**3. Types of toxins and intoxications**

Chemicals that disrupt the mammalian nervous system can occur naturally (neurotoxins) or be produced (neurotoxicants). While the term "neurotoxins" refers to substances with neurotoxic potential, this is not an inherent quality of the chemicals but rather a description of the effect that may occur when the tissue concentration surpasses a certain threshold. Neurotoxic biological substances usually demonstrate a high level of target selectivity and toxicity. Microorganisms, reptiles, and vertebrates exhibit direct or indirect neurotoxic effects that are wellunderstood mechanistically (**Table 1**) [3, 4].

Other naturally occurring compounds with less strong qualities have been shown to cause neurotoxicity when administered in high concentrations for a sustained length of time. Metals (arsenic, lead, and mercury) and other elements and compounds, such as selenium and vitamin B6, come into this category. While these chemicals are neurotoxic in high concentrations, they are required in trace levels to maintain proper physiological function, particularly in the nervous system. Natural enzymes (thiaminase) that metabolize necessary chemicals (thiamine) are also associated with neurological disorders in both animals and humans. Synthetic chemicals with neurotoxic potential are most frequently obtained through a


#### **Table 1.**

*Natural mammalian neurotoxic potential substances [3, 4].*


#### **Table 2.**

*Potentially neurotoxic heavy metals and synthetic substances [3, 4].*

prescription (vincristine, ethambutol, isoniazid) and over-the-counter (bismuth preparations) pharmaceutical aisles; (pyridethione) products used in antidandruff shampoos; (2,6-dinitro-3-methoxy-4-tet-butyltoluene) fragrance raw materials; and (acrylamide) pyrolysis products used in broiled, baked. Others are associated with particular applications, such as chemical warfare in military and civilian settings (sarin). Directly neurotoxic substances are supplemented by medications that change neurological function due to their effects on another organ system on which the brain relies for proper operation. This class of medications includes those that target the lung, kidney, and liver particularly, as well as drugs that disrupt the nervous system's constant supply of oxygen (cyanide, azide) and glucose (glucose) (6-chloro-6-deoxyglucose). Chronic liver failure and manganese toxicity are associated with increased signal abnormalities in the basal ganglia on T1-weighted magnetic resonance images, implying that the metal accumulates due to the liver's general inability to eliminate it (**Table 2**) [3, 4].
