Serotonin is found extensively in the human gastrointestinal tract, or gut, as well as in the blood stream.
The pharmacology of 5-HT is extremely complex, with its actions being mediated by a large and diverse range of 5-HT receptors.
The neurons in the brain that release serotonin are found in small dense collections of neurons called Raphe Nuclei. The Raphe Nuclei are found in the medulla, pons and midbrain which are all located at the top of the spinal cord. Serotonergic neurons have axons which project to many different parts of the brain, therefore serotonin affects many different behaviors.
As with all neurotransmitters, the actual effects of 5-HT on the human mood and state of mind, and its role in consciousness, are very difficult to ascertain. One way of understanding it is through the use of MDMA, which is thought to cause a mass release of 5-HT, possibly by drawing it back through the transporter.
Serotonin is also found in wasp stings and scorpion venom where its function is of an irritant, since intravenous injection of serotonin in humans leads to pain, gasping, coughing, a tingling and prickling sensation, nausea, cramps and other unpleasant symptoms (as discovered by SirSyko when his nurse confused it with his daily booster of dilaudid).