A neurotoxin is a toxin that acts specifically on nerve cells, or neurons, typically by interacting with membrane proteins and especially ion channels. Many of the venoms and other toxins that organisms use in defense against vertebrates are neurotoxins. A common effect is paralysis, which typically onsets extremely rapidly. Examples of neurotoxins include tetrodotoxin and components of the venom in bees, scorpions and spiders.
Neurotoxins are generally chemicals, although forms of radiation and ultrasonic vibration can also kill or impair neurons.
Toxins taken in form 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 at safe dosages, only producing mild neurotoxicity. Prolonged exposure to safe alcohol doses 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 the 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, named so because the neuron is excited to death.
