Introduction:

Parkinson's disease (PD) is a slowly progressive disorder that affects movement, muscle control, and balance. Parkinson's disease is referred to as idiopathic, which means that the cause is unknown. This term distinguishes the primary disease from parkinsonism, which are the symptoms occurring from a known cause. In addition to its effects on motor control, Parkinson's disease is now recognized as a broader condition that can include cognitive and behavioral disturbances, sleep disorders, speech difficulties, and other problems.

Parkinson's Disease and Dopamine Loss

Parkinson's disease occurs from the following process in the brain:

• PD develops as cells are destroyed in certain parts of the brain stem, particularly the crescent-shaped cell mass known as the substantia nigra.

Parkinson's disease is a slowly progressive disorder that affects movement, muscle control, and balance. Part of the disease process develops as cells are destroyed in certain parts of the brain stem, particularly the crescent-shaped cell mass known as the substantia nigra. Nerve cells in the substantia nigra send out fibers to tissue located in both sides of the brain. There the cells release essential neurotransmitters that help control movement and coordination.

• Nerve cells in the substantia nigra send out fibers to the corpus stratia, gray and white bands of tissue located in both sides of the brain.
• There the cells release dopamine, an essential neurotransmitter (a chemical messenger in the brain). Loss of dopamine in the corpus stratia is the primary defect in Parkinson's disease.

Dopamine. Dopamine deficiency is the hallmark feature in PD. It is one of three major neurotransmitters known as catecholamines, which help the body respond to stress and prepare it for the fight-or-flight response. Loss of dopamine negatively affects the nerves and muscles controlling movement and coordination, resulting in the major symptoms characteristic of Parkinson's disease. Dopamine also appears to be important for efficient information processing, and deficiencies may also be responsible for problems in memory and concentration that occur in many patients.

Although it is clear that dopamine deficiency is the primary defect in Parkinson's disease, it is not clear what causes dopamine loss. The culprit is less likely to be a single cause than a combination of genetic and biologic factors, which are triggered by some environmental assault.

Other Changes. The PD disease process also appears to impair nerve endings in the heart, causing dysautonomia-- changes in the autonomic (also called sympathetic) nervous system. Such changes may impair the release of norepinephrine, a hormone that regulates blood pressure, pulse rate, perspiration, and other automatic responses to stress. Evidence suggests this may be responsible for the abrupt drops in blood pressure when standing that occur in PD. Further research is underway to determine if the loss of nerve terminals is confined to the heart or if it affects other organs as well.

Other Biologic Factors

Research suggests that three molecules are critical in the development of inherited PD: alpha synuclein, parkin, and ubiquitin, which all interact in the normal brain. Normally, parkin and ubiquitin, are involved in the natural process of programmed cell death called apoptosis. If this process goes awry, for instance, with a defective parkin gene, cell death fails to occur. If synuclein is not eliminated in these cells, it builds up and becomes toxic to dopamine. In such cases, synuclein accumulates in Lewy bodies, the deposits of fibrous tissue found in all patients with PD.

Lewy bodies are the hallmark signs of Parkinson's disease. They are found in the substantia nigra, the place in the brain where dopamine is first released. These substances are also present in other diseases that cause dementia, such as Alzheimer's, and can occur in people without neurologic symptoms.

Another protein, beta amyloid, also increases the build-up of synuclein. Beta amyloid is a known factor in Alzheimer's disease, and may help explain the co-existence between Alzheimer's and Parkinson's disease in many patients.

Other biologic factors that may play a role in Parkinson's disease are also being investigated.

Genetic Factors

Specific genetic factors appear to play a strong role in early-onset Parkinson's disease, an uncommon form of the disease. Recent research suggests that multiple genetic factors may also be involved in some cases of late-onset Parkinson’s disease.

Environmental Triggers

Environmental toxins, infections, and other triggers can provoke excessive production in the body of oxygen free-radicals, damaging particles that may play a major role in the deterioration of nerve cells that lead to Parkinson's.

Infectious Organisms. Some research has identified immune factors that suggest a viral presence in the Lewy bodies and swollen nerve pathways of Parkinson's brains. Influenza and other potent viruses have long been known to be a cause of parkinsonism. In one well-known example, a major flu epidemic causing encephalitis in the early twentieth century left many of its victims with parkinsonism.

Environmental and Industrial Chemicals. Intense exposure to certain environmental and industrial chemicals is also being studied.

• Pesticides and Herbicides. Some evidence implicates pesticides and herbicides as important factors in many cases of Parkinson's disease. A higher incidence of parkinsonism has long been noted in people who live in rural areas, particularly those who drink private well water or are agricultural workers.
• Other Chemicals. Intense exposure to other industrial chemicals and metals (manganese, copper, lead, iron, mercury, zinc, aluminum, and others) has also been linked with parkinsonism, a cause that is often reversible. The role of long-term exposure in the development of Parkinson's disease is unclear. High levels of iron content observed in critical parts of the brain in PD are under particular scrutiny.

Aging Process

Most Parkinson's victims are elderly. Some studies indicate that the very elderly are not susceptible to the disease, indicating that the aging process itself is not the major player in the disease. Aging does appear to reduce the concentration of dopamine in structures called dopamine transporters, which carry the neurotransmitter back and forth between nerve cells. Some researchers posit that any excessive stress on these transporters might trigger Parkinson's disease in the aging, and more vulnerable, brain.