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Here is a simplified explanation of how SSRI's work.
These drugs are very neat. They are used in many conditions including migraine, depression, anxiety disorders, panic disorders, obsessive-compulsive disorder, eating disorders, chronic pain, post traumatic stress disorder, irritable bowel syndrome and premature ejaculation. The group of available SSRIs include citalopram, fluoxetine, fluvoxamine, paroxetine, dapoxetine, sertraline and many more. Serotonin is a chemical produced from the amino acid tryptophan. Serotonin is also called 5-HT (5-hydroxytryptamine) and is found all over the place in our bodies. Serotonin is strongly associated with mood.
Here is a simplified example of what SSRIs do in the nervous system. Messages are sent round the nervous system by a mechanism involving electric charge. So when one end of a nerve (a synapse) is stimulated an impulse runs along the length of the nerve to the other end. It's not the same as an electric current in a wire but the message flows pretty quickly. When this pulse reaches the synapse at the other end of the nerve it causes it to release certain molecules. These molecules spread into the gap (the synaptic gap) between this nerve end and the begining of the next one. The change in the chemical content in this gap causes the next nerve to fire a pulse along it's length. Serotonin is a chemical used in the transmission of information across the synaptic gap. For the nerve system to work efficiently these serotonin molecules need to be cleared up or the second nerve would continue to fire pulses along it's length producing something analogously akin to feedback from an electric guitar. So, as a consequence of producing the serotonin molecules, the original nerve also sucks them back. How does the synapse suck the molecules back? A simple analogy is that the molecules have a certain shape like a key and the synapse presents receptor sites like a keyhole. The Brownian motion which is constantly buffeting these molecules around at ridiculous speeds causes the serotonin molecules to rapidly fit into these "keyholes" thereby removing all the serotonin from the gap. The cell conveniently gets it's serotonin back too.
As you might imagine the speed and reliability of these impulses have a significant effect on the workings of the whole system. If, for some reason, the strength or reliability of these impulses is reduced you could perhaps find a way of getting more serotonin into the gap to "oil" the works so to speak. Well a very clever little mechanism was discovered whereby what you do is you put molecules into the system which just fit these serotonin keyholes. Provided the molecules you use have little or no other effect than to fit the keyholes then the only consequence is that they temporarily reduce the number of available keyholes and as a consequence there is more available serotonin to boost the second nerve impulse. It's a bit like cleaning the terminals on your battery to get a better connection and better electric flow in your circuit. That is why this group of drugs are called selective serotonin reuptake inhibitors. They slow down the reuptake of the serotonin in the synaptic gap.
Of course you can find out more about this and related subjects from the experts who contribute to the Encyclopaedia Britannica.
If you want to see what the serotonin molecule looks like you can see it in beautifully rendered interactive 3D on Karl Harrison's website 3dchem.com. Click this link to view the --> Serotonin Molecule.