What is tetanus as a contraction phenomenon, and how is it produced?

• A. A rapid, single twitch.
• B. A contraction with incomplete relaxation.
• C. A voluntary movement causing fatigue.
• D. A sustained, smooth contraction produced by high-frequency stimulation resulting in continuous calcium presence and no relaxation.

Answer: (D)

Tetanus is a sustained, smooth contraction that happens when muscle fibers are stimulated so rapidly that there isn’t time for the muscle to relax between stimuli. The reason this occurs is that high-frequency nerve impulses keep calcium ions in the cytosol, keeping the cross-bridge cycling between actin and myosin ongoing. With each quick impulse, more cross-bridges are formed before the previous contraction can relax, so the tension summates into a continuous, fused contraction.

This is different from a rapid, single twitch, which results from a lone stimulus and then relaxation. It’s also different from incomplete tetanus, where there’s high frequency but some relaxation still occurs between contractions. And voluntary movement fatigue is a broader concept related to endurance, not the defining pattern of a tetanus contraction.