When neurons are undisturbed, which rarely occurs in the functioning body, they sit at their resting membrane potential, which for most neurons is around -70 mV. As effortless as the term makes it sound, neurons are required to actively pump K+ in and Na+ out of their membranes to maintain such resting potential.
When positive current is injected inside the neuron, the membrane potential becomes more positive, or in other words is depolarized. If the depolarization is large enough to take the neuron's membrane potential above its threshold value, voltage-gated sodium channels start to open, allowing an influx of Na+ ions (as you saw in the animation). Because Na+ has a positive charge, this in turn causes the membrane potential to depolarize even more, which causes more channels to open, which causes more Na+ ions to flow in, which causes the membrane potential to depolarize even more (you get the picture!!!)... Eventually all available channels are open and the membrane potential is so positive that it causes the Na+ channels to inactivate, a process known as becoming refractory.
Now K+ channels activate, causing a massive exodus of K+ ions from the neuron, which serve to push the membrane potential to more negative values. This part of the action potential is called repolarization. Additional K+ currents cause an afterhyperpolarization, also termed refractory period, that pushes the membrane potential to values more negative than the resting potential before stabilizing itself back at the resting potential.
In this simulation we will explore the relationship between the amount of current injected into a neuron and the corresponding changes in membrane potential.
Producing an action potential
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Stimulus duration: 1 ms
At 1 ms stimulus duration, what is the minimum stimulus strength needed to produce an action potential?