Fig. 2

Schematic diagram of the interplay among aging, genetic susceptibility and environmental toxins in the pathogenesis of PD. The red line represents the threshold of DA neuronal stress over which PD will manifest. With increasing age, impairment in cellular pathways such as oxidative stress, mitochondrial dysfunction increases the susceptibility of DA neurons to degenerate and thus lower the threshold for developing PD. During younger age, genetic susceptibility likely plays a more major role in causing PD, whereas cumulative exposure to environmental toxins likely has more significant contribution to PD development in old age