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A neurotoxin is a toxin that acts specifically on nerve cells – neurons – usually by interacting with membrane proteins such as ion channels. Many of the venoms and other toxins that organisms use in defense against vertebrates are neurotoxins. A common effect is paralysis, which sets in very rapidly. The venom of bees, scorpions, pufferfish, spiders and snakes can contain many different toxins. Many neurotoxins act by affecting voltage-dependent ion channels. For example, tetrodotoxin and batrachotoxin affect sodium channels, maurotoxin, agitoxin, charybdotoxin, margatoxin, slotoxin, scyllatoxin and hefutoxin act on potassium channels, and calciseptine, taicatoxin and calcicludine act on calcium channels.

Toxins ingested from the environment are described as exogenous and include gases (such as carbon monoxide), metals (such as mercury), liquids (ethanol) and an endless list of solids. When exogenous toxins are ingested, 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^{[citation needed]}.

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^{[citation needed]}. This whole process is called excitotoxicity, so named because glutamate normally acts as an excitatory neurotransmitter at lower levels.^{[citation needed]}

A potent neurotoxin such as batrachotoxin affects the nervous system by causing depolarization of nerve and muscle fibres due to increased sodium ion permeability of the excitable cell membrane.

A number of artificial neurotoxins, known as nerve agents, have been developed for use as chemical weapons.

A very potent neurotoxin is tetrodotoxin. This chemical acts to block sodium channels in neurons, preventing action potentials. This leads to paralysis and eventually death.

• Neurotoxicity
• Teratogenesis, many birth defects are essentially caused by neurotoxicity in developing neurons
• Penitrem A