Parkinson's Disease is probably the most widely known movement disorder. It is characterized by the triad of tremor (rhythmic shaking, often involving one or both hands), bradykinesia (slowness of movement), and rigidity (increased muscle tone making movement difficult). The disease is the result of loss of neurons in a small area of the brain known as the substantia nigra.

When functioning normally, these neurons are responsible for acting on a complex neuronal circuit known as the basal ganglia with a neurotransmitter called dopamine. By the time a patient becomes symptomatic, the majority of the cells in the substantia nigra have died, markedly reducing the amount of dopamine acting on the basal ganglia.

This causes a chemical imbalance that leads to a complex cascade of changes in the functioning of the nuclei that make up the basal ganglia. The primary result of these changes is an inhibition of normal movement. This leads to the 'off' symptoms of bradykinesia and rigidity.

In the early stages of Parkinson's a patient's symptoms may be mild and often affect only one side of the body. Parkinson's disease is a progressive illness, however, and over time most patient's symptoms become progressively more severe.