excitotoxicity

Excitotoxicity is what happens when neurons (the brain's wire-like signaling cells) are stimulated so hard, for so long, that they end up killing themselves. The trigger is usually too much glutamate, the brain's main "go" signal — the chemical one nerve cell uses to tell the next one to fire. A normal pulse of glutamate is a quick, useful nudge. But a flood of it is like jamming a finger down on the accelerator and never letting go: the receiving cell is whipped into a frenzy of activity it cannot switch off, and that relentless overexcitement, rather than the glutamate itself, is what does the damage. The word literally packs in the idea — to excite to the point of poisoning.

Here is the mechanism. When glutamate lands on certain docking sites called receptors — especially one named the NMDA receptor — it opens gates in the cell membrane that let calcium, a charged particle the cell normally keeps scarce inside, pour in. A little calcium is a vital messenger; a torrent is a wrecking signal. The flood of calcium switches on enzymes that chop up the cell's own proteins, fats, and DNA, and it overwhelms the cell's tiny power plants, the mitochondria, until they fail and the energy supply collapses. Starved of power and torn apart from within, the neuron dies. Because dying cells spill their own glutamate, the damage can spread outward to neighbors, turning a small injury into a widening patch of loss.

Excitotoxicity is a major way neurons are lost in sudden brain emergencies like stroke and head trauma, where the surrounding zone of barely surviving cells is finished off by this glutamate flood. It is also suspected of contributing, more slowly, to long-running diseases such as ALS, Huntington's, and Alzheimer's. This is why some treatments aim to dial glutamate signaling down — for example, the Alzheimer's drug memantine gently blocks the NMDA receptor — trying to quiet the overexcitement without silencing the normal signaling the brain still needs.