Parkinson's disease, the second most common degenerative brain disease (after Alzheimer's), affects 1 million Americans and typically begins between the ages of 50 and 79. It occurs when neurons, or nerve cells, that control movement start to die off for unknown reasons. The result is a shortage of the brain-signaling chemical dopamine, which triggers the muscles that allow fluid body movements, such as lifting an arm or walking. The decline in dopamine levels leads to tremor, slowness of movement, stiffness of the limbs and trunk, difficulty walking and lack of facial expression.

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