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PCAT Biological Processes – Action Potential Peak

At the peak of an action potential, which of the following statements describes the flow of ions across the neuronal cell membrane?

A. Sodium is flowing in, potassium is flowing out
B. Sodium is flowing out, potassium is flowing in
C. Sodium is not flowing, potassium is flowing out
D. Sodium is flowing out, potassium is not flowing

Click for Explanation

C is correct. At the peak of an action potential, the membrane potential of the neuron is approximately +40 mV, which is depolarized. At this positive voltage, sodium ceases to flow into the cell (sodium channels close), while voltage-gated potassium channels open to allow potassium to flow out of the cell. As positive charge exits the cell, the neuron repolarizes and then hyperpolarizes as the membrane potential drops below the resting potential. Finally, the membrane potential returns to the resting potential as a result of the activity of the sodium-potassium pump.

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