Reduction of presynaptic vesicles in VA motor neurons in unc-4(e120). Vesicles were counted in EM photos of VA and VB cross sections showing PSDs (arrows). A, B, Presynaptic regions of VA (A) and VB (B) motor neurons in wild type are shown. C, D, Fewer vesicles are seen in the presynaptic region of VAs (C) versus VBs (D) in unc-4(e120). The circumference of individual VA and VB motor neuron processes is outlined with a heavy black border. Handwritten numbers on these prints were used to trace individual neurons through these serial sections for reconstruction of the wild-type andunc-4(e120) ventral nerve cords (White et al., 1986,1992). E, Plot of vesicles per PSD in VA and VB motor neurons in wild-type versus two unc-4-mutant animals [unc-4(1), unc-4(2)] is shown. Application of the Mann–Whitney statistical test indicates that the decreased number of vesicles in VA motor neurons inunc-4(e120) is significantly different from that of the VBs (*p < 0.005; **p ≪ 0.001). Error bars indicate SD.