Neurons "talk" to each other by releasing glutamate at a synapse that binds to NMDA receptors on the surface of the "listening" neuron. If the listening neuron is strongly excited, magnesium ions are expelled from the channel of NR1/2A receptors, one NMDA receptor subtype. Calcium ions then can flow through the open channel into the listening neuron at the synapse, causing the synapse to be strengthened and you to remember that "hot stove = pain."
Another type of NMDA receptor that is thought to help sculpt memories is called NR1/2D. Although NR1/2A receptors require strong excitation to let calcium ions flow across the membrane, NR1/2D receptors respond even to weak inputs.
In this paper, Johnson and Qian further elucidate how NR1/2D receptors do this. Johnson was most surprised to find that the magnesium ion, which strongly blocks the channels of NR1/2A receptors, flows much more easily in NR1/2D receptors. In those receptors, magnesium acts more like a "permeant" ion, which means it can flow through the channel without getting stuck in the middle.
In addition to computer modeling, the researchers used the technique of patch clamp recording, which takes a tiny piece of cell membrane and measures the charge that flows through one open channel. They were able to use the remarkable precision of the patch clamp to see exactly when the magnesium ion entered and exited the ion channel.
"Because of our currently limited understanding of NR1/2D receptors, drawing a direct link to human disease and memory is speculative, but I am confident that the links will become firmer as research progresses," Johnson said.