Fig. 9. Significantly more current was necessary to elicit small action potentials than large action potentials. (A) Action potentials elicited by a series of 50 ms current steps, increasing by +25 pA from bottom to top. Passive electrical responses were compensated by adding the responses of two pre-pulses of opposite polarity and half the amplitude to each sweep. The large action potentials (filled arrowheads) required a current of 175 pA, while the small action potentials (open arrowheads) were first elicited by a 225 pA step. The action potentials marked with open circles were considered to be spontaneous, since either no action potential was present in the next sweep or the action potential occurred outside the current step. (B) The small action potentials required significantly more current (244 pA) than the large ones (208 pA). Values are means + S.E.M., N=123 sets of five step protocols without drug injection. The asterisk indicates a significant difference from the control (P<0.01, Student's t-test). For both classes of action potential, no significant changes were detected after the injection of any dose of octopamine or serotonin (not shown).