Multiple nerve cords connect the arms of octopuses, providing alternative paths for inter-arm signaling

Octopuses are remarkable in their ability to use many arms together during behavior (e.g., see Levy et al., ¹ Mather,² Byrne et al.,³ and Hanlon et al.⁴). Arm responses and multi-arm coordination can occur without engagement of major brain regions,⁵ which indicates the importance of local proprioceptive responses and peripheral connections. Here, we examine the intramuscular nerve cords (INCs),^6,7,8,9 the key proprioceptive anatomy in the arms. INCs are understood to include proprioceptive neurons, multipolar neurons, and motoneurons (reviewed by Graziadei¹⁰) and are thought to contribute to structuring whole-arm movement.¹¹ There are four INCs running the full length of each arm (e.g., see Guérin-Ganivet,⁶ Martoja and May,⁸ and Graziadei⁹); we focused on the pair closest to the suckers, called the oral INCs. In tracking the oral INCs, we found that they extend proximally and continue beyond the arm, through the arm's base. Each oral INC bypasses two adjacent arms and is continuous with the nearer oral INC of the third arm over. As a result, an arm connects through oral INC pathways to arms that are two arms away to the right and left of it. In addition to connecting distant arms, nerve fibers project from the central region of the INCs, suggesting function in local tissues. The other two INCs, paired aboral INCs, also extend proximally beyond the arm's base with trajectories suggestive of the oral INC pattern. These data identify previously unknown regions of the INCs that link distant arms, creating anatomical connections. They suggest potential INC proprioceptive function in extra-arm tissues and contribute to an understanding of embodied organization for octopus behavioral control.^12,13,14,15.