The structure of the fourth abdominal ganglion of the crayfish, Procambarus clarki (Girard). II. Synaptic neuropils.

Four discrete regions of synaptic neuropil in the crayfish fourth abdominal ganglion are described by light and electron microscopy. The largest is the horseshoe neuropil, a horseshoe-shaped mass of synaptic glomeruli that lies horizontally in the ventral ganglionic core. This neuropil has a substructure of three rings of fused glomeruli associated with the entry of small axons from the first and second nerve roots. The lateral neuropils are large, paired bulges of neuropil that define the sides of the ganglionic core. They contain neuronal profiles of various sizes, filled with clear or dense-cored vesicles. The neurons are randomly oriented except for occasional dendritic bundles. The tract neuropil is ultrastructurally similar to the lateral neuropils but it is distributed among the largest axons of the through-tracts and commissures. The midline neuropils are small, U-shaped clumps of uniformly sized neuronal profiles that contain large numbers of dense-cored vesicles and distinctive lamellar inclusions.