Reduced size of the functional vesicle pool within motor-nerve terminals contributes to lowered quantal content of MuSK-MG-affected TS preparations.

(A) Representative time course of quantal content for EPCs in control or MuSK-MG-affected TS muscles that were stimulated at 50 Hz; X axis is the stimulus number (log 2 scale). (B) Representative graph showing the summation of quantal content during the 2 minute, 50 Hz train. The Y axis value is the summation of all quantal contents prior to the EPC number plotted on the X axis. The final value, or Y axis endpoint, of each curve is the total number of vesicles released during the 2 minute, 50 Hz stimulus train. (C) The total number of quanta released during the 2 minute, 50 Hz stimulus train is significantly (* P<0.05) less for MuSK-MG-affected (8 NMJs, 4 mice) muscles than that for vehicle-injected control muscles (7 NMJs, 3 mice). (D) Distribution of mEPC amplitudes as a percent of the total mEPCs used to calculate the mean amplitude shown in (Figure 2B). While the modal value remains equivalent to that of control, reduced numbers of mEPCs larger than 2–3 nA accounts for the decline of mean mEPC of MuSK-MG-affected TS preparations.