It’s not just a movement disorder. Besides causing tremors and other motion-related symptoms, Parkinson’s disease affects memory, learning, and behavior.

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Dopamine is a neurotransmitter, a chemical used to carry messages between neurons. Dopamine is produced in several areas of the brain, including the substantia nigra.

Dopamine's effects are complex and poorly understood, but dopamine appears to play a role in signaling reward in the brain. For example, "pleasurable" events such as eating, drinking, and having sex are all associated with increased brain dopamine levels, while individuals experiencing depression or anxiety may have lowered brain dopamine levels. Many drugs of abuse which give "pleasurable" or "calming" highs, such as cocaine and nicotine, appear to work by mimicking dopamine in the brain.

Parkinson's disease destroys dopamine-producing neurons in the substantia nigra, and causes motor symptoms (dyskinesia, tremor, rigidity) as well as cognitive symptoms. The motor symptoms can be treated by drugs that increase brain dopamine levels.

Schizophrenia is a disorder associated with abnormally high levels of brain dopamine; symptoms may include disordered thought, hallucinations and social withdrawal. These symptoms may be ameliorated by drugs that decrease brain dopamine levels.

Parkinson's patients given too much dopaminergic medication may develop schizophrenia-like symptoms, while schizophrenic patients given too much anti-dopaminergic medication may develop motor problems reminiscent of Parkinson's disease. Apparently, the brain requires a very delicate balance of dopamine to function normally.