by the all-or-none principle.
