Reactions don't need to happen, the magnesium ion is a single atom that interacts with the NMDA receptor to block it at specific potentials. When the postsynaptic neuron gets enough input at a synapse, there is enough change in the charge inside the cell where it allows the Mg2+ ion to be displaced from the pore to allow cations to pass through
Our cells use single atoms, usually in the form of charged ions, on a regular basis and we would not survive without them
