Neuroscience MCAT Practice Question
A 68-year-old man with mild cognitive impairment undergoes neuropsychological testing as part of a research study on memory and synaptic plasticity. Researchers use hippocampal slice electrophysiology to investigate his learning mechanisms. During high-frequency stimulation of Schaffer collateral inputs, postsynaptic neurons show rapid depolarization mediated by AMPA receptors. As depolarization progresses, NMDA receptor-mediated currents begin to contribute significantly to the postsynaptic response. The researchers note that at resting membrane potential (approximately -70 mV), NMDA receptors show minimal current flow despite glutamate binding, but this changes dramatically once the membrane potential becomes less negative. Which of the following best explains the voltage-dependent gating mechanism that initially prevents NMDA receptor contribution during resting conditions?
Answer choices
- AMagnesium ions competitively bind to the glutamate recognition site on NMDA receptors, preventing agonist-induced channel opening
- BMagnesium ions physically occlude the NMDA receptor ion channel in a voltage-dependent manner, requiring depolarization to dislodge them and allow calcium influxCorrect answer
- CMagnesium ions allosterically inhibit the glycine co-agonist binding domain, preventing NMDA receptor activation regardless of membrane potential
- DMagnesium ions phosphorylate serine/threonine residues on NMDA receptors, which must be dephosphorylated during depolarization for channel opening
- EMagnesium ions activate GABAergic inhibitory interneurons that tonically suppress NMDA receptor signaling until high-frequency stimulation overcomes this inhibition
- FMagnesium ions chelate intracellular calcium stores, preventing the calcium-calmodulin-dependent signaling cascade necessary for NMDA receptor activation
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