The majority of psychiatric drugs act by altering one or more neurotransmitter systems. Typically the medications do not contain the actual neurotransmitters but instead contain various chemicals that regulate the action of neurotransmitters. For example, the SSRI antidepressants block the re-absorption of serotonin. This keeps the serotonin molecules in the synapse longer, giving them more time to bind to receptor sites and, therefore, stimulate the firing of the post-synaptic neuron. To visualize this process, imagine someone standing at your door, continuously pressing his or her finger on your doorbell.