I am curious about what the calcium ions in these neurons are doing when they replace sodium ions. I think of sodium ions as primarily being responsible for propogating the "action potential" which occurs when a neuron is activated. Both calcium and sodium ion concentrations are much higher outside cells than inside.

What fascinates me is the fact that the major sources of calcium ions in tissues is the extracellular fluid and the mitochondria. The cell membrane at rest is relatively impermeable to both calcium and sodium ions, where any "leakage" of these ions is reversed by active channels, or pumps. During an action potential, the membrane becomes transiently more permeable to sodium ions. I'm not sure about what happens to calcium ions then.

I plan to get the Nature paper and read it to find out more.
Robert