The reasoning: two ways to lose the safety factor
- Myasthenia gravis (MG): antibodies to AChR (~85% of generalised MG; complement‑mediated destruction of junctional folds, receptor internalisation), MuSK or LRP4 → fewer functional receptors → smaller MEPPs and EPPs. Each normal rundown of release in a train then drops fibres below threshold → decrement and fatigability.
- Lambert–Eaton myasthenic syndrome (LEMS): antibodies to presynaptic P/Q‑type VGCC (≈ 50–60% paraneoplastic, mostly small‑cell lung cancer) → low quantal content; MEPPs normal; high‑frequency stimulation or brief exercise accumulates terminal Ca²⁺ → increment.