Brain enemies +

Depression seems to be a function of stunted cell growth.

For reasons scientists still don't understand, cells that produce a brain chemical called dopamine begin to die in a region of tissue that facilitates muscle movements.

When dopamine levels drop, neurons start to fire abnormally, causing the classic Parkinson's tremors.

One percent of the U.S. population over 60 has Parkinson's disease. Only stroke and Alzheimer's cause more deaths as a result of brain dysfunction. The disease is still incurable.

Bungled genetic instructions can cause proteins to tangle and to build up inside or in between nerve cells.

The result is that neurons die, leading to memory loss, dementia, and eventually death.

Certain genes increase the odds of Alzheimer's, but the most critical known risk factor is simply old age.

About 4.5 million Americans have the disease. That number is expected to triple to more than 13.2 million by 2050, when 20 percent of the U.S. population will be over 65.

Alzheimer's is the seventh leading cause of death in the United States.

A stroke is a heart attack in a person's head; an interruption of blood caused either by a clot or a rupture in one of the millions of blood vessels that feed the brain.

Either way, brain cells die, resulting in any number of symptoms, from partial paralysis to loss of speech or vision, depending on the location of the damage.

An estimated 700,000 Americans suffer from stroke annually, and 150,000 die, making it the third leading cause of death in the United States, behind heart failure and cancer.

Strokes are a major cause of physical disability and dementia.

Some parts of the brain, like the frontal lobe and the amygdala, may develop too fast in early childhood, possibly because cells become overwrapped in an insulating layer that may facilitate growth, says neuroscientists David Amaral of the M.I.N.D. Institute at the University of California at Davis.

Exactly how these changes cause symptoms; social impairment, repetitive behaviors, poor language skills, is uncertain, although the frontal lobe is known to regulate behavior and the amygdala, anxiety.

Researchers believe that autism has both genetic and environmental causes.

Autism afflicts one in 150 kids by age eight. Diagnoses are on the rise, although whether because of better recognition or a growing frequency is unclear.