The activity of single motor fibres in arthropods. I. The dragonfly nymph.

Although myoneural mechanisms of arthropods have been extensively studied, comparatively little work has so far been carried out on the central connexions of motor units. The effect of motor discharges has usually been investigated with nerves isolated from their ganglia, and hence the role of the efferent synapses in modifying the information transmitted to the muscle is relatively unknown. On the other hand the synapses relaying sensory input to the central nervous system have received considerable attention both in crustaceans (Preston & Kennedy, i960; Hughes & Wiersma, 1960) and insects (Roeder, 1948; Fielden, 1960,1963 a; Fielden & Hughes, 1962). It therefore appeared a profitable approach to employ the dissection technique for isolating single fibres in studying some of the synaptic junctions of arthropod motor neurones. The work covered by this paper concerns the responses of efferent fibres in nymphs of the dragonflies, Anax imperator and Aeschna brevistyla. A subsequent article will describe motor fibre activity in a crustacean, the Australian yabbie, Cherax destructor. Relatively few studies have involved the reflex responses of motor fibres in insects. Discharges of motor neurones have been investigated by Hagiwara & Watanabe (1956) in the cicada, and by Luco (1963) in the cockroach. In addition the activity of efferent fibres in whole nerves has been studied in the cockroach and mantis (Roeder, Tozian & Weiant, i960) and in the dragonfly nymph (Mill, 1963). Some observations have also been made on the responses of motor fibres, with intact central connexions, in the control of leg muscles (Pringle, 1940; Hoyle, 1957; Usherwood, 1962), of flight muscles (Wilson, 1961, 1964; Neville, 1963) and of spiracular muscles (Hoyle, i960; Miller, i960). However, the main emphasis of these workers has been on the neuromuscular events and there is comparatively little reference to the properties of motor neurone synapses. In the present work responses to both mechanically and electrically induced afferent volleys have been studied in single units isolated from the segmental nerves. The results show that both inhibition and temporal summation/facilitation are important at motor junctions and these are discussed in relation to the control of insect muscle.