Distribution of fine neurites of stomatogastric neurons of the crab Cancer borealis: evidence for a structured neuropil.

The neuropil of the stomatogastric ganglion of the crab Cancer borealis contains many neuronal processes that may be arranged either at random or in some form of orderly structure. In this study, we provide evidence for two types of order in the neuropil, a segregation of the processes based on their size and a cell-specific distribution to the fine neurites. Identified neurons were injected with Lucifer yellow, fixed, and imaged as whole mounts with a confocal microscope. Four cell types were analyzed using the serial images, two pyloric neurons, one mixed pyloric/gastric neuron, and one gastric neuron. All of the neurons consisted of a approximately 60-microns-diameter soma, a approximately 20-microns-diameter primary neurite projecting into the center of the neuropil, a number of < 10-microns-diameter medium-sized neurites radiating away from the center, and many < 3-microns-diameter fine neurites around the periphery of the neuropil. The neuropil can, therefore, be divided into three layers, a central core containing the largest neurites, an intermediate region containing both medium-sized and fine neurites, and a peripheral neuropil containing mostly fine neurites. The distribution of the fine neurites was mapped using a three-dimensional grid. We found that the fine neurites were distributed not at random within the neuropil but in consistent, cell-specific patterns.