You’ve probably probably heard the term “neurotransmitter” before, but what does this really mean? Neurotransmitters are chemical messengers within the brain that facilitate communication between nerve cells. Let’s illustrate with serotonin. Figure 1 depicts the junction between two nerve cells.

Packets of serotonin molecules are released from the end of the presynaptic cell (the axon) into the space between the two nerve cells (the synapse). These molecules may then be taken up by serotonin receptors of the postsynaptic nerve cell (the dendrite) and thus pass along their chemical message. Excess molecules are taken back up by the presynaptic cell and reprocessed.

Several things might potentially go wrong with this process and lead to a serotonin deficit. Just to enumerate a few possibilities:

 
• Not enough serotonin is produced,  
• There are not enough receptor sites to receive serotonin,  
• Serotonin is being taken back up too quickly before it can reach receptor sites,  
• Chemical precursors to serotonin (molecules that serotonin is manufactured from) may be in short supply, or  
• Molecules that facilitate the production of serotonin may be in too short supply.

As you can see, if there is a breakdown anywhere along the path, neurotransmitter supplies may not be adequate for your brain’s needs. Inadequate supplies lead to the symptoms that we know as depression.