Why does botulinum toxin paralyse muscles while tetanus toxin makes them rigid? · Grey Matter

Both toxins are enzymes that cut SNARE proteins, and without intact SNAREs vesicles cannot fuse.

Why does botulinum toxin paralyse muscles while tetanus toxin makes them rigid?

Both toxins are enzymes that cut SNARE proteins, and without intact SNAREs vesicles cannot fuse. Botulinum toxins act on motor nerve terminals at the neuromuscular junction, so acetylcholine release stops and the muscle goes limp; that is also how cosmetic and medical botulinum injections relax a chosen muscle. Tetanus toxin is carried backward up the motor axon into the spinal cord, where it cuts synaptobrevin in the inhibitory interneurons that release glycine and GABA onto motor neurons. With that brake gone the motor neurons fire unchecked and the muscles lock in spasm, so the same molecular cut gives opposite symptoms depending on which synapse loses it.