An action potential is a rapid, temporary change in the membrane potential of a neuron that allows electrical signals to travel along axons. This process is crucial for communication in the nervous system. It involves a precise sequence of changes triggered by voltage-gated ion channels and electrochemical gradients.
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1. Resting State (~ -70 mV)
Before an action potential begins, the neuron is at resting membrane potential:
- Inside the cell: Negatively charged due to trapped anions and K⁺.
- Outside the cell: Positively charged with abundant Na⁺.
- Maintained by Na⁺/K⁺ ATPase and leak channels.
At this stage, voltage-gated sodium (Na⁺) and potassium (K⁺) channels are closed.
Learn more about resting potential from Khan Academy’s resting membrane tutorial.
2. Depolarization
When a stimulus reaches the neuron and crosses a threshold (~ -55 mV):
- Voltage-gated Na⁺ channels open.
- Na⁺ rushes into the cell, making the inside more positive.
- Membrane potential rises to about +30 mV.
This rapid positive shift in voltage is called depolarization.
3. Peak Phase
At the peak of the action potential:
- Na⁺ channels close (inactivate).
- Voltage-gated K⁺ channels open.
This phase ends the influx of Na⁺ and prepares the cell for the next step.
See this phase illustrated in Visible Body’s action potential animation.
4. Repolarization
With Na⁺ channels closed and K⁺ channels open:
- K⁺ flows out of the cell.
- The membrane potential drops, returning to negative values.
This phase restores polarity, with the inside of the neuron becoming negative again.
5. Hyperpolarization
As K⁺ continues to exit:
- The membrane potential becomes more negative than -70 mV.
- This is called the afterpotential or hyperpolarization.
Eventually, K⁺ channels close, and the Na⁺/K⁺ pump restores the original ion balance.
6. Refractory Period
During and after the action potential, neurons enter a refractory period, during which:
- They cannot fire another action potential immediately.
- This ensures one-way signal transmission and prevents overlap.
Learn about the phases of the action potential in this detailed TeachMeAnatomy guide.
Summary Table
| Phase | Description | Key Ions Involved |
|---|---|---|
| Resting | Cell at -70 mV, ready to fire | Na⁺ out, K⁺ in |
| Depolarization | Na⁺ rushes in, membrane becomes positive | Na⁺ |
| Peak | Na⁺ channels close, K⁺ channels open | Na⁺, K⁺ |
| Repolarization | K⁺ exits, restoring negative charge | K⁺ |
| Hyperpolarization | K⁺ overshoots, membrane dips below -70 mV | K⁺ |
| Refractory | Recovery phase | Na⁺/K⁺ pump |
Conclusion
The action potential is a precise sequence of membrane changes involving ion channel activity and charge shifts. Starting from depolarization and ending in hyperpolarization, these changes allow neurons to transmit signals quickly and accurately. This process is fundamental to neural communication and underlies everything from reflexes to thought processes.
For a dynamic view, check out InnerBody’s interactive neuron model.