Oct 17, 2017

This is a pretty challenging question that requires mastery of neuron physiology.

#### Explanation:

Action potentials occur when neurons become depolarized, usually.

Depolarization occurs when the resting potential (usually $- 70 m V$) increases beyond the threshold.

This occurs because voltage gated sodium channels open and the intracellular fluid is flooded with sodium, which is favored by both the concentration and electrochemical gradient.

Near the peak of the action potential, voltage gated potassium channels open, and potassium ions diffuse through the membrane because this is favored (at this point) by both the concentration and electrochemical gradient. At the absolute peak, sodium channels close and sodium is largely diffused through the membrane.

All the while, refractory periods occur due to the polarization of the intracellular fluid during an action potential, which inhibits (or completely stops) the cell from generating another action potential until resting potential is reattained.

From here, we can deduce it isn't (C).

In most cases, (D) is a nonsense answer.

Notice I never mentioned a pump, so (A) is false.

Largely, it will travel in one direction because the former segments of axon are in their refractory periods. So once it "chooses" to travel one way, it will continue to be "one-way".

I really don't like the available answers, but (E) is probably the right answer. (B) is false because the potassium channels won't really contribute to the neuron's resting potential.