Figure 1.

PSCs during synaptic competition at the mammalian NMJ. A, Diagram illustrating the experimental situation where transmitter release from a dually innervated NMJ was recorded with a sharp microelectrode and synaptic responses evoked by independent stimulation of two different ventral roots. B, Labeling of presynaptic terminals (green), PSCs (blue), and postsynaptic nAChR (red) of P7 Soleus NMJs. At this age, most NMJs were poly-innervated (as shown in C1 and marked with “*”), but some NMJs were already mono-innervated. C1, At dually innervated NMJs, two independent axonal branches (in green, arrows) converged at the same endplate area (red). Also note that a single PSC (blue) covered the whole endplate area and both nerve terminals. PSCs counted (arrowheads) were positive for S100β (blue) and DAPI (gray) staining and were located at the endplate (red). C2, At mono-innervated NMJs, one axon (in green, arrow) was observed at the endplate area (red). This NMJ was from another muscle preparation than in B and C1. D, Histogram of the number of PSCs at poly-innervated and mono-innervated NMJs. Note that there were more PSCs at mono-innervated than poly-innervated NMJs. Unpaired t test, p < 0.0001. E, Distribution of the number of PSCs at mono-innervated and poly-innervated NMJs. Note that most poly-innervated NMJs had 1 or 2 PSCs. Scale bars: B, 10 μm; C1, C2, 5 μm.