Aspects of the organization of central nervous pathways in Aplysia depilans.

There has been little recent work on the relationship between the behaviour of gastropod molluscs and the activity of their nervous systems. Older workers (e.g. ten Cate, 1928), studied the effects of electrical stimulation of different nerves on the movements of parts of the animal. More recently observations on the spontaneous activity and electrical properties of individual cells in isolated ganglion preparations have been extensively carried out (Arvanitaki, 1942; Tauc, 1955, 1957a, b, 1958, 1960a; Hagiwara & Saito, 1959), but little attention has been paid to the importance of these phenomena in the life of the animal. Furthermore, deductions concerning the behaviour of gastropods based on observations of the activity in isolated ganglia are necessarily tentative (Hughes & Kerkut, 1956). The most recent study of neuronal pathways is that of Turner & Nevius (1951) on Ariolimax columbianus. They used external electrodes in stimulation and recording experiments and made tentative proposals concerning the layout of some of the typical neuronal connexions. As they point out, little is known of the connexions between neurones, and anatomical methods are unable to provide sufficient information about the nature of neuronal connexions over long distances. Nisbet (1961) has recently recorded impulses in the pallial nerves of Archachatina in response to tactile stimulation and has studied conduction in the nerves and across ganglia. Techniques similar to those of Turner & Nevius have been used in the present work but in addition intracellular micro-electrodes have been employed for recording the activity of individual cells in whole animal preparations. This technique is extremely valuable as the pattern of activity of single units, now well known from such cells in isolated preparations (Tauc, 1960a), gives additional information about the paths of single neurones over quite large distances. A preliminary account of results of such investigations on the so-called giant cells of Aplysia has already been published (Hughes & Tauc, 1961). In the present paper special attention is given to the general nature of pathways within the central nervous system and to the activity of cells in the abdominal ganglion while this retains its connexion with the rest of the animal. This preparation enables a study to be made of the influence of peripheral stimulation on the spontaneously active cells. Studies have also been made in isolated preparations of the different type3 of activity found in cells of the pleural ganglion and these are compared with what is already known about abdominal ganglion cells.