Botulinum toxin: A toxin produced by the bacterium Clostridium botulinum that is the most poisonous biological substance known. Botulinum toxin acts as a neurotoxin. It binds to the nerve ending at the point where the nerve joins a muscle, blocking the release by the nerve of the chemical acetylcholine (the principal neurotransmitter at the neuromuscular junction), preventing the muscle from contracting. The result is weakness and paralysis of the muscle. The muscle atrophies. The blockage of acetylcholine release is irreversible. Function can be recovered by the sprouting of nerve terminals and the formation of new synaptic contacts, which usually takes 2 to 3 months.
Very small amounts of botulinum toxin can cause botulism in one of two ways. One way is by ingesting the toxin itself (food borne botulism), as in canned foods. The other way is by infection with the bacterial spores that produce and release the toxin in the body (infectious botulism). The infection may occur in the intestine (intestinal botulism), as in a newborn (infant botulism), or deep within a wound (wound botulism).
There is more than one type of botulinum toxin. Different strains of the bacteria produce eight distinct neurotoxins. All eight types have a similar molecular weight and structure, consisting of a heavy chain and a light chain joined by a disulfide bond (most publications recognize only seven types; there are eight if the subtypes of C, C1 and C2, are counted as separate types. All eight types act in a similar manner. Only types A, B, E and F are known to cause botulism in humans. The toxin is heat labile and can be destroyed if heated at 80 C (175 F) for 10 minutes or longer although the CDC recommends boiling for 10 minutes or longer.
Purified botulinum toxin A was the first bacterial toxin to be used as a medicine. The US Food and Drug Administration (FDA) approved purified botulinum toxin A (Botox) in 1989 for treating two eye conditions: blepharospasm and strabismus. Botox has since found other medical and cosmetic uses and was FDA approved in 2002 for treatment of moderate-to-severe frown lines between the eyebrows known as glabellar lines. Botox is often used in other areas of the face for cosmetic purposes. It has been used in the treatment of other medical conditions as well, including migraine headaches, cervical dystonia ( a muscle disorder of the neck muscles), and primary hyperhidrosis (excessive sweating).
See also: Clostridium botulinum, botulism
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