NBME 21 Answers : Block 4 : #45 = During an experiment, an investigator observes that ...

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submitted by cantaloupe5(34),

This one was tricky but I think you could’ve done this one without knowledge of NMDA receptors. Stem told you that glutamate activates both non-NMDA and NMDA receptors but it activated only non-NMDA receptors in the early phase. That means NMDA receptors activate after non-NMDA receptors. That means something was delaying NMDA receptor activating and the only answer that made sense as the Mg inhibiting NMDA at resting potential. Once the cell is depolarized by non-NMDA receptors, NMDA receptors can be activated.

hungrybox I forgot/didn't know this factoid and narrowed it to the correct answer and a wrong answer. Guess which one I chose? +4

yotsubato >That means something was delaying NMDA receptor activating and the only answer that made sense as the Mg inhibiting NMDA at resting potential. What makes the fasting gating kinetics choice incorrect then? +1

imgdoc NMDA receptors are both voltage gated and ligand gated channels. Glutamate and aspartate are endogenous ligands for this receptor. Binding of one of the ligands is required to open the channel thus it exhibits characteristics of a ligand channel. If Em (membrane potential) is more negative than -70 mV, binding of the ligand does NOT open the channel (Mg2+ block on the NMDA receptor). IF Em is less negative than -70 mV binding of the ligand opens the channel (even though no Mg2+ block at this Em, channel will not open without ligand binding. Out of the answer choices only NMDA receptors blocked by Mg2+ makes sense. Hope this helps. +