A Map of Distal Leg Motor Neurons in the Thoracic Ganglia of Four Decapod Crustacean Species

We describe the numbers, central positions, and axonal exit routes of the distal leg motor neurons of four decapod species: squat lobsters (Munida quadrispina), spiny sand crabs (Blepharipoda occidentalis), mole sand crabs (Emerita analoga), and signal crayfish (Pacifastacus leniusculus). As predicted by previous physiological and anatomical identification of axons at the periphery in crayfish and lobsters, cobalt backfills reveal about seventeen cell bodies, which are found in four areas in the ganglion. By comparing their positions and neurite morphologies with the previously identified neurons, functional identifications could be assigned to most of them. The common inhibitor and stretcher inhibitor are located posterior-medial. An anterior-lateral cluster of about twelve somata includes the opener identical to stretcher excitor, one of two bender excitors (bender excitor alpha), four flexor excitors, and two excitors each to the extensor, reductor, and closer muscles. Three cell bodies are posterior-lateral. Of these, the opener inhibitor and the second bender excitor (bender excitor beta) are on about the same dorsoventral plane. The third posterior-lateral cell, the accessory flexor excitor, is noticeably more dorsal than the other two posterior-lateral cell bodies. The reductor muscle is innervated by at least three neurons: the putative common inhibitor and fast and slow excitors. None of the leg motor neurons project into the contralateral hemiganglion. The most variable feature across species is the nerve through which motor axons exit the ganglion: axons leave the ganglia via different routes in each of the four species examined. These differences in the axons' pathway, however, are insufficient to explain the differences in motor output and behaviour of these four species.