4 Steps of Generating an Action Potential

Question 1

1. Resting State

Answer 1

All gated Na+ and K+ channels are closed
- Only leakage channels for Na+ and K+ are open – this maintains the resting membrane potential
- Each Na+ channel has 2 voltage-sensitive gates
- Activation Gates: closed at rest; opens with depolarization to allow Na+ to enter
- Inactivation Gates: open at rest; blocks channel when its open to prevent more Na+ from entering cell
- Each K+ channel has 1 voltage-sensitive gate
Closed at rest; opens slowly with depolarization

Question 2

2. Depolarization

Answer 2

Na+ Channels Open
- Depolarizing local currents open voltage-gated - Na+ channels and Na+ rushes into the cell
Na+ activation and inactivation gates are open
- Influx of Na+ causes more depolarization and opening of more Na+ channels
- ICF becomes less negative
- At threshold (-55 to -50 mV), positive feedback causes opening of all Na+ channels
- Results in large action potential spike
- Membrane polarity jumps to +30 mV

Question 3

3. Repolarization

Answer 3

Na+ channels are inactivating, and K+ channels open
- Na+ channel inactivation gates close – membrane permeability to Na+ declines to resting state
- AP spike stops rising
- Voltage-gated K+ channels open and K+ exits the cell moving down its concentration gradient
- Membrane returns to resting membrane potential

Question 4

4. Hyperpolarization

Answer 4

some K+ channels remain open, and some Na+ channels reset
- Open K+ channels allow excessive efflux of K+
- Inside of membrane becomes more negative than in resting state
- Na+ channels also begin to reset – activation gates closed and inactivation gates open