In Parkinson’s disease, nerve cells in the brain called neurons slowly break down or die. Many Parkinson’s disease symptoms are caused by a loss of neurons that produce a chemical messenger in the brain. This messenger is called dopamine.

Decreased dopamine leads to irregular brain activity. This causes movement problems and other symptoms of Parkinson’s disease. People with Parkinson’s disease also lose a chemical messenger called norepinephrine that controls many body functions, such as blood pressure.

The cause of Parkinson’s disease is unknown, but several factors seem to play a role, including:

• Genes. Specific genetic changes are linked to Parkinson’s disease. But these are rare unless many family members have had Parkinson’s disease.
• Environmental factors. Exposure to certain toxins or other environmental factors may increase the risk of later Parkinson’s disease. One example is MPTP, a substance that can be found in illegal drugs and is sometimes sold illegally as “synthetic heroin.” Other examples include pesticides and well water used for drinking. But no environmental factor has proved to be a cause.

Many changes happen in the brains of people with Parkinson’s disease. Researchers are studying why the changes happen and the roles they play. These changes include:

• The presence of Lewy bodies. Clumps of proteins in the brain are associated with Parkinson’s disease. These are called Lewy bodies, and researchers believe these proteins hold an important clue to the cause of Parkinson’s disease.
• Alpha-synuclein found within Lewy bodies. Alpha-synuclein is a protein found in all Lewy bodies. It occurs in a clumped form that cells can’t break down. This is currently an important focus among Parkinson’s disease researchers. Alpha-synuclein has been found in the spinal fluid of people who later have Parkinson’s disease.
• Altered mitochondria. Mitochondria are powerhouse compartments inside cells that create most of the body’s energy. Changes to mitochondria can cause cell damage. These changes have been found in the brains of people with Parkinson’s disease.