Within a few hours after injection, BTX attaches to the nerve endings of motor muscles and blocks the release of acetylcholine, a neurotransmitter that depolarizes the motor end plate of the muscle to cause a contraction. Therefore, blocking acetylcholine will inhibit or lessen the muscle contraction, depending on the dose, and results in smoothing of the skin overlying these muscles. The physiology of the action is due to the fact that BTX inhibits the release of acetylcholine at the neuromuscular junction.8 The paralyzing effect generally takes 2-10 days to occur and there is no loss of sensation. The effects last approximately 3-4 months depending on such factors as the amount injected, the metabolic rate of the patient, and lifestyle choices – smokers may notice a more rapid return of wrinkles around the mouth and active/healthy individuals may experience as shorter duration of treatment effects due to a more rapid repair of synaptic contacts. After treatment, the muscles initiate new acetylcholine receptors and neural branches to form new synaptic contacts. This causes the muscle to gradually return to full function with no side effects.8 Timing of retreatment is dependent on return of symptoms.1
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