From the raphe nuclei outward
The serotonin-producing neurons of the brain are clustered in the raphe nuclei along the brainstem midline. From there their axons fan out remarkably widely, reaching the cortex, the limbic system, the basal ganglia, and back down the spinal cord. This broad distribution is why serotonin is linked to so many functions at once, including mood, sleep, appetite, and the regulation of pain.
Because the same system touches many circuits, serotonin signalling is best understood as a broad modulator rather than a single switch for any one feeling.
The large pool outside the brain
Most of the body's serotonin, commonly cited as around ninety percent, is actually produced in the gut, where it regulates intestinal movement and other functions. This peripheral serotonin does not freely cross into the brain, so gut and brain serotonin operate as largely separate pools, a point often blurred in popular claims about boosting serotonin through diet.
This is general educational information, not medical advice. It does not recommend any supplement or treatment; mood and digestive concerns should be discussed with a qualified professional.