Neurotoxicology is the subdivision of toxicology that examines the impact of endo- and exotoxins on the nervous system. All toxic agents are not equally neurotoxic. In other words, some toxic substances have particularly strong effects on the nervous system and its activity. For example, mercury, a heavy metal toxicant, has been shown to undergo incorporation into the amino acid cysteine, forming a new molecule called methylmercury cysteine. Accumulations of this molecule are associated with a variety of nervous system disorders, including alterations in vision, ataxias, and paresthesias.
Nerve cells have specialized receptors for receiving neurotransmitted signals.
In the brain, nervous system activity depends upon proper balance of these neurotransmitter substances and receptor activity that is neither excessive nor deficient. Overstimulation of the neurotransmitter receptors has been shown to be involved with a variety of neurological disorders, including stroke, epilepsy, and Huntington's disease. One series of key links in the process appears to be overstimulation of receptors by excitatory amino acids. When an electrical impulse traveling along the nerve reaches the axon, the neurotransmitter is released and travels across the synapse, either prompting or inhibiting continued impulses. There are more than 300 known neurotransmitters, including the endorphins and acetylcholine.