Final answer:
Apoptosis via NMDA receptors is primarily caused by elevated intracellular calcium levels, as the removal of magnesium allows for calcium entry, leading to cellular cascades that result in neuronal cell death.
Step-by-step explanation:
Apoptosis mediated by the N-methyl-d-aspartate (NMDA) receptor complex is most likely caused by elevated intracellular levels of calcium ions (Ca2+). The NMDA receptor is normally blocked by a magnesium ion, which is removed upon strong depolarization. This removal allows the influx of calcium into the cell, which can trigger a cascade leading to excitotoxicity and ultimately apoptosis. This calcium influx can activate processes including the opening of mitochondrial permeability transition pores and the release of reactive oxygen species, making it central to excitotoxic cellular damage and neuronal death.