At which of the points along the illustrated action potential can a second action potential be produced, but only with a stimulus significantly greater than the one that produced the first?

The best answer to the question: At which point along the illustrated action potential can a second action potential be produced, but only with a stimulus significantly greater than the one that produced the first? would be: During the relative refractory period.

Explanation:

When a nervous impulse is initiated, and a cell starts an action potential due to depolarization, then normally the cell´s membrane potential will change as sodium and potassium move in and out of the cell. As this happens, the cell membrane starts to "charge" itself, and once it reaches the proper amount of energetic depolarization, then the summatory effects of further ion exchanges will produce what is known as the action potential. Once the action potential happens, the cell´s control systems kick in to prevent further depolarization. Thus, the cell starts to exchange both sodium and potassium, restore them to their pre-depolarization state, to restore its polarity. During this time, called the absolute refractory period, no matter what stimulus is given, or in what amount, the cell will not respond. However, after this period ends, a new period known as the relative refractory period begins, and at this time, if a stimulus is strong enough, then the cell might me encouraged to depolarize again and produce a new action potential.

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