Which toxin is responsible for the signs of tetanus by blocking relaxation of muscles?

• A. Botulinum toxin
• B. Staphylococcal enterotoxin
• C. Tetanospasmin
• D. Diphtheria toxin

Answer: (C)

Tetanus signs come from a toxin that prevents muscles from relaxing by blocking inhibitory signals in the nervous system. The toxin tetanospasmin, produced by Clostridium tetani, travels from the infection site into the spinal cord where it blocks the release of the inhibitory neurotransmitters GABA and glycine by cleaving the synaptic protein synaptobrevin. With inhibition lost, motor neurons fire unchecked, producing the sustained muscle contraction, rigidity, and painful spasms that define tetanus.

Botulinum toxin, in contrast, blocks acetylcholine release at the neuromuscular junction, causing flaccid paralysis rather than rigidity. Staphylococcal enterotoxins act as superantigens and drive GI symptoms and shock, not the specific tetanic muscle relaxation blockade. Diphtheria toxin inhibits protein synthesis by ADP-ribosylating elongation factor-2, leading to tissue damage rather than the targeted disruption of inhibitory signaling in motor pathways.