Nerve agents are substances that disrupt the chemical communications through the nervous system. One mechanism of disruption, utilized by the G, GV, and V series of chemicals is caused by blocking the acetylcholinesterase, an enzyme that normally destroys and stops the activity of acetylcholine, a neurotransmitter. Poisoning by these nerve agents leads to an accumulation of acetylcholine at the nerve axon, producing a perpetual excited state in the nerve (e.g. constant muscle contraction). The eventual exhaustion of muscles leads to respiratory failure and death. A separate class of nerve agents are related to Tetrodotoxin, frequently abbreviated as TTX, is a potent neurotoxin with no known antidote. Tetrodotoxin blocks action potentials in nerves by binding to the voltage-gated, fast sodium channels in nerve cell membranes, essentially preventing any affected nerve cells from firing by blocking the channels used in the process.