An electrophysiological study of parapodial innervation patterns in Aplysia fasciata.

Parapodial extensions of the foot are particularly well developed in Aplysia fasciata, in which species they function during swimming. As was indicated in earlier studies (Hughes & Tauc, 1962), many of the nerves which innervate the foot sensu stricto of other species supply the parapodia of Aplysia fasciata. In fact there is no detailed account of the branching of the nerves in A. fasciata; Mazarelli (1893) compared the different species, but not in this respect, and based his account of the nerves mainly on A. depilans. Hoffman (1939) has compared these results with those of Eales (1921) on A. punctata and those of McFarland (1909) on A. dactylomela and A. tervinus. Nevertheless, in our first studies with whole animal preparations (Hughes & Tauc, 1962), it was clear from experiments using electrical stimulation and recording that the account given by Eales (1921) based upon A. punctata was not strictly applicable, especially as the posterior pedal nerves in that species are clearly homologous with the posterior parapodial nerves which innervate the parapodia of A. fasciata. Accordingly, a division of nerves leaving the pedal ganglia was made into pedal nerves proper innervating the foot, of which there are three, and the other main nerves which innervate the parapodia. These latter nerves were separated into anterior, middle and posterior, a subdivision based partly on the relative positions of origin, but especially on the region of the parapodium which they appear to innervate. During the present study it became clear that a considerable overlap exists in the sensory area supplied by different parapodial nerves, and this contrasts with the more sharply defined dermatomes of mammals and the sensory innervation of the abdominal segments in the crayfish (Hughes & Wiersma, i960) and of the earthworm (Prosser, ) The main objective of this study was to distinguish different afferent units in the main nerves and to plot the distribution of the main regions of the parapodium which they innervate. During the course of the work further investigations were made on certain units which, in addition to responding to mechanical stimulation of the parapodial surface, were also responsive following electrical stimulation of distal branches of other parapodial nerves. Thus, following contraction of the parapodial muscles, such responses might provide some sort of feedback and have a proprioceptive function. This study is also of relevance to work being carried out in which recordings have been made from individual nerve cells in the pedal and pleural ganglia following peripheral stimulation (Hughes, Weevers & Hartley, 1969).