Parasol cells of the hemiellipsoid body in the crayfish Procambarus clarkii: dendritic branching patterns and functional implications.

Multimodal, higher-order sensory integration in decapod crustaceans occurs in local interneurons (parasol cells) within a structure in the lateral protocerebrum, the hemiellipsoid body, which is located dorsal to the terminal medulla. The hemiellipsoid body is targeted by projection neuron inputs by means of the olfactory globular tract from bilateral deutocerebral neuropils, the accessory lobes, which receive secondary visual, mechanosensory, and olfactory inputs. Parasol cell dendrites arborize extensively within the two neuropils of the hemiellipsoid body and possibly have some neurites within another neuropil at its base. The two neuropils of the hemiellipsoid body, neuropils I and II, are known to receive asymmetrical inputs from the contralateral and ipsilateral accessory lobes, and our current study addresses the question of the distribution of parasol cells within these two neuropils. Three anatomic methods were used to analyze this distribution: intracellular filling of cells with neurobiotin and visualization of the cells by using either a fluorescent or a peroxidase avidin conjugate, or placement of a fluorescent lipophilic tracer within a lobe of the hemiellipsoid body. All of these methods demonstrated that single parasol cells exclusively arborize within one of the two lobes of the hemiellipsoid body, but not in both lobes. Electrophysiological recordings from pairs of parasol cells with dendrites in the same or different lobes confirm a functional separation between neuropils I and II. Comparisons are made between insect and crustacean systems, emphasizing the inputs to the hemiellipsoid body and the mushroom body and similarities between extrinsic cells in insects and parasol cells in decapod crustaceans.