The inhibitory motoneurons of crayfish thoracic limbs: Identification, structures, and homology with insect common inhibitors

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Abstract
The three inhibitory motoneurons supplying crayfish thoracic limbs were identified, stained, and compared structurally. The inhibitors to the walking leg muscles (in Orconectes) were identified anatomically by a combination of immunocytochemical staining for gammaaminobutyric acid (GABA) or glutamate decarboxylase and differential backfill staining with nickel and cobalt ions. The cheliped inhibitors were identified intracellularly and injected with Lucifer Yellow (Pucifustucus) or cobalt (Procumbarus). The common inhibitor (CI) in each thoracic segment has a medial or slightly contralateral soma near the ganglion's posterior boundary, a gently curving primary neurite, an extensive ipsilateral dendritic tree, and an axon emerging through the anterior root. The stretcher‐closer inhibitor (SI) has a soma slightly anterior and ipsilateral to the CI's, a sharply bent proximal neurite, a smaller dendritic tree, and an axon in the posterior root. The opener inhibitor (OI) lies more laterally and often posterior to the CI; its diagonally directed neurite enters the posterior root. The inhibitors' structures were related to major neuroanatomical landmarks within the ganglion, to soma positions of excitatory motoneurons revealed by backfilling, and to soma locations of inhibitory interneurons revealed by GABA‐like immunoreactivity. In their peripheral distributions to the leg muscles and in their central structures, these crayfish limb inhibitors show striking similarities with those of the locust. Crayfish and locust thoracic ganglia also show more general neuroanatomical similarities. These observations suggest that the crayfish CI, SI, and 01 are, respectively, homologous with the locust CI1 CI2, and CI3. The implications of such a homology for arthropod phylogeny are discussed.