Parkinson's disease is a neurological disease affecting the "substantia nigra," a small area of cells in the mid-brain. Degeneration of these cells as a result of free radical activity results in a reduction in levels of the neurotransmitter dopamine and upsets the balance between dopamine and another brain chemical, acetylcholine. The most familiar signs of the disease are tremor, or trembling in hands, arms, legs, jaw, and face; a generalized slowness of movement, stiff limbs, rigid facial expressions, and problems with balance or gait. Depression often precedes the physical signs, and mental function can deteriorate in advanced cases.