Development of Parkinson's Disease

How Parkinson's disease develops:
Generally, in Parkinson's disease, brain cells called neurons, which produce a substance called dopamine -- a chemical that helps the nervous system control muscle activity -- begin to deteriorate. The exact reason for this is not known. When the neurons are destroyed, dopamine is not produced at the normal rate, and an abnormally low supply of dopamine causes Parkinson's symptoms to appear.

More specifically, a small area in the brain called the substantia nigra produces dopamine. Messages pass between the cells in the substantia nigra and the cells in another part of the brain called the striatum with the aid of dopamine, which acts as a transmitter.

In Parkinson's disease, the cells in the substantia nigra begin to die. In turn, less dopamine is made and less is transported to the striatum, a coordination center for the various chemicals. When there is not enough dopamine, a chemical imbalance occurs, which effects movement, balance, and walking. In later stages of the disease, cells in other portions of the brain and nervous system may also degenerate.

Initially, the brain may adjust to the loss of these cells in the substantia nigra through increased activity of the remaining cells, or through an increase in the sensitivity of the cells in the striatum to the actions of dopamine. Additional amounts of dopamine may also be made by other cells in the striatum, which do not produce dopamine under ordinary conditions.

These are called compensatory mechanisms and may cause a reduction or delay in the appearance of PD symptoms. Eventually, however, these mechanisms often fail and symptoms reappear.

There may also be a dopamine deficiency in other parts of the brain. It is believed that the dopamine deficiency in the striatum is responsible for the primary symptoms of the disease, while the dopamine deficiency in other parts of the brain may cause some of the secondary symptoms.