Repeating ultrastructural motifs provide insight into the organization of the octopus arm nervous system

The peripheral nervous system of the octopus is among the most complex of any animal. In each arm, hundreds of serial ganglia form a central core of nervous tissue processing sensory input, issuing motor commands, and exchanging information with the central brain.^1,2,3,4,5 In addition to the central cord, there are two other types of neural elements: fine intramuscular nerve cords (INCs)^6,7 and small sucker ganglia at the base of each sucker.^2,6,8,9 Connections between these different elements and the structural organization of the arm nervous system remain poorly understood, despite decades of interest and a more recent explosion of studies of the cephalopod nervous system.^{8,10,11,12,13,14,15} Here, we use serial blockface electron microscopy to reconstruct large volumes of an arm from Octopus bocki at the base and toward the tip, mapping connections between the various neural elements and their relationship to the muscle and skin. We show that the ganglia follow an alternating mirror-image pattern along the arm, where the left or right-sided location of successive suckers determines ganglionic orientation. We also describe previously unrecognized patterns in (1) continuity of oblique connectives between the INCs that encircle the arm; (2) repeatable structures of the major blood vessel branches and nerve connectives within each ganglion; (3) clustering of rare, unusually large neurons within the cell body layers; and (4) division of the cortex into repeating columns. These new findings from the first 3DEM reconstruction of the arm should greatly facilitate future studies of octopus neurobiology, particularly sensori-motor integration and arm control.