Medical News Today looks at present understanding of the biochemical mechanisms that lead to Parkinson's disease, "in which nerve cells in part of the brain called the substantia nigra die, resulting in the loss of dopamine, a nerve-signaling molecule that helps control muscle movement. The absence of dopamine from these cells, called dopaminergic neurons, causes a loss of muscle control, trembling and lack of coordination. ... The molecule that prevents damage to the substantia nigra is an enzyme called GST pi ... This molecule stands like a sentry at the crossroads of several biochemical pathways, any one of which can lead to Parkinson's disease ... The study sheds light on the cause of most cases of Parkinson's disease, which currently are unexplained. ... The majority of these cases of Parkinson's disease appear to arise because individuals who have a genetic susceptibility to the disease are exposed to environmental toxins such as pesticides and herbicides, which trigger the formation of free radicals that kill dopaminergic neurons in the substantia nigra. We also know that GST pi blocks the process of cell suicide triggered by stresses that the cell can't overcome, such as an increase in the presence of free radicals or a loss of the cell's ability to produce energy."