A muscle physiologist records the isometric tension produced by a motor unit at different stimulation frequencies. At 10 Hz, individual twitches are visible. At 50 Hz, a smooth fused tetanus is produced with much higher tension. What accounts for the marked increase in tension during tetanus compared to a single twitch?
- A Successive action potentials arrive before cytosolic Ca²⁺ has returned to baseline, allowing Ca²⁺ to accumulate and sustain troponin C saturation, maintaining cross-bridge cycling at maximum rate ✓
- B High-frequency stimulation recruits larger, type II motor units that were inactive at 10 Hz
- C Tetanic stimulation releases additional acetylcholine reserves, sensitizing the end-plate potential
- D Ca²⁺ re-enters the cytosol from extracellular space through L-type channels during tetanic stimulation
Correct answer: A. Successive action potentials arrive before cytosolic Ca²⁺ has returned to baseline, allowing Ca²⁺ to accumulate and sustain troponin C saturation, maintaining cross-bridge cycling at maximum rate
Explanation
During a single twitch, Ca²⁺ is released from the SR, briefly activates troponin C, then is rapidly re-sequestered by SERCA pumps before maximum tension is developed. At tetanic frequencies (>40 Hz), the interval between stimuli is shorter than the Ca²⁺ re-uptake time, causing cytosolic Ca²⁺ to accumulate to levels that keep troponin C continuously saturated. This allows maximal and sustained cross-bridge cycling, generating 3–5× greater tension than a twitch.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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