In a patient with myasthenia gravis, the decremental response on repetitive nerve stimulation is diagnostic. Which of the following correctly explains the electrophysiological basis of the decremental response at 3 Hz stimulation?

• A Repeated stimulation depletes immediately available acetylcholine from the readily releasable pool faster than it is replenished; in a normal NMJ there is a large safety margin of ACh quanta per impulse; in MG with reduced AChRs, the safety margin is already critically low, so depletion with repetitive stimulation fails to generate action potentials in an increasing fraction of fibers → decrement ✓
• B Anti-AChR antibodies cause Ca²⁺ channel blockade at the presynaptic terminal, progressively reducing ACh release with each stimulus
• C AChR complement-mediated destruction accelerates during repetitive stimulation due to increased antibody-receptor cross-linking exposing more complement epitopes
• D Repetitive stimulation recruits additional motor units in MG, causing summation that paradoxically reduces individual end-plate potentials

Explanation

Normal NMJs have a large safety factor: each nerve impulse releases far more ACh quanta than needed to reliably trigger a muscle action potential. During repetitive stimulation, the readily releasable ACh vesicle pool is partially depleted between stimuli (especially at 3–5 Hz), reducing quantal content per impulse. In normal NMJs this depletion is within the safety margin and muscle response is maintained. In myasthenia gravis, anti-AChR antibodies (or anti-MuSK) have already reduced functional AChR density, critically narrowing the safety factor. The additional reduction in ACh release with repetitive stimulation crosses below threshold for EPP-to-action potential conversion in an increasing proportion of muscle fibers, producing the characteristic >10% decrement in CMAP amplitude. Option B incorrectly attributes presynaptic Ca²⁺ channel blockade to MG antibodies (this is the Lambert-Eaton mechanism). Option C is a pathological process, not an electrophysiological mechanism. Option D is anatomically incorrect.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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