Synapse distribution of olfactory interneurons in the procerebrum of the snail Helix aspersa.

The procerebrum is believed to be important for processing olfactory information and storing olfactory memories in terrestrial pulmonate molluscs. Previous results have demonstrated that the procerebral cell population is morphologically heterogeneous. In the present study, serial sections and electron microscopy were used to investigate differences in synapse distributions. The results demonstrate that procerebral neurons with different sites of arborization have distinct patterns of synapse distribution that probably reflect different functional contributions to the olfactory pathway. Cells that have all their arborizations in the procerebrum, but none in the internal mass, have multiple large varicosities that are specialized for output. On the other hand, 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 cells appear to transmit information from the procerebral cell body mass to other central nervous system regions, e.g., the internal mass and the mesocerebrum. The implications of these data are twofold. Firstly, the procerebrum directly participates in distributing processed olfactory information to more central regions of the nervous system. Secondly, the procerebral neuronal population may be divisible into two subgroups: 1) intrinsically arborizing interneurons; and 2) projection neurons. This is significant because the neural organization of the procerebrum may now be compared with that of olfactory systems in other organisms.