The convergence of depression and pain is reflected in the circuitry of the nervous system. In the experience of pain, communication between body and brain goes both ways. Normally, the brain diverts signals of physical discomfort so that we can concentrate on the external world. When this shutoff mechanism is impaired, physical sensations, including pain, are more likely to become the center of attention. Brain pathways that handle the reception of pain signals, including the seat of emotions in the limbic region, use some of the same neurotransmitters involved in the regulation of mood, especially serotonin and norepinephrine. When regulation fails, pain is intensified along with sadness, hopelessness, and anxiety. And chronic pain, like chronic depression, can alter the functioning of the nervous system and perpetuate itself.
The mysterious disorder known as fibromyalgia may illustrate these biological links between pain and depression. Its symptoms include widespread muscle pain and tenderness at certain pressure points, with no evidence of tissue damage. Brain scans of people with fibromyalgia show highly active pain centers, and the disorder is more closely associated with depression than most other medical conditions. Fibromyalgia could be caused by a brain malfunction that heightens sensitivity to both physical discomfort and mood changes.