Small Motor Axons in Orthopteran Insects: A Reinvestigation of the Innervation of the Femoral Retractor Unguis Muscle in a Stick Insect and two Species of Locust

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ABSTRACT The motor innervation of the femoral retractor unguis muscle in a stick insect and two species of locust was investigated morphologically and electrophysiologically. In all three insects a somewhat variable number of axons with diameters less than 2 μm, some even less than 0·5 μm, accompany much larger ones in the motor nerve supplying the muscle. In the stick insect Carausius probably more than six motoneurones, including at least one inhibitory, control the muscle. In the two species of locust two populations of muscle fibres, ‘red’, and ‘white’, can be distinguished according to their respectively high and low levels of succinate dehydrogenase and their different patterns of innervation. The ‘red’ muscle fibres in Schistocerca have, in addition to one or two ‘fast’ excitors, two inhibitors, and in Locusta ‘red’ fibres receive a further one or sometimes two ‘slow’ excitors, giving a more complex innervation pattern of the ‘red’ compared to the ‘white’ fibres. An additional neurosecretory control by at least one neurone is indicated by the presence of characteristic nerve endings throughout the muscle. Of the two ‘fast’ motor axons in both locusts, one innervates the ‘red’ and the other the ‘white’ muscle fibres but both axons, to a lesser extent, also synapse with muscle fibres of the other type. The synaptic responses they evoke here are, however, only of small or intermediate amplitude. The existence of ‘slow’ excitatory innervation in Locusta ‘red’ fibres shows that functional synaptic contacts from one motoneurone can be restricted to a small region of a muscle fibre whilst those from other motoneurones are present along almost the entire length of the same fibre. Some of the difficulties in comparing morphologically demonstrated numbers of axon profiles with electrophysiologically derived numbers of motoneurones are discussed.