Botulinum toxin can be injected into humans in extremely small concentrations and works by preventing signals from the nerve cells reaching muscles, effectively leaving the muscles without instructions to contract, therefore paralyzing them.24
In order for muscles to contract, nerves release a chemical messenger, acetylcholine (a neurotransmitter), at the junction where the nerve endings meet muscle cells. The acetylcholine attaches to receptors on the muscle cells and causes the muscle cells to contract or shorten. Injected botulinum toxin prevents the release of acetylcholine, preventing contraction of the muscle cells. The effect of botulinum toxin causes a reduction in abnormal muscle contraction allowing the muscles to become less stiff.