Fig. 6.

The number of endocytic sites in a bouton is fixed. Data from three snakes (circles,triangles, squares) are shown. Each of five muscles from each snake received a precise number of stimuli in the presence of SR101 (5 Hz; x-axis inA–D). Dots and large structures were analyzed in 33–161 boutons from each muscle. A, The number of visible small endocytic sites increased rapidly with initial stimulation and then remained at a density corresponding to ∼26 sites per average-sized bouton. B, The mean brightness of sites increased monotonically with stimulation. C, The number of visible large structures (putative endosomes) was 0 for 25 and 50 stimuli and then rose monotonically with further stimulation.D, Large structures were brighter than endocytic sites, but their brightness was relatively insensitive to stimulation time; note the expanded brightness scale compared with that inB.