What is the action of botulinum toxin at the neuromuscular junction?

Botulinum toxin exerts its effect at the neuromuscular junction primarily by inhibiting the release of acetylcholine. When botulinum toxin enters the presynaptic terminal of the motor neuron, it cleaves proteins essential for the fusion of the synaptic vesicles that contain acetylcholine. This prevents the vesicles from releasing their contents into the synaptic cleft, effectively blocking the ability of the motor neuron to communicate with the muscle fiber. As a result, there is a temporary paralysis of the muscle because acetylcholine is necessary for muscle contraction.

This mechanism is crucial for understanding various clinical applications of botulinum toxin, such as in the treatment of muscle spasticity, cosmetic applications to reduce wrinkles, and in managing certain headache types. The inhibition of acetylcholine release is a clear and specific action that differentiates botulinum toxin from other neuroactive substances that may have different mechanisms of action.