Final answer:
NMDA receptors are activated during LTP when postsynaptic depolarization expels Mg2+ ions, allowing Ca2+ ions to enter and trigger a signaling cascade. This leads to insertion of more AMPA receptors and stronger synaptic connections.
Step-by-step explanation:
In order for an NMDA receptor to become activated during long-term potentiation (LTP), two conditions must be met. First, the postsynaptic neuron must be depolarized to expel the inhibitory magnesium (Mg2+) ions that normally block the receptor. Once depolarization occurs, the NMDA receptor can be activated by the neurotransmitter glutamate, and calcium (Ca2+) ions can then enter the cell. This influx of Ca2+ ions initiates a signaling cascade that leads to the insertion of more AMPA receptors into the postsynaptic membrane. Subsequent binding of glutamate to both NMDA receptors and the newly inserted AMPA receptors results in more efficient depolarization. Enhanced signaling at the synapse strengthens the synaptic connection, a key process in forming memories and learning.
Strong depolarization is necessary to fully expel the magnesium plug from NMDA receptors, which allows substantial calcium entry and LTP. In contrast, weaker depolarization only partially displaces magnesium, resulting in a lower influx of calcium through NMDA receptors and may lead to long-term depression (LTD), a process where synaptic strength is reduced.