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Botulinum toxin acts on the end of nerves that control either muscles or certain glands such as sweat glands. In the very small doses that it is used clinically, it has to be injected into the area where the effect is required. If it were swallowed, in the very small doses used clinically, the stomach acid would destroy it and it would have little or no effect whatsoever.
When Botulinum toxin is injected, it affects the nerves in the area just around where it is injected. Usually the effect only spreads for a few millimetres or up to a centimetre. This spread can be controlled by changing the volume injected and the amount of toxin in the solution. A trained clinician will therefore consider both the volume they are injecting as well as the dose of Botulinum toxin.
The Botulinum toxin goes in to the local nerves (called motor neurons) that control the muscles or glands, where it destroys the very end of them. This action is at the very junction where the nerve connects with the muscle or gland at a place called the synapse.
The Botulinum toxin permanently destroys the end of the nerve. This stops it releasing the chemical messenger (called the neuron-transmitter). Therefore the nerve impulse does not reach the muscle or gland. This means that the muscle supplied by that nerve is paralysed or the gland supplied by that nerve stops secreting.
We know clinically that Botulinum toxin wears off usually after some three months to one year, depending where it is being used or what dose is being given. This is not due to the nerve starting to work again but is due to the fact that the nerve grows a new end. This new end buds out and passes round the original end, binding again with the muscle or gland. However, this new bud is never as effective as the original nerve end and so the muscle gland never regains its full function once one dose has been carried out. This is usually beneficial in the clinical setting.
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