Feedforward afferent excitation of peripheral inhibitors in the crayfish escape system

1. Each abdominal ganglion of the crayfish contains peripheral inhibitors of the fast flexor muscles. These flexor inhibitors (FIs), which can effectively inhibit tension development in the tailflip powerstroke muscles, are excited by a delayed central pathway from the same giant axons which trigger escape (33). The FIs also received sensory input, which increases in efficacy in the more posterior segments (4), but until now neither the origin of the input nor its central pathways had been well described. We have used intracellular recording and staining techniques to investigate the afferent input onto the two telson flexor inhibitors (F16 and F17), which receive more powerful sensory input than any of their anterior homologs (4). 2. Both F16 and F17 showed a delayed (3.7 ms) compound postsynaptic potential (PSP), which peaked at long latency when any afferent nerve in the abdomen was stimulated. The amplitude of these slow PSPs waned rapidly with repeated stimulation at 1 Hz and was increased by hyperpolarization and decreased by depolarization of the FI. The PSPs are most likely to be mediated chemically, via polysynaptic pathways. 3. When any afferent nerve from the telson was stimulated, both telson FIs showed an additional fast-rising, short-latency (1.4 ms) PSP, which preceded the slow component. This fast component was not produced by afferent nerves innervating any region other than the telson. The fast PSPs of the two FIs were similar, but in F16 the fast component was always subthreshold, whereas in F17 it often elicited an impulse at short latency. 4. The amplitude of the fast component was not affected by changing the membrane potential of the FIs, suggesting electrical transmission. In spite of its short latency, the fast component is unlikely to be mediated monosynaptically, since it was variably present even in the same animal, and occlusion was observed when any two of the four telson nerves that evoked the response were stimulated simultaneously. 5. Although occlusion was seen among responses produced by stimulating afferents from any source, the responses summated linearly with the compound excitatory postsynaptic potential evoked in FI by the lateral giant escape command axons. Thus at least two separate suprathreshold pathways converge onto the telson FIs.