The reflex behaviour and innervation of the tergo-coxal retractor muscles of the stick insectCarausius morosus

Summary

The retractor coxae muscles of the mesothorax (Fig. 3) were examined by reflex stimulation and simultaneously recording intracellular muscle fiber potentials and the activity in nerve nl₅. Five axons were identified by their action potential amplitudes and reflex characteristics (Figs. 4, 5, 6). The smallest axon 1 is also present in nl₂ and nl₃ and has several of the properties of a common inhibitor (Figs. 6, 9). Four other axons produce depolarisations of the muscle fibers and were classified as slow 2, semi-fast 3, 4 and fast 5 on the basis of the amplitude of the junction potential.

All five axons were active in a resistance reflex produced by rotation of the coxa about the coxothoracic axis in the stimulus range 0.01–20 Hz (Figs. 1, 4, 10–12). Each axon showed a different dependence upon the frequency of the stimulus but the strongest responses occurred in the range 0.5–10 Hz.

A similar reflex was present in the metathorax. Axon 2 was often spontaneously active in the rear legs and a weak synchronizing reflex could sometimes be found between right and left hind legs. This reflex could only be detected by the modulation of ongoing activity (Fig. 4). No other interleg reflex pathways were found. Records from nl₂ show that the protractor muscles in both segments have resistance reflexes with higher thresholds (Fig. 4).

Cobalt back filling was used to show the morphology of both groups of neurons (Fig. 9). The soma position and dendrite structure of the retractor neurons are similar to a homologous neuron in locust.