What is an Action Potential?
An action potential is a rapid, temporary reversal of the electrical potential across a neuron's membrane — the mechanism by which nerve impulses are transmitted.
Learning Goals:
- Describe the resting potential and how it is maintained.
- Explain the four phases of an action potential.
- Distinguish between the absolute and relative refractory periods.
- Describe saltatory conduction in myelinated neurons.
Resting Potential
At rest, the inside of the neuron is −70 mV relative to the outside. This is maintained by the Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in) and K⁺ leak channels.
Four Phases
| Phase | Voltage Change | Ion Movement |
|---|---|---|
| 1. Resting | −70 mV | K⁺ leak out, Na⁺/K⁺ pump active |
| 2. Depolarization | −70 → +30 mV | Na⁺ channels open → Na⁺ rushes in |
| 3. Repolarization | +30 → −70 mV | Na⁺ channels close, K⁺ channels open → K⁺ rushes out |
| 4. Hyperpolarization | Below −70 mV | K⁺ channels slow to close → overshoot |
Threshold and All-or-Nothing
- Threshold: ~−55 mV. If stimulus reaches threshold → full action potential fires.
- All-or-nothing: Action potentials always have the same amplitude (+30 mV). Signal intensity is encoded by frequency, not amplitude.
Refractory Periods
| Type | Duration | What happens | Significance |
|---|---|---|---|
| Absolute | ~1 ms | Na⁺ channels inactivated | No second AP possible |
| Relative | ~2 ms | Some channels recovered | Stronger stimulus needed |
Refractory periods ensure unidirectional propagation and limit firing frequency.
Saltatory Conduction
In myelinated neurons, action potentials "jump" between Nodes of Ranvier (gaps in myelin sheath). This increases conduction speed from ~2 m/s to ~120 m/s.
Worked Examples
Example 1: Why can't an action potential travel backwards?
The region just behind the action potential is in its absolute refractory period — Na⁺ channels are inactivated and cannot reopen. So the impulse can only move forward.
Example 2: Multiple sclerosis and conduction
MS destroys the myelin sheath → action potential must travel continuously along the bare axon → speed drops dramatically → symptoms include muscle weakness and numbness.
Common Mistakes
- "Depolarization means K⁺ rushes in" — Na⁺ rushes in during depolarization. K⁺ moves out during repolarization.
- "Bigger stimuli = bigger action potentials" — All-or-nothing: amplitude is constant. Bigger stimuli increase frequency.
- Forgetting hyperpolarization — K⁺ channels close slowly, causing a brief undershoot below −70 mV.
Exam Tips
- Draw and label a voltage-time graph with all four phases.
- Specify which ions move and in which direction at each stage.
- Saltatory conduction = myelinated = faster. Continuous conduction = unmyelinated = slower.
Related Topics
- Membrane Transport — Ion channels and pumps also control membrane transport.
- Enzyme Kinetics — Na⁺/K⁺ ATPase is an enzyme.