Practice question · Put in order
Put the phases of a single action potential in the correct order.
- The membrane depolarizes rapidly toward its +40 mV peak.
- Na+ channels inactivate and voltage-gated K+ channels open, so K+ flows out.
- K+ channels are slow to close, briefly overshooting below -70 mV (hyperpolarization).
- Leak channels and the Na/K pump restore the resting potential.
- The membrane reaches threshold; voltage-gated Na+ channels open and Na+ rushes in.
Hints
- Sodium entry drives the rise; potassium exit drives the fall - order those causes first.
- The undershoot below rest comes AFTER repolarization, because K+ channels linger open.
Show the answer
- The membrane reaches threshold; voltage-gated Na+ channels open and Na+ rushes in.
- The membrane depolarizes rapidly toward its +40 mV peak.
- Na+ channels inactivate and voltage-gated K+ channels open, so K+ flows out.
- K+ channels are slow to close, briefly overshooting below -70 mV (hyperpolarization).
- Leak channels and the Na/K pump restore the resting potential.
Why
Na+ influx depolarizes to the peak, K+ efflux repolarizes, sluggish K+ channels overshoot into hyperpolarization, and the pump plus leaks reset rest. Getting the ion for each phase right is the point - Na+ up, K+ down.
Practise Action Potential
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