Tracing neural pathways in snail olfaction: from the tip of the tentacles to the brain and beyond.

The anatomical organization of the olfactory system of terrestrial snails and slugs is described in this paper, primarily on the basis of experiments using the African snail Achatina fulica. Behavioral studies demonstrate the functional competence of olfaction in mediating food finding, conspecific attraction, and homing. The neural substrate for olfaction is characterized by an extraordinarily large number of neurons relative to the rest of the nervous system, and by the fact that many of them are unusually small. There exist multiple serial and parallel pathways connecting the olfactory organ, located at the tip of the tentacle, with integrative centers in the central nervous system. Our methods of studying these pathways have relied on the selective neural labels horseradish peroxidase and hexamminecobaltous chloride. One afferent pathway contains synaptic glomeruli whose ultrastructure is similar to that of the glomeruli seen in the mammalian olfactory bulb and the insect olfactory lobe. All of the olfactory neuropils, but especially the tentacle ganglion, contain large numbers of morphologically symmetrical chemical synapses. The procerebrum is a unique region of the snail brain that possesses further features analogous with olfactory areas in other animal groups. Olfactory axons from the tentacle terminate in the procerebrum, but the intrinsic neurons do not project outside of it. An output pathway from the procerebrum to the pedal ganglion has been identified and found to consist of inter-ganglionic dendrites. The major challenge for future studies is to elucidate the pattern of connectivity within, rather than between, the various olfactory neuropils.