Fig. 3. Effect of presynaptic inhibition on Ca entry and transmitter release. (A) The relationship between Ca entry (observed as a change in fluorescence), during stimulation of the excitor alone (black) or the excitor and inhibitor (grey). Presynaptic inhibition caused a significant reduction in the fluorescence transient (average 20±1%) in excitor terminals (paired t-test, P<0.001, 122 terminals from 12 animals). (B) An example of excitatory junction potentials (EJPs) recorded from a muscle fibre when stimulating only the excitor (black) or the excitor and inhibitor together (grey). (C) Pooled data shows that, on average, the first, second and third EJPs were inhibited by 42±4%, 51±4% and 59±2%, respectively (N=27 cells from 18 animals). (D) Facilitation of EJPs resulting from stimulation of the excitor alone (black) or the excitor with the inhibitor (grey) was calculated by dividing the amplitude of the second and third EJP by the amplitude of the first. There was significantly less facilitation of the third EJP with inhibition (paired t-test, P<0.01, N=27 cells from 18 preparations).