http://en.wikipedia.org/wiki/Serotonin Look at the receptor list. There are some in the PNS, the CNS, blood vessels, all over the body. Some work on certain receptors more than others.

Receptors downregulate in number due to an excess in signaling. If you take an excess of an SSRI, the receptors downregulate to get back to normal. It's like if you take caffeine, you need more to get a stimulant effect after awhile since the receptors downregulated.