Under normal circumstances, a nerve comes into contact with a muscle at a neuromuscular junction. The nerve releases a neurotransmitter called acetylcholine.  Acetylcholine travels to the muscle resulting in contraction of the muscle.

Botox keeps the neurotransmitter from stimulating the muscle, thereby keeping the muscle from contractin.

This is the reason that Botox® is helpful for "dynamic" wrinkles, or wrinkles that result from the contraction of muscles. With time and with repeated injections of Botox®, the muscles that produce unwanted wrinkles will become smaller, and this may result in treatments that last longer or in treatments that require few units of the toxin.