Loss of nerve cells
Parkinson's disease is caused by a loss of nerve cells in the part of the brain called the substantia nigra.
Nerve cells in this part of the brain are responsible for producing a chemical called dopamine. Dopamine acts as a messenger between the brain and the nervous system, and helps control and co-ordinate body movements.
If these nerve cells become damaged or die, the amount of dopamine in the brain is reduced. This means that the part of the brain controlling movement cannot work so well, which causes movements to become slow and abnormal.
The loss of nerve cells is a slow process. The level of dopamine in the brain falls over time. Only when 80% of the nerve cells in the substantia nigra have been lost will the symptoms of Parkinson's disease appear and gradually become more severe.
It is not known why the loss of nerve cells associated with Parkinson's disease occurs.
Research is ongoing to identify potential causes.
Genetics
Rarely Parkinson's disease can run in families; in this case abnormal genes are responsible but the exact role genetics plays in causing ordinary (sporadic) Parkinson's disease is unclear
So far at least nine genetic mutations have been identified as increasing a person’s risk of developing Parkinson’s disease (a genetic mutation is when the instructions carried in all living cells become scrambled in some way meaning that one or more functions of the body does not work in the right way).
However it is thought that in most cases genetics is not solely responsible for Parkinson’s disease and there needs to be an environmental factor to trigger it in genetically susceptible people.
Environmental factors
Some researchers think that exposure to toxins (harmful chemicals) could be the environmental trigger. Possible toxins could include:
- pesticides and herbicides used in farming
- toxin release by industrial plants
- air pollution related to road traffic
Arguably the most compelling evidence that toxins play a role is that drug users who injected themselves with a heroin substitute called MPTP went on to quickly develop Parkinson-like symptoms.
It was found that once MPTP crossed into the brain it started killing brain cells. It is possible other toxins could have a similar effect.
