Neurotransmitters are chemicals produced by the nerve cells in the brain that send messages back and forth across the space between the cells (synapse). When the normal balance of these neurotransmitters is upset, headache, depression, or other mental health problems may develop.

The neurotransmitters that are believed to play a role in mental functioning are serotonin, norepinephrine, dopamine, and gamma-aminobutyric acid (GABA).