What Causes Parkinson's Disease?

Coronal view of a human brain with Parkinson's disease
Coronal view of a human brain with Parkinson's disease. Sherbrooke Connectivity Imaging Lab (SCIL)/Cultura/Getty Images

Parkinson's disease occurs when certain nerve cells (which are called neurons) located in a particular part of your brain stop working properly or die. These neurons normally produce an important brain chemical called dopamine, which helps to control muscle activity.

When you don't have enough dopamine because these neurons aren't producing it, you can't direct or control your muscle movements normally. That's because the area of your brain that houses these dopamine-making neurons (a brain region known as the substantia nigra) can't transmit signals to the next so-called "relay station" of the brain, the corpus striatum.

Studies have shown that Parkinson's patients have a loss of 80% or more of dopamine-producing cells in the substantia nigra. This lack of dopamine leads to the jerky, stiff movements found in Parkinson's disease.

What Causes Loss of Dopamine in Parkinson's?

Scientists don't know why the brain cells in the substantia nigra part of your brain stop making dopamine, but there are several theories.

Genetics may play a role in some cases of Parkinson's disease. Some 15% to 25% of those diagnosed with Parkinson's also have a relative who has the condition, indicating a possible genetic link. In addition, there are certain types of Parkinson's disease that do run in families, and some of the genes involved have been identified.

But most people with Parkinson's disease don't seem to have a strong family history of the condition, and so researchers are looking elsewhere for the condition's cause.

Theories on Root Causes

One theory involving the root cause of Parkinson's disease — the destruction of the nerve cells that make dopamine — holds that the cells become damaged due to free radicals in the body. Free radicals are unstable, potentially damaging molecules made by normal chemical reactions in the body.

Free radicals react with neighboring molecules (especially metals such as iron) in a process called oxidation. Oxidation is thought to cause damage to tissues, including neurons. Normally, the free radical damage is kept under control by antioxidants, chemicals that protect cells from this damage.

Patients with Parkinson's disease have been found to have increased brain levels of iron, especially in the substantia nigra, and decreased levels of ferritin, a protein found in the body which surrounds the iron and isolates it, thereby protecting the body's tissues from it.

Another theory involves pesticides and other toxins. Some scientists have suggested that Parkinson's disease may occur when a toxin in the environment destroys the neurons that make dopamine. There are a number of toxins (1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine, or MPTP, is one) that can cause symptoms of Parkinson's disease.

So far, though, no research has provided firm proof that a toxin is the cause of the disease.

Yet another theory proposes that Parkinson's disease occurs when, for unknown reasons, the normal, age-related wearing away of dopamine-producing neurons speeds up in certain individuals. This theory is supported by the idea that we slowly lose the mechanisms that protect our neurons as we age.

Many researchers believe that a combination of these four mechanisms — oxidative damage, environmental toxins, genetic predisposition, and accelerated aging — may ultimately be shown to cause the disease.

View Article Sources
  • Parkinson's Disease Foundation. Causes fact sheet.
  • Parkinson's Disease Foundation. Environmental Factors and Parkinson's: What Have We Learned? fact sheet.