Postsynaptic Inhibition of Crayfish Tonic Flexor Motor Neurones By Escape Commands

Abstract

ABSTRACT The crayfish abdomen contains separate slow and fast neuromuscular systems that mediate posture and escape tailflips (Kennedy & Takeda, 1965 a,b). It was recently demonstrated that impulses in the medial and lateral giant axons, which trigger escape responses, also inhibit both spontaneous and evoked activity in the tonic, or slow, flexor motor neurones (Kuwada & Wine, 1979). The evidence for inhibition was based exclusively on extracellular recordings of the spontaneously active tonic flexor motor neurones. We have now used intracellular recordings from the somata and neuropilar processes of the tonic flexor motor neurones to clarify the nature of inhibition. Our main findings are that synaptic potentials (from unknown sources) appear to underlie the spontaneous activity of the tonic flexor motor neurones, and that impulses in the giant axons cause large-amplitude, hyperpolarizing IPSPs in the tonic flexor motor neurones and EPSPs or spikes in the peripheral inhibitor. Of the six tonic flexor neurones in each half ganglion, we have recorded from f 3, f5, f6 and possibly f4 (the motor neurones are numbered 1−6 according to increasing size, with f5 being the peripheral inhibitor). The motor neurones were identified by the size of their extracellularly recorded axon spikes and by the positions of their somata (Wine, Mittenthal & Kennedy, 1974). Results are from six preparations. General methods were the same as those used earlier (Kuwada & Wine, 1979). For intracellular recordings we used 3 M-KCI electrodes of 15−80 MΩ, inserted into desheathed ganglia of isolated nerve cords. The giant axons were stimulated via focal suction electrodes placed directly over them in sheathed connectives. Examples of tonic flexor motor neurone activity and inhibition of it by giant axon spikes are shown in Fig. 1. The following points can be made.