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The procerebrum of terrestrial molluscs is an important processing centre for olfaction. While the physiology of the procerebrum is relatively well characterized, the procerebrum's structure and organization has not been previously investigated in detail. The goal of this thesis is to better characterize the structural organization of the procerebrum and to understand how it compares with other olfactory systems. / The morphology of the procerebral neurons in the snail Helix aspersa was investigated through intracellular injections of biocytin. The population of cells is heterogeneous, but no formal categorization of neuronal types was possible. The main difference among cells lies in the placement of the cells' neurites. Furthermore, contradicting previous results, certain neurons were found to have neurite projections outside the procerebrum, travelling as far as the contralateral cerebral ganglion. / To investigate if differences in sites of arborization represent functional differences, the distribution of synaptic contacts on labelled cells was studied using serial sections and electron microscopy. Neurons with different sites of arborization have distinct patterns of synapse distribution. Cells with arborizations in the procerebrum but not in the internal mass have large varicosities specialized for output. Cells that arborize in the internal mass or outside the procerebrum have mostly input synapses proximal to the soma and mostly output synapses in the terminal region of the neurites. These latter cells appear to transmit information from the procerebral cell body mass to other brain regions. The implications of these data are, firstly, that the procerebrum directly distributes processed information throughout the nervous system. Secondly, the procerebral neuron population may be divisible into two subgroups: intrinsically arborizing interneurons and projection neurons. / These results suggest a novel mechanism by which compartmentalization could be achieved in the procerebrum. Compartmentalization is believed to be important for processing olfactory information, is present in most olfactory centres but has not previously been described in the molluscan olfactory system. I propose that varicosities on the local interneurons generate foci of activity in the procerebrum which, in turn, activate specific subsets of output neurons, similar to what happens in other olfactory systems.

The morphology of C3, a motoneuron mediating the tentacle withdrawal reflex, was investigated in the snail Helix aspersa by intracellular injections of the tracers Neurobiotin and biocytin. Axonal projections were identified in the optic nerve, the olfactory nerve, the internal peritentacular nerve, the external peritentacular nerve, the cerebral-pedal connective and the cerebral commissure. A rare characteristic of the cell was the multibranching of axons in the neuropil and the exiting of this bundle of fibres into the cerebral-pedal connective. Dendritic arborizations were observed branching from the cell body, the axon hillock and the dorsal main axon. In addition, tufts of dendrites were seen to branch from the ventral axon. Based on its morphology, C3 is probably a central component in the avoidance behaviour, receiving sensory input at extensive dendritic sites and sending axons to a number of key effector sites to co-ordinate the chain of reactions that constitutes the snail's avoidance behaviour.

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