The distal limb motor neurons in the thoracic ganglion of the spiny lobster

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Abstract
The 16 motor neurons which innervate the muscles in the limb distal to the autotomy plane have been found in the third thoracic ganglion of the southern rock lobster, Jasus novaehollandiae. They were traced from the autotomy plane to the ganglion by stripping them from the small and large leg nerves to the edge of the ganglion where they were filled with cobalt dye. Cell bodies of 11 of these neurons lie in an anterior cluster of somata, and the remainder lie in a ventral rind of somata. The neurites from these somata expand into enlarged central segments, 400 μm long and 10–30 μm in diameter. These segments run parallel and in apposition to each other as they course through the neuropile a few cell layers beneath the dorsal surface of the ganglion. Near their origin some central segments send branches both medially between fibers of the interganglionic connectives, and anteroventrally into the connective. Distally the segments give rise to side branches which project around the anterior and posterolateral borders of the neuropile. The central segments constrict near the lateral edge of the neuropile, rise to its surface, and form axons. Eight axons, including those which innervate the flexor muscle of the carpus, enter an anterior motor tract which exists the side of the ganglion in a first root, the source of the small leg nerve. The remaining eight axons, which include those that innervate the extensor muscle of the carpus, enter a posterior motor tract which exits the side of the ganglion in a second root, the source of the large leg nerve. All the motor fibers which innervate the muscles of the limb, with the exception of the retractor of the coxa, run in these two tracts within the ganglion. Fibers which innervate the retractor of the coxa and the muscles of the thorax leave the ganglion in a third root at the posterodorsal border of the ganglion. The central segment, as the common structure of confluence of neuropilar branches, is the likely candidate for the integrating region of each neuron.