Action potential experiments

Action potential phases

Action potentials can roughly be divided into four phases:

1. Depolarization: Na+ ions are rushing into the neuron (down the electro-chemical gradient for Na+), raising the neuron's membrane potential to more positive values.
2. Repolarization: the high membrane potential causes the Na+ channels to inactivate and then K+ channels open in a time-dependent manner, letting K+ flow out of the neuron (down its electro-chemical gradient) and consequently lowering the membrane potential back to negative values.
3. Afterhyperpolarization: K+ channels are still open and keep letting K+ out for a little while after the neuron reaches its resting potential, producing an overshoot or hyperpolarization.
4. Return to RMP: The active Na+/K+ pump moves K+ back into the cell and Na+ back out of the cell, gradually returning the membrane potential back to the resting value (the RMP).

For the first millisecond or so after phase 1 the voltage-gated sodium channels are still inactive, which means that they will not open in response to depolarization. This is called the absolute refractory period, and it is impossible to evoke another action potential in this period.

Meanwhile, the voltage-gated sodium channels are slowly transitioning back to their resting state, i.e. closed, but no longer inactive; getting ready to produce a new action potential should the right amount of depolarization come their way. But voltage gated potassium channels are slow to close, meaning that even after the sodium channels are ready to go again the membrane will require a greater depolarization to reach threshold, giving rise to the relative refractory period. In this period it is possible to evoke another action potential, but it requires a greater stimulus.

In this simulation we will inject two current pulses separated by a variable interstimulus interval, and vary the strength of the second pulse to learn more about refractory periods.

Exploring the refractory period