Nature vs Nurture

This is the second of three articles based on the Surgeon General's Report on Mental Health:

We know that depression is a physical phenomenon. Thanks to modern imaging, we can see it actually taking place, like a tiny squid secreting dark ink in the left prefrontal regions of the brain. We also know that genes play a factor, that in twins there is a co-occurrence that cannot be explained away by chance, and that the condition often has a family history spanning several generations.

But life, let alone depression, is never that simple.

Yes, our mental well-being is dependent on the brain's 100 billion neurons and how they connect and interact with other neurons, but it takes a push from the outside environment to start the process, creating a stimulus and response relationship that defines who we are. According to the Surgeon General's Report:

"When a neuron forms a synapse with its target cell, the pattern of activity, usually furnished by external environmental stimulation, strengthens or weakens the developing synapse. Only strengthened synaptic connections survive early development to form enduring connections, while weakened synaptic connections are eliminated ... Later in the course of development, established patterns of connections still can be altered by the environment - through learning."

One indicator of environment at play is depression in teen-age girls, which is significantly higher than in boys. According to the Report, one explanation may be that girls are more socially oriented, more dependent on positive social relationships, and more vulnerable to losses of social relationships than boys. Also, girls seem to (not) cope with stress by thinking a lot more.

Then there are stressful life events, such as marriage breakups, death in the family, economic hardships, or racism and discrimination. For example, 30 to 40 percent of those undergoing divorce report a significant increase in symptoms of depression and anxiety. Single mothers face twice the risk of depression as do married mothers. Victims of domestic violence must often bear their considerable emotional burdens while battling incapacitating depression.

But the brain doesn't merely react through its neuronic pathways or, more specifically, the neurotransmitters such as dopamine and serotonin that service them. More complex interactions are thought to be at play. For one, stress is thought to mobilize the sympathetic nervous system that triggers the fight-or-flight response and a host of physiological reactions. It also acts on two key regulatory centers found in the cerebral hemispheres of the brain - the hippocampus (which is smaller in depressed individuals) and the amygdala. These, in turn, are believed to activate the hypothalamic-pituitary-adrenocortical (HPA) axis. The hippocampus and amyygdala are major nuclei of the brain's limbic system which underlies emotions.

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