The effects of various toxicants on the body tissues cause tissue abnormalities resulting in dystrophic changes and necrosis. The nervous system is the most vulnerable to the effects of exogenic substances, both chemical and biological, due to high metabolic activity and the cells' incapability of self-renewal. Neurotoxicants lead to disturbances of cellular nutrition and eventually to neurodegeneration. Neurons can die due to both apoptosis and necrosis.