The NMDA receptor plays a critical role in long-term potentiation (LTP). Which property of the NMDA receptor channel is essential for its role as a 'coincidence detector' in LTP induction?

• A Constitutive Ca²⁺ conductance that is enhanced by glycine binding without requiring simultaneous depolarisation
• B Voltage-dependent Mg²⁺ block that is removed only when the postsynaptic membrane is simultaneously depolarised by AMPA receptor activation ✓
• C Rapid receptor desensitisation that prevents sustained Ca²⁺ entry and limits LTP magnitude
• D Selective permeability to Na⁺ that generates a large EPSP sufficient to trigger LTP independently

Explanation

At resting potential, Mg²⁺ blocks the NMDA receptor channel pore in a voltage-dependent manner, preventing ion flow even when glutamate and glycine (co-agonist) are bound. When the postsynaptic neuron is depolarised (typically by AMPA receptor activation from repeated presynaptic stimulation), Mg²⁺ is expelled, allowing Na⁺, K⁺, and critically Ca²⁺ to flow through. The Ca²⁺ influx activates CaMKII, leading to AMPA receptor insertion (LTP). This dual requirement — glutamate binding AND postsynaptic depolarisation — makes the NMDA receptor a molecular coincidence detector for Hebbian plasticity.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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