Parkinson's disease is a serious brain condition that results from nerve damage in certain regions of the brain that regulate the body's voluntary muscles. Also referred to as "PD," Parkinson's disease is a movement disorder that causes muscle rigidity, shaking, and slow difficult walking.(1, 2) PD usually strikes in mid to late adult life, although 30 percent of people with the disease experience symptoms before age 50.(3) Another 40 percent develop the disease between ages 50 and 60. PD is a slowly progressive and incurable disease.

Like all cells in the nervous system, brain neurons function in response to electrical impulses that travel with lightning speed from cell to cell. These impulses need assistance in order to jump from one neuron to the next. They get this help in the form of neurotransmitters, chemicals in the body that carry messages across the gaps between adjacent neurons. Without neurotransmitters, brain neurons would be isolated and alone, wholly unable to communicate with each other and with the rest of the body.

The hallmark feature of PD is a loss of neurons governed by a neurotransmitter called "dopamine."(4) These cells are found in areas of the brain that allow us to control voluntary muscles. When we want to move, walk, write with a pen, throw a ball, swing a golf club, drive a car, or do just about anything, the brain sends a message to the muscles that perform the given task. The muscles contract properly and movement occurs. As dopamine-containing neurons die off in PD, signals from the brain that coordinate movement and muscle function are transmitted too slowly.(5) When the signals are received, the body is unable to respond normally.

The first sign of PD is often a slight tremor in one hand. This tremor is most visible when the individual with PD is standing or sitting still. The shaking is generally not noticeable when the hands are moving or doing something. However, tremor is absent in a small percentage of people with PD.(6) Other classic symptoms include slow movements, "shuffling" while walking, muscle stiffness, stooped posture, and a blank facial expression.(7, 8) Before these symptoms begin, a person with oncoming PD may experience vague, aching pain in the limbs, neck, or back and decreased spinal flexibility. Other early and subtle symptoms include impaired handwriting ability and low speaking volume.(9) As the disease progresses, cognitive function often suffers, and some patients develop dementia. In the later stages, people with PD often become depressed, which is certainly not difficult to understand.

Although much is known about the brain degeneration that occurs in Parkinson's disease, why this happens remains a mystery.(10) Viral infections and exposure to certain substances have been implicated, but in most cases PD is an "idiopathic" disease, one with no known cause.