Neuroanatomy of a crayfish thoracic ganglion: Sensory and motor roots of the walking-leg nerves and possible homologies with insects

The internal organization of the third and fourth thoracic ganglia of the crayfish, Pacifastacus leniusculus, was studied in serial sections stained with osmium ethyl gallate. The aims were 1) to provide an anatomical framework for studies of sensorimotor integration in the walking system and 2) to explore possible homologies with abdominal ganglia in crayfish and with the thoracic ganglia of insects. Crayfish thoracic ganglia show several intersegmental homologies with the unfused ganglia of the abdominal nervous system: 1) Longitudinal tracts and dorsal commissures are arranged similarly, allowing use of the same nomenclature. 2) Paired lateral neuropils are located dorsolaterally and contain many large neurites including those of leg motor neurons and of nonspiking, proprioceptive afferents from the basal limb joints. They resemble the lateral neuropils of abdominal ganglia. 3) Neuropil lying more ventrally is fine textured and receives projections from other leg afferents. This ventral neuropil resembles the "horseshoe neuropil" of abdominal ganglia. The functional implications of this organization are discussed. Compared to the abdominal ganglia, however, thoracic ganglia also show specific intersegmental differences: 1) They have more ventral commissures; 2) the ventral neuropil undergoes a large bilateral extension; 3) distinct anteromedial regions of the ventral neuropil receive specific afferent projections; and 4) recognizable dorsoventral "T-tracts" occur. Moreover, these "thoracic" features show a striking resemblance to structures found in thoracic ganglia of orthopteran insects. These correspondences provide further indications that the neuropil of segmental ganglia may be organized in homologous ways in crustaceans and in insects.