The organization of a cephalopod ganglion

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Abstract
The stellate ganglion of cephalopods is sharply divided into a ventral part containing only large cells and a dorsal part where there are also microneurons (amacrine cells). Axons proceed from the larger cells of the ganglion to the stellar nerves in distinct dorsal and ventral roots, which join as they leave the ganglion. The ventral roots contain only large motor fibres, one arising from each of the 30000 ventral cells. The input to this part is from less than 2000 large fibres of the pallial nerve. These fibres branch abundantly in the ventral neuropil. After severing the pallial nerve massive degeneration occurs there, producing shrinkage of the whole ganglion. There is also degeneration in the dorsal neuropil, which therefore also has input from the pallial nerve. The dorsal roots contain some large fibres, being the axons of the larger dorsal cells. In addition, they contain numerous small fibres. These include efferent chromatophore fibres, which degenerate after severing the pallial nerve and therefore pass through the ganglion presumably without synapse. There are also afferent fibres from the periphery in the dorsal roots and, after severing stellar nerves, degeneration appears in the outer layers of the dorsal neuropil and in the pallial nerve. No degeneration occurs in the central stumps of the ventral roots after this operation. The trunks of the small cells of the dorsal part form characteristic bundles of fine fibres in the outer dorsal neuropil and dorsal roots. These bundles carry varicosities and make plexuses in the bases of the dorsal roots, intertwined with collaterals of the outgoing large fibres and branches of the incoming afferents from the periphery. Probably these microneurons terminate within the ganglion and are concerned with reflex modulation of the output of the dorsal neuropil. The arrangement of the dorsal and ventral divisions of the ganglion and roots of the stellar nerves is similar in Sepia and Loligo to that in Octopus. There are more numerous large terminal knobs in the neuropils of these decapods and these endings are also found within the cell layers, especially in the hind part of the dorsal region. The course of degeneration within the ganglion was followed after section of the pallial and stellar nerves in all three species, more in detail in Octopus. Degeneration of terminations is already advanced 15 h after severing the pallial nerve (at about 24 °C); break-up within the nerve trunks comes later. Degeneration granules have mostly disappeared 3 days after the lesion. Severed stellar nerves of Octopus show very abundant sprouting from the central stump, the fibres turning back to invade the ganglion and form terminal knobs in the neuropil and throughout the cell layers.

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