Parkinson

Parkinson disease develops as a part of the brain known as the substantia nigra degenerates. The substantia nigra is located in the midbrain, halfway between the cerebral cortex and the spinal cord. In healthy people, the substantia nigra contains certain nerve cells, called nigral cells, that produce the chemical dopamine. Dopamine travels along nerve cell pathways from the substantia nigra to another region of the brain, called the striatum. In the striatum, dopamine activates nerve cells that coordinate normal muscle activity. In people with Parkinson disease, nigral cells deteriorate and die at an accelerated rate, and the loss of these cells reduces the supply of dopamine to the striatum. Without adequate dopamine, nerve cells of the striatum activate improperly, impairing a person’s ability to control movement.

A study published in 2000 found that people with Parkinson disease have a decreased number of nerve fibers in the heart. These results suggest that the disease affects nerves in organs outside the brain and may explain symptoms common in people with Parkinson disease, such as a drop in blood pressure when a person stands up, constipation, and difficulty urinating.

Scientists do not understand the mechanisms underlying nerve cell death in Parkinson disease. Most researchers believe that Parkinson disease results from a combination of factors involving genetics, environmental agents, and abnormalities in cellular processes.