Where does the brain's serotonin come from, and how does it reach so many regions? · Grey Matter

Nearly all of it comes from the raphe nuclei, clusters of neurons along the midline of the brainstem; the upper ones (the dorsal and median raphe) project to the forebrain and the lower ones to the spinal cord.

Where does the brain's serotonin come from, and how does it reach so many regions?

Nearly all of it comes from the raphe nuclei, clusters of neurons along the midline of the brainstem; the upper ones (the dorsal and median raphe) project to the forebrain and the lower ones to the spinal cord. Like the noradrenaline and dopamine systems, a small population of cells sends highly branched axons that release serotonin over wide territories, often outside classical synapses. Serotonin then acts through more than a dozen receptor types, almost all metabotropic, so the same release can excite some cells and inhibit others. This layout makes serotonin a modulator of state (mood, sleep, appetite, the response to stress) more than a carrier of specific messages.