Neuronal segmentation in cephalopod arms.

Prehensile arms are among the most remarkable features of the octopus, but little is known about the neural circuitry controlling arm movements. Here, we report on the cellular and molecular organization of the arm nervous system, focusing on its massive axial nerve cords (ANCs). We found that the ANC is segmented. In transverse cross sections, the ANC cell body layer wraps around the neuropil with no apparent segregation of sensory and motor neurons. In longitudinal sections, however, ANC neurons form segments, setting up a modular organization to the adjoining neuropil. ANC nerves exit in the septa between segments, and for each sucker, form a spatial topographic map ("suckerotopy"). A strong link between ANC segmentation and flexible sucker-laden arms is confirmed by comparative study of squid arms and tentacles. These ANC modules offer a template for modeling the motor control of soft tissues and provide a compelling example of nervous system segmentation in molluscs.