MCAT Behavioral Sciences Question 234: Answer and Explanation

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Question: 234

4. Which of the following statements is LEAST likely to be true?

  • A. Drugs that increase Cl influx into the postsynaptic cell could disrupt LTP.
  • B. Drugs that increase K+ efflux from the postsynaptic cell would result in hyperpolarization of the cell and would increase LTP.
  • C. The insertion of new AMPA receptors in the postsynaptic cell membrane would increase the rate at which Mg2+ is displaced from NMDA receptors upon subsequent stimulation by glutamate.
  • D. The influx of Na+ upon initial stimulation by glutamate depolarizes the postsynaptic cell in order to displace Mg2+.

Correct Answer: B

Explanation:

The passage states that the NMDA receptor is blocked by extracellular Mg2+ that must be displaced by a sufficient change in membrane potential. It is fair to assume that this change must be a depolarization, since the initial receptor activated is the AMPA receptor, and according to the figure, that receptor allows an influx of Na+ (choice D is likely to be true and can be eliminated). Additional AMPA receptors would increase the rate of depolarization and thus the rate of Mg2+ displacement. This is in fact the basis for LTP; the first stimulation leads to the effects shown in the figure, including new receptors in the membrane, thus the effect of subsequent stimulation is enhanced (choice C is likely to be true and can be eliminated). Anything that would lead to a hyperpolarization of the cell would not displace Mg2+ and would disrupt the calcium influx and all associated events, including LTP. An increase of Cl? influx would hyperpolarize the cell and disrupt LTP (choice A is likely to be true and can be eliminated), but an increase in K+ efflux, while it would hyperpolarize the cell, would not increase LTP (choice B is unlikely to be true and is the correct answer choice).

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