Slow synaptic responses and excitability in sympathetic ganglia of the bullfrog.

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Abstract
Requirements were established for preganglionic inputs which could effectively elicit slow IPSP (inhibitory postsynaptic potential) and slow EPSP [excitatory, PSP] in frog''s sympathetic ganglion. It was found possible to completely dissociate B from C fiber preganglionic inputs. Preganglionic C fiber volleys can elicit a relatively large slow IPSP, with repetition of 10 per sec. B fiber volleys elicit only small IPSP, but can also elicit a long (30 sec) slow EPSP. Both slow PSP''s resist curarizing agents but are completely abolished by atropine (0.1 [mu]g/ml). In uncurarized ganglia atropine reduced the posttetanic after-postivity indicating a substantial contribution by the IPSP. The latent period of the slow IPSP indicates an extraordinarily long synaptic delay of about 100 msec. Excitability-recovery curves determined without and with atropine indicated that the slow IPSP can in fact inhibit ganglion cell discharge by orthodromic excitatory volleys, at least when the margin of safety for firing by the EPSP is below its maximal level.