Certain chemicals (toxins) affect different systems within the body. Determining the effect of chemicals on the nervous system (neurotoxicants) relies on changes in the functional and cognitive behavior of the victim. This reliance is based on the changes that the toxicant imparts on the different physical attributes of the brain, spinal cord and other peripheral nerves. It is because of the changes in functional and cognitive behavior that neurotoxicologists are able to determine the different manifestations of toxins on the nervous system.
Neuronopathies or Toxins that Affect the Neurons
Certain toxins directly affect the neurons. In extreme cases these toxins can completely destroy the neuron; an effect that is irreversible. The destruction of a neuron leads to the destruction of its related parts which include the cytoplasm, dendrites and eventually the axons. Methyl mercury is an example of a neuron toxicant. This was apparent in the case where a population in Minamata Bay, Japan was exposed to large amounts of methyl mercury through their diet. The methyl mercury in this case destroyed the neurons in the brain responsible for sight and other neurons responsible for motor functions. Residents in the Minamata Bay area suffered from ataxia as a result of the poisoning.
Axonopathies or Toxins that Affect the Axons
These toxins affect the axons within the nervous system. Exposure to these toxins cause the axons along with the myelin sheath to die back. Since only the axons are affected and the neurons are left intact, these toxic injuries can be reversed. Organophosphorous esters are a good example of axonotoxicants. These chemicals are found in pesticides and are also used as additives in plastic products. They enter the nervous system and their effect is not noticeable for at least 7 to 10 days. Axonopathy presents symptoms similar to a patient suffering from multiple sclerosis.
Myelinopathies or Toxins Affecting Myelin
Myelin is important in the transmission of electrical impulses in the nerves. If myelin is damaged, nerve signals are slowed. Lead is a classic example of a neurotoxicant that affects myelin. Exposure to high levels of lead can cause encephalopathy (diseases that results in the disruption of brain function) such as cerebral edema. In lower concentrations lead also disrupts brain function; affecting intellectual ability.
Toxins that Affect Neurotransmitters
These toxins affect the way neurotransmitters work. They work to block, interrupt, or modify the way messages are transmitted. Nicotine and cocaine are examples of toxins that affect neurotransmitters. Nicotine binds to specific receptors that result in an elevated heart rate, increased blood pressure and the constriction of surface blood vessels. Cocaine on the other hand blocks the reuptake of certain neurotransmitters at the nerve ending.
Most of the lethal toxins produced by organisms are neurotoxins. They are complex molecules with effects that range through the different neurotoxin's effects. The majority are however, toxins that affect neurotransmitters. Exposure to these toxins results in serious illness, paralysis or death. These specific nervous system effects have led to researchers seeking practical applications for neurotoxins as therapeutic agents
Sources
Douglas C. Anthony et. al. "Toxic Responses of the Nervous System." Casarett & Doull's Toxicology: The Basic Science of Poisons (Klaassen CD, ed). 5th ed. New York:McGraw Hill. pp. 463-482.
