Question

Describe the falling phase of an action potential. What other channels are opening and what ions are flowing out? When do these channels get inactivated? What occurs at the undershoot, when the membrane is hyperpolarized?

Answer 1

During the falling phase of an action potential, voltage-gated potassium channels open, allowing K+ to exit the cell, leading to repolarization followed by hyperpolarization. Sodium channels inactivate and will not reactivate until the membrane potential approaches resting levels, resulting in a refractory period. The sodium-potassium pump later restores the resting potential.

Step-by-step explanation:

The falling phase of an action potential occurs after the peak depolarization. During this phase, voltage-gated potassium (K+) channels open, allowing K+ ions to flow out of the cell, countering the previous influx of sodium (Na+) ions. This results in the repolarization of the membrane, where the internal charge of the neuron becomes more negative again. These K+ channels eventually close, but because they close slowly, more K+ ions leave the cell than necessary to achieve the resting potential. This overcompensation creates a temporary state called hyperpolarization, which is the undershoot phase, wherein the membrane potential is even more negative than the resting potential. Hyperpolarization during the undershoot makes the neuron less likely to fire another action potential since the membrane potential is further from the threshold. Voltage-gated sodium (Na+) channels, which previously inactivated shortly after opening, remain inactive until the membrane potential nears the resting level again. This period, where the neuron is less susceptible to stimulation, is known as the refractory period. The sodium-potassium pump eventually restores the resting membrane potential, which is approximately -70mV, preparing the neuron for the next potential action.