What is a neurotoxin, and where are they coming from?

Many types . A neurotoxin is any compound that causes dysfunction or death of nerve cells. Neurotoxins range from elements such as lead which affects the function of certain enzymes, to botulinum toxin that paralyzes muscles to acetylcholinesterase inhibitors (in insecticides) that overstimulate the nervous system. The amount of the compound also matters e.g. Botox can be used medically or can kill you.