Dye injection and electrophysiological mapping of giant neurons in the brain of Archidoris

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Abstract
The morphology of 7 giant neurons in the brain of the nudibranch mollusk A. pseudoargus was studied by electrophysiological mapping techniques and by intracellular injection of Procion dyes. The neurons studied included 2 pairs of pedal giant cells symmetrically placed in right and left pedal ganglia, and the paired cerebral giant neurons and a single giant cell in the right pleural ganglion. Electrophysiological mapping showed which nerve trunks carried axons of the giant neurons. This was confirmed by dye injection, which also demonstrated the branching architecture of the cells in the neuropile areas of the brain. After injection with dye the neurons were photographed as whole mounts and subsequently reconstructed from serial sections. The left dorsal pedal giant neuron had a single peripheral axon and discrete dendritic branches. The remaining 6 neurons showed complex branching of the axon and did not have finely branched dendritic processes. The pedal giant neurons showed unilateral distribution of axon branches to pedal nerve trunks. Of these neurons the ventral giant cells formed a symmetrical pair; the dorsal cells did not. The paired cerebral giant neurons showed bilateral and symmetrical distribution of axon branches to pedal, pleural and cerebral nerves. Dye injection also demonstrated secondary division of axon branches within the nerve trunks.