So, where does the brain come into the picture and what does it have to do with this genetic material? The brain is key, although people didn't always know how important the brain actually was.

Ancient Egyptians tossed out the brain when they were mummy-making. They didn't consider it a necessary part of the body. What we've learned since that time is, well, mind-boggling. And we've barely scratched the surface.

What do you remember from your biology class? You probably can recall some brain terms, such as medulla oblongata, cerebellum, and cerebrum, but there are more parts, and some of them lie deep within. Scientists are still in the early stages of unraveling the secrets of the human brain.

As far as depression goes, the National Institute of Mental Health (NIMH) is a good source of information on brain research, and it reports that two parts of your brain, the amygdala and the hippocampus, are of interest to researchers studying depressive disorders.

The Amygdala

Your amygdala is one of those brain parts located deep in the brain. It's shaped sort of like an almond, and scientists believe it functions as a communications hub between the parts of the brain that process incoming sensory signals and the parts that interpret these signals.

For example, if you hear noises in the night, your amygdala serves as a neuro-911. It sends out the danger message, and you experience a heightened sense of anxiety as a result. This is the beginning of the fight-or-flight response. Next, you'll analyze the danger and decide whether to ignore it, confront it, or make tracks.

In addition to being Mission Control, the amygdala also warehouses these emotional memories. When you experience the same situation again or encounter a situation that's similar, your amygdala kicks in and accesses that prior knowledge, and you become anxious. In essence, your brain's been trained.

The Hippocampus

You also have a hippocampus in your brain, and your hippocampus has been in the news recently. New research has discovered that the dentate gyrus, a specific part of the hippocampus, is the source of the eerie déjà vu effect. Your hippocampus, or your dentate gyrus, works with your amygdala to do some further work on that anxiety-provoking situation you just experienced.

The hippocampus encodes these threatening events into memories. Since memories of traumatic events are the triggers for post-traumatic stress disorder (PTSD), researchers are interested in exploring this further. Also, the hippocampus appears to be smaller in some people who were victims of child abuse or who served in military combat. What causes the hippocampus to shrink? Why does it shrink?

These are questions that, when answered, may explain flashbacks and lapses in memory centered around the traumatic event. By learning more about how the brain creates fear and anxiety, scientists may be able to devise better treatments for anxiety disorders and the depression that accompanies them.

The Chemical Process

Your brain is a complex organ, and its chemistry is not yet fully understood. Researchers do know quite a bit, though, and are adding to that knowledge daily. They know that certain chemicals send impulses across nerve endings to other nerves or muscles or organs. These chemicals are called neurotransmitters.

When everything is working smoothly, these neurotransmitters operate efficiently and in correct amounts. When things are not going according to plan, neurotransmitter levels may fluctuate. This is not good. The neurotransmitters of interest here are norepinephrine, serotonin, and dopamine. They're the neurotransmitters associated with major depression.

• Norepinephrine is also known as noradrenaline. It has a stimulating effect and promotes alertness and a sense of well-being. If norepinephrine is overproduced, fear and anxiety can result.

• Serotonin is a key neurotransmitter for maintaining mental and emotional health. If your serotonin levels drop, due to prolonged stress or illness or malnutrition, depression can follow.

• Dopamine is the precursor to norepinephrine. It's also a neurohormone (see the next section). Scientists are studying the connection between unbalanced dopamine and schizophrenia and Parkinson's disease.

It may seem strange to think of your brain as a chemistry experiment, but that's exactly how researchers are approaching the problem. They're looking at fear and anxiety as chemically induced responses. If they find out that this is indeed true, then discovering the chemistry behind those responses is the first step toward developing medications to alter those chemicals, reducing the fear or anxiety response, and ultimately curing or even preventing the onset of depression. This is exciting terrain for a scientist.

Add a Hormone or Two

Neurotransmitters have help. They need it! They work hard at what they do. They're aided in their tasks by other chemicals, called neurohormones, that may also play a role in depression. One of these chemicals, corticotrophin-releasing hormone (CRH) gets you on the alert when a real or perceived danger looms. Research suggests that trauma during childhood can negatively affect the functioning of CRH and the hypothalamic-pituitary-adrenal axis (HPA axis) throughout the course of your life.

So much of what people have learned about the brain reinforces the importance of the early years in creating good mental and physical health. Many illnesses, and that includes depression, may have their roots in childhood and in childhood experiences.

Studies have shown that people who have dysthymia (chronic depression) typically have increased levels of CRH. Antidepressants and electroconvulsive therapy are both known to reduce these high CRH levels. As CRH levels return to normal, depressive symptoms recede.