2 EXCITATORY MOTONEURONS DIFFER IN QUANTAL CONTENT OF THEIR JUNCTIONAL POTENTIALS IN ABDOMINAL MUSCLE-FIBERS OF THE CRICKET, GRYLLUS-BIMACULATUS

In abdominal muscles 202 and 203 of the cricket, Gryllus bimaculatus, large and small excitatory junctional potentials (l- and s-EJPs) with similar durations can be recorded from the same muscle fibers, At the normal extracellular calcium ion concentration ([Ca2+](o)) of 5 mM, th e amplitudes of l-EJPs in both muscles were larger than the threshold membrane potential for muscle action potentials, which is about -40 mV , Below 0.75 mM [Ca2+](o), the amplitudes became much smaller and were below the firing level for the action potentials, At 0.5 mM, they flu ctuated and decreased to 10.3 and 1.9 mV in muscles 202 and 203, respe ctively, and at 0.25 mM frequent failures occurred, The amplitudes of s-EJPs at 5 mM [Ca2+](o) were 13.3 and 5.1 mV in muscles 202 and 203, respectively, and the fluctuating amplitudes were far below the thresh old for muscle action potentials, Below 0.75 mM, s-EJPs were rarely ob served, The relation between log (EJP amplitude) and log ([Ca2+](o)) w as linear within a certain range of [Ca2+](o) and the slopes of the li nes for l-EJPs were about twice as steep as those for s-EJPs in both m uscles, In muscle 202, the amplitude distribution of l-EJPs obtained a t 0.25 mM and that of s-EJPs at 0.75 mM both showed peaks at once and twice the voltage at the first peak, which were coincident with the vo ltages at the peaks of amplitude distributions of miniature EJPs recor ded simultaneously, The reversal potentials for l- and s-EJPs in muscl e 202 were +1.02 and +0.22 mV, respectively, In muscle 202, the decrea ses in amplitude of both EJPs by L-glutamate were similar and concentr ation-dependent. The results suggest. that the difference in amplitude between l- and s-EJPs is attributable mainly to the difference in qua ntal contents. (C) 1997 Elsevier Science Ltd. All rights reserved.