Doctors aren't sure what causes depression, but a prominent theory is altered brain structure and chemical function. Brain circuits that regulate mood may work less efficiently during depression. Drugs that treat depression are believed to improve communication between nerve cells, making them run more normally. Experts also think that while stress -- such as losing a loved one -- can trigger depression, one must first be biologically prone to develop the disorder. Other triggers could include certain medications, alcohol or substance abuse, hormonal changes, or even the season.
Illustrated here are neurons (nerve cells) in the brain communicating via neurotransmitters.