Fig. 4.
Force-frequency curve for control and proctolin groups. A: representative traces shown above depict the effect of varying the intraburst frequency on evoked-contractions from control animals. B: effect of 10−9 M proctolin on nerve-evoked contractions was determined for various intraburst frequencies (motor neuron stimulation) in third-instar larvae. Proctolin enhanced nerve-evoked contractions at higher impulse frequencies but not at low intraburst frequencies. Maximal force was estimated using a single 50-Hz burst (250-ms duration) at the end of each trial. Statistical comparisons between contractions with and without proctolin were made using a Mann-Whitney test (*P < 0.05; **P < 0.01). Error bars, SD; n = 7–10.