Thoracic output of crayfish giant fibres - I. Pereiopod promotor motor neurones

A crayfish (Pacifastacus leniusculus) thoracic pereiopod promotor motor neurone (PMM) receives direct input from the ipsilateral medial giant fibre (MG) via a 1:1 rectifying electrical synapse. The evidence for this is as follows. (1) PMM spikes follow MG spikes at very short latency. (2) Negative current injected into the PMM propagates antidromically to the MG. (3) Positive voltage perturbations in the PMM (antidromic spikes) fail to propagate to the MG. (4) There is very close anatomical apposition between the PMM and MG as observed in wholemount preparations following staining with Lucifer Yellow. (5) Dye coupling between the MG and a neurone with anatomy similar to the PMM has been observed in very young crayfish, although not in adults. The electrical input from MG to PMM can be gated by depolarizing IPSPs impinging on the latter. The PMM produces EJPs in the leg promotor muscle which show initial massive antifacilitation, followed by slow facilitation. Homologous neurones occur in each of the 5 thoracic ganglia innervating the legs and claws.