A substance that damages or destroys neurons (nerve and brain cells). Neurotoxins can originate outside the body, such as through exposure to chemicals such as polychlorinated biphenyls (PCBs) and to certain heavy metals, pesticides, and herbicides. Some medications also can have neurotoxic effects, even when taken at recommended dosages. Neuro toxins also can arise within the body, such as from bacteria (endotoxins and exotoxins) and oxidants. There are thousands of known neurotoxins that cause various symptoms. Those that have been linked to Parkinson’s disease or Parkinson’s-like symptoms include PCBs, the pesticide rotenone, MPTP the inadvertent contaminant of illicit attempts to synthesize narcotics and metals introduced through industrial or pollutant exposure such as mercury or manganese, and metals such as copper or iron that may accumulate due to genetic defects. Some neurotoxic damage reverses when the neurotoxin exposure ends, but much of it is permanent.