Action Potential
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ACTION POTENTIALS
Summary
See: Action Potential
Overview
- All-or-nothing (like firing a gun)
Definitions
Depolarization
- Decrease in membrane potential relative to resting potential (membrane becomes more positive)
Hyperpolarization
- Increase in membrane potential relative to resting potential (membrane becomes more negative)
Graded potentials
- Depolarization signals that operate over short distances
Action potentials
- Depolarization signals that operate over long distances
4 STEPS OF AN ACTION POTENTIAL
1) Resting state - ~70mV membrane potential
- Voltage-gated sodium channels closed
- Voltage-gated potassium channels closed
2) Depolarization phase
- Graded potentials cause slight depolarization until threshold (~ -55mV) when voltage-gated sodium channels open (voltage-gated potassium channels still closed)
- Sodium rushes into the cell and membrane potential rapidly increases and peaks at about +30mV
3) Repolarization phase
- Inactivation gate of sodium channel "plugs" the channel, stopping sodium from entering the cell
- Potassium gates open allowing potassium out of the cell and membrane potential decreases
4) Hyperpolarization phase
- Sodium channels transition from inactivated to closed
- Some potassium channels are still open so the membrane potential overshoots the resting state and becomes more negative
- About 4ms after the original change in membrane potential, resting potential is restored
REFRACTORY PERIODS
Absolute Refractory Period
- Time from when sodium channels open to when they reset (no stimulus, no matter how large, can cause another action potential during this period)
Relative Refractory Period
- Time when most of the sodium channels have reset but some potassium channels are still open (it is possible for a stronger than normal stimulus to cause an action potential during this period)