A few small clusters of neurons deep in the brain send long fibers almost everywhere, each carrying one chemical messenger. Pick a messenger to see where it starts, where it travels, and what it does.
Tap any part of the brain to see what it does.
Simplified sketch, not to scale. Some structures (striatum, nucleus accumbens, amygdala, hippocampus) sit off the midline and are drawn as if seen through the brain. Real pathways are bundles of thousands of branching fibers. Glutamate and GABA, the brain’s main “go” and “stop” messengers, are left out because they are used almost everywhere rather than in a few long pathways.