Graph
Reading File
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Reading File
Here is the well-labeled graph of the Action Potential of a Nerve for section 21.1:
Key Labels to Know for Your Practical
| Phase | Voltage | Ionic Basis |
|---|---|---|
| Resting Membrane Potential | -70 mV | K⁺ leak channels open; Na⁺/K⁺ ATPase active |
| Threshold | -55 mV | Minimum stimulus needed to trigger AP |
| Depolarization (Rising phase) | -55 mV → +40 mV | Voltage-gated Na⁺ channels open → Na⁺ rushes IN |
| Overshoot / Peak | +40 mV | Na⁺ channels inactivate |
| Repolarization (Falling phase) | +40 mV → -70 mV | Voltage-gated K⁺ channels open → K⁺ flows OUT |
| Afterhyperpolarization (Undershoot) | ~-90 mV | K⁺ channels slow to close |
| Return to resting potential | -70 mV | Na⁺/K⁺ ATPase restores balance |
Refractory Periods
- Absolute Refractory Period - during depolarization and most of repolarization; no stimulus, however strong, can generate another AP (Na⁺ channels inactivated)
- Relative Refractory Period - during afterhyperpolarization; a stronger-than-normal stimulus can trigger an AP (some K⁺ channels still open)
Important Notes for OSPE / Viva Voce
- The AP follows the all-or-none law - it either fires fully or not at all
- The AP is self-propagating along the axon
- Duration of nerve AP is approximately 1 ms (much shorter than cardiac AP ~200-400 ms)
- The Na⁺/K⁺ ATPase pump restores ionic balance after the AP but is not responsible for repolarization - that is done by K⁺ efflux
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