Common Inhibitory Motoneurones in Insects

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Abstract
The innervation patterns of the metathoracic posterior coxal levator muscles of the cockroach, Periplaneta americana, have been investigated, and the mechanical effects produced by activity in the axons to these muscles studied. These muscles are innervated by four excitatory and two inhibitory axons. The three smallest excitatory axons may be classified as slow and the largest as fast. Some single muscle fibres are quintuply innervated by all but the fast axon, and some may be innervated by all six. One of the inhibitory axons is a branch of a common inhibitory neurone. This neurone sends branches out all but one of the ipsilateral nerve trunks to innervate synergic and antagonistic muscles. A similar common inhibitory motoneurone has been found in both the locust species, Locusta migratoria and Schistocerca gregaria. In the locust two branches of this common inhibitory neurone correspond to the inhibitory axon innervating the extensor tibiae muscle and the inhibitory-conditioning axon innervating the anterior coxal adductor muscle. Possible functions of common inhibitory neurones in insects are discussed. In the cockroach this neurone may have a dual function: (a) to regulate leg position when the animal is standing, and (b) to facilitate relaxation of the depressor muscles; this allows a more rapid and stronger leg levation when the animal is walking.