Fig. 4. Effect of presynaptic inhibition on Ca entry into terminal varicosities. (A) The complex anatomical relationship between excitor (black) and inhibitor (grey) axons and variation in the amount of inhibition between terminals. Ca entry was reduced by 0-40% at different terminals with inhibition. (B) Greater inhibition was not observed along a string of terminal boutons. (C) Greater inhibition at a terminal on a quaternary branch compared with those on tertiary branches. Inhibition at the six terminals on the tertiary branch ranged from 12 to 28%, while the terminal on the quarternary branch (Q) experienced approximately 40% inhibition. Inhibitor terminals are located in the vicinity of the bottleneck structure forming the branch of the excitor axon. Overall, there was slightly more inhibition at terminals on quaternary branches (24±2%, N=36 terminals) than on those on tertiary branches (18±1%, N=86 terminals, two-sample t-test, P<0.05). Fluorescence transients measured from the excitor axon are depicted next to the terminals that they represent (arrows). Scale bars indicate 2% F/F (vertical) and 50 µm (horizontal). The black trace resulted from stimulating the excitor alone, while the grey trace resulted from stimulating the excitor and inhibitor together. The action potential was conducted from the right to the left in all of the images.