glutamate

Glutamate is the brain's main "go" signal — the chemical messenger that one nerve cell uses most often to tell the next cell to fire. When neurons (the brain's wire-like signaling cells) talk to each other, they don't touch; they shoot tiny puffs of a chemical across the narrow gap between them, called a synapse. Glutamate is the chemical released at the vast majority of those excitatory "speed up" connections. If you think of the brain as a city of switches, glutamate is the most common finger that flips a switch to ON.

Here is how it works. One neuron stores glutamate in tiny bubbles and squirts it into the synapse. The glutamate drifts across and lands on docking sites called receptors on the receiving cell. These receptors are gates: when glutamate plugs in, the gates pop open and let positively charged particles rush into the cell, nudging it closer to firing its own signal. This handoff is extremely fast — a fraction of a thousandth of a second — which is why glutamate drives quick thoughts, sensing, and movement. It is also the workhorse of learning and memory, because connections that use glutamate get stronger with repeated use, the way a footpath gets worn smoother the more it is walked.

Because glutamate is so powerful, too much of it is dangerous: a flood can overexcite neurons until they exhaust themselves and die, a kind of damage called excitotoxicity that happens in strokes and some brain injuries. So the brain keeps glutamate on a tight leash, mopping it back up from the synapse within milliseconds. Its calming counterpart is GABA, the brain's main "stop" signal; healthy thinking depends on a constant balance between the two.