An Electrophysiological Study of the Anatomical Relations of Two Giant Nerve Cells in Aplysia Depilans.

Histological studies of the central nervous system of invertebrates using methyleneblue and silver-staining techniques, alone or in conjunction with degeneration experiments, have provided a considerable amount of information about neuronal pathways. The use of electrophysiological methods, however, makes it easier to trace the path of individual neurones over long distances and can be especially helpful in invertebrates where the path of an axon is complicated by its passage through the neuropiles. In addition, they may give some insight into the types of functional connexion which exist between neurones. The technique of dissecting connectives between pairs of ganglia and recording activity of single units in small bundles has proved successful in some arthropods (Wiersma, 1958; Hughes & Wiersma, i960; Fielden & Hughes, 1962), but many molluscan nerves are not so amenable to such an approach because they are not easy to split. However, the use of microelectrodes inserted into single ganglion cells makes it possible to investigate single units in such animals. From this point of view the material used in the present studies was very suitable, partly because the ganglia are widely separated, and also because some of the nerve cell bodies are so large. Furthermore, a considerable amount of information is available about the different sorts of electrical activity which may be recorded from individual ganglion cells in isolated preparations (Tauc, 1955,1962a), and in addition some account has already been given of the general nature of responses recorded from this ganglion in the intact animal (Hughes & Tauc, 1962). The present paper is concerned particularly with the anatomy and function of the so-called 'giant cell' of the abdominal ganglion, which has been used more than any other in studies of isolated ganglion preparations. An interesting finding in the present work was the great ramification of the axon of this giant cell, which extends throughout nerves innervating both the foot and parapodia on the right side. A homologous giant neurone on the left side of the animal has also been found and shown to have similar anatomical relations. Because of their size and distribution and by analogy with other large neurones (e.g. Mauthner) it seemed justifiable to ascribe some motor function to the cells and to suggest that they might operate, for instance, during escape responses. However, from the results obtained, no indications were found for such a function. A preliminary report of this work has already been published (Hughes & Tauc, 1961). • Department of Zoology, Cambridge, England.