Mechanism of action of botulinum neurotoxin: Unexpected consequences.

Botulinum neurotoxin (BoNT) is a widely used therapeutic in part because its mechanism of action is much wider than initially expected. Since BoNT is taken up more avidly in active presynaptic terminals, there is some selectivity for weakening muscles involved in frequent involuntary movements. BoNT blocks gamma motoneurons as well as alpha motoneurons, hence reducing afferent spindle activity which appears to have a favorable effect. Some BoNT is retrogradely transported in the motor axons, leading at least to reduction in recurrent inhibition mediated by the Renshaw cell. There are also central nervous system changes after BoNT injections and these may be due to brain plasticity.