Question

Botulinum toxin inhibits muscle contraction presynaptically by decreasing the amount of ACh released into the neuromuscular junction. In contrast, curare acts post-synaptically, blocking the nicotinic ACh receptors and preventing the excitation of the muscle cell membrane. Tetrodotoxin blocks voltage-sensitive Na⁺ channels, impacting both the initiation and the propagation of action potentials in the motor neuron. Both ACh and neostigmine stimulate muscle contraction. What is the mechanism of action of botulinum toxin?

1) Inhibits the release of ACh into the neuromuscular junction
2) Blocks the nicotinic ACh receptors
3) Blocks voltage-sensitive Na⁺ channels
4) Stimulates muscle contraction

Answer 1

Botulinum toxin inhibits muscle contraction by preventing the release of acetylcholine from motor neurons, interrupting the signal needed for the muscles to contract.

Step-by-step explanation:

The mechanism of action of botulinum toxin involves inhibiting the release of acetylcholine (ACh) from motor neurons at the neuromuscular junction. This is accomplished by the light A subunit of the toxin, which functions as a protease to cleave proteins essential for the synaptic vesicles to fuse to the membrane and release ACh. As a result, the signal for muscle contraction is removed, leading to muscle relaxation, and in severe cases, this can be life-threatening due to cease of breathing.