The generation of an action potential within a cell is shown in the graph below. Which of the following statements is true of the membrane during repolarization at the indicated by the arrow?
A. The voltage-gated sodium channels close stopping the flow of sodium into the cell and the voltage-gated potassium channels open allowing the flow of potassium out of the cell.
B. The voltage-gated sodium channels close stopping the flow of sodium out of the cell and the voltage-gated potassium channels open allowing the flow of potassium into the cell.
C. The voltage-gated potassium channels close stopping the flow of potassium into the cell and the voltage-gated sodium channels open allowing the flow of sodium out of the cell.
D. The voltage-gated potassium channels close stopping the flow of potassium out of the cell and the voltage-gated sodium channels open allowing the flow of sodium into the cell.
Click for Explanation
This question asks the examinee to identify the cellular response during an action potential at the time of repolarization. The time preceding that indicated by the arrow corresponds to the depolarization of the cell. During depolarization, the voltage-gated sodium channels are open and sodium flows down its concentration gradient into the cell.
After the cell depolarizes, the cell reestablishes the negative resting membrane potential by closing the voltage-gated sodium channels preventing the flow of sodium into the cell, and opening the voltage-gated potassium channels allowing the flow of potassium down its concentration gradient and out of the cell. The flow of positively charged potassium ions during repolarization results in the net loss of positive ions within the cell, which in effect creates a negatively charged interior. Thus, choice A is the correct answer.
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