Parkinson's disease is one of a larger group of neurological conditions called motor system disorders. Historians have found evidence of the disease as far back as 5000 B.C. It was first described as "the shaking palsy" in 1817 by British doctor James Parkinson. Because of Parkinson's early work in identifying symptoms, the disease came to bear his name.

In the normal brain, some nerve cells produce the chemical dopamine, which transmits signals within the brain to produce smooth movement of muscles. In Parkinson's patients, 80 percent or more of these dopamine-producing cells are damaged, dead, or otherwise degenerated. This causes the nerve cells to fire wildly, leaving patients unable to control their movements. Symptoms usually show up in one or more of four ways:

• tremor, or trembling in hands, arms, legs, jaw, and face
• rigidity, or stiffness of limbs and trunk
• bradykinesia, or slowness of movement
• postural instability or impaired balance and coordination.

Though full-blown Parkinson's can be crippling or disabling, experts say early symptoms of the disease may be so subtle and gradual that patients sometimes ignore them or attribute them to the effects of aging. At first, patients may feel overly tired, "down in the dumps," or a little shaky. Their speech may become soft and they may become irritable for no reason. Movements may be stiff, unsteady, or unusually slow.

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