Toxins taken in from the environment are described as exogenous and include gases (eg carbon monoxide), metals (such as mercury), liquids (ethanol) and an endless list of solids. When exogenous toxins are taken in, the effect on neurons is largely dependent on dosage. Thus ethanol (alcohol) is inebriating in low doses, only producing mild neurotoxicity. Prolonged exposure to "safe" alcohol levels slowly weakens and kills neurons.
Neurotoxicity also occurs from substances produced within the body - endogenous neurotoxins. A prime example of a neurotoxin in the brain is glutamate, which is paradoxically also a primary neurotransmitter. When the glutamate concentration around a neuron reaches a critical point the neuron kills itself by a process called apoptosis. This whole process is called excitotoxicity, so named because glutamate normally acts as an excitatory neurotransmitter at lower levels.
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