The pedal neurons of Aplysia punctata.

There are no detailed accounts of the connexions and branching of the axons of neurons in the pedal ganglia of opisthobranchs. Most of the experiments on these ganglia have been limited to cutting and stimulating nerve trunks, and using these techniques, Frohlich (1910) demonstrated the role of the pedal ganglia in inhibiting the tonic contraction of the isolated foot of Aplysia limacina. Ten Cate (1928) considered that the wave-like swimming movements of the parapod of this species were also maintained by inherent patterns of activity in the neurons of the pedal ganglia, and found that sectioning the pedal commissure eliminated the response of the parapodia to contralateral stimulation. Herter (1931) and Turner & Nevius (1951) both studied pathways in the nervous system of pulmonate gastropods which involved the pedal ganglia, but the nature of their experiments could not reveal the precise configuration of the pathways they described. Nevertheless, they demonstrated several types of pathways through the pedal ganglia and found that they were influenced by activity of the cerebral ganglia descending in the cerebro-pedal connective. Unit activity of cells in the pedal ganglia of Agriolimax, and subsequent action potentials in the pedal nerves were recorded by Hughes & Kerkut (1956), who found that many cells exhibited a slow spontaneous activity that was sensitive to small changes in the concentration of the bathing medium. The ability to distinguish antidromic from orthodromic action potentials whilst making intracellular recordings from neurons (Tauc, 1957), has been used in mapping the ramification of the branched axons of the giant cells in the abdominal and pleural ganglia of Aplysia depilans (Hughes & Tauc, 1962, 1963), and in the cerebral and pleural ganglia of Tritonia hombergii (Dorsett, 1967). These authors found neurons with many-branched axons which were distributed in a symmetrical fashion in nerves on both sides of the body. The present paper is a study made on the distribution of the axons and synaptic pathways to 175 neurons sampled from the pedal ganglia of 15 specimens of Aplysia punctata.