Neuronal morphology and neuropil structure in the stomatogastric ganglion of the lobster, Homarus americanus.

The stomatogastric nervous system (STNS) has long been used as a model system for the study of central pattern generation, neuromodulation, and network dynamics. Anatomical studies of the crustacean stomatogastric ganglion (STG) in different species have mostly been restricted to subsets of neurons and/or general structural features. For the first time, we describe the morphology of all STG neurons belonging to the two circuits that produce the well-described pyloric and gastric rhythms in the lobster, Homarus americanus. Somata sit on the dorsal and lateral surface of the STG and send a single primary neurite into the core of the neuropil, which is mostly made up of larger lower order branches. The perimeter of the neuropil consists mostly of finer higher order branches. Immunohistochemical labeling for synaptic proteins is associated with the small diameter branches. Somata positions are not constant but show preferred locations across individuals. The number of copies is constant for all neuron types except the PY and GM neurons (PY neuron number ranges from 3 to 7, and GM neuron number ranges from 6 to 9). Branch structure is largely nondichotomous, and branches can deviate substantially from cylindrical shape. Diameter changes at branch points can be as large as 20-fold. Clearly, the morphology of a specific neuron type can be quite variable from animal to animal.