Scientists discover ‘whole new way to engineer a nervous system’

Octopuses are not like humans: they are invertebrates with eight arms and are more closely related to clams and snails. Despite this, they have evolved complex nervous systems with as many neurons as in the brains of dogs, allowing them to display a wide range of complex behaviors.

This makes them an interesting topic for researchers such as Melina Hale, Ph.D., William Rainey Harper Professor of Organismal Biology and Vice Chancellor of the University of Chicagowho want to understand how alternative nervous system structures can perform the same functions as humans, such as sensing limb movement and controlling movement.

In a recent study published in current biologyHale and colleagues discovered a surprising new feature of the octopus’s nervous system: a structure that allows intramuscular nerve cords (INCs), which help the octopus sense arm movement, to connect the arms on opposite sides of the animal

The surprising discovery provides new insights into how invertebrate species have independently evolved complex nervous systems. It may also provide inspiration for robotics engineering, such as new autonomous underwater devices.

“In my lab, we study mechanosensation and proprioception: how movement and position of limbs is perceived,” Hale said. “These INCs have long been thought to be proprioceptive, so they were an interesting target to help answer the kinds of questions our lab is asking. So far, not much work has been done on them, but previous experiments had indicated that they are important for arm control.”

Thanks to cephalopod research support from the Marine Biological Laboratory, Hale and his team were able to use young octopuses for the study, which were small enough to allow the researchers to image the bases of all eight arms at once . This allowed the team to trace the INCs through the tissue to determine their path.

“These octopuses were about the size of a nickel or maybe a quarter, so it was a process to place the specimens in the correct orientation and get the right angle during sectioning. [for imaging]” said Adam Kuuspalu, senior research analyst at UChicago and lead author of the study.

Initially, the team was studying the larger axial nerve cords in the arms, but began to notice that the INCs did not stop at the base of the arm, but continued out of the arm and into the animal’s body. Realizing that little work had been done to explore the anatomy of INCs, they began tracing the nerves, expecting them to form a ring on the octopus’s body, similar to axial nerve cords.

Using imaging, the team determined that in addition to running the length of each arm, at least two of the four INCs extend into the octopus’ body, where they bypass the two adjacent arms and merge with the third arm’s INC . This pattern means that all the arms are connected symmetrically.

It was difficult, however, to determine how the pattern would hold in all eight arms. “As we were imaging, we realized they weren’t coming together as we expected, they all seemed to be going in different directions, and we were trying to figure out if the pattern held for all the arms, how would that work?” Hale said. “I even got out one of those kids’ toys, a spirograph, to play with what it would look like, how it would connect at the end. It took a lot of pictures and playing around with drawings as we racked our brains about what might be going on before it became clear how it all fits together.”

The results were not at all what the researchers expected to find.

“We think this is a new design for a limb-based nervous system,” Hale said. “We haven’t seen anything like this in other animals.”

Researchers don’t yet know what function this anatomical design might serve, but they have some ideas.

“Some older articles have shared interesting ideas,” Hale said. “A study from the 1950s showed that when you manipulate an arm on one side of the octopus with damaged brain areas, you’ll see the arms respond on the other side. So it could be that these nerves allow decentralized control of a reflexive response or behavior That said, we also see that fibers exit nerve cords to muscles along their tracts, so they can also allow continuity of proprioceptive feedback and motor control along their lengths.

The team is currently conducting experiments to see if they can gain insight into this question by analyzing the physiology of INCs and their unique design. They are also studying the nervous systems of other cephalopods, including squid and cuttlefish, to see if they share similar anatomy.

Ultimately, Hale believes that in addition to shedding light on unexpected ways in which an invertebrate species might engineer a nervous system, understanding these systems can aid in the development of new engineering technologies, such as robots.

“Octopus can be a biological inspiration for the design of autonomous underwater devices,” Hale said. “Think about their arms: they can bend anywhere, not just at the joints. They can twist, extend their arms, and actuate their suction cups, all independently. The function of an octopus arm is much more sophisticated than ours, so understanding how octopuses integrate sensorimotor information and control movement can support the development of new technologies.”

Reference: “Multiple nerve cords connect octopus arms, providing alternative pathways for arm-to-arm signaling” By Adam Kuuspalu, Samantha Cody, and Melina E. Hale, Nov. 28, 2022 current biology.
DOI: 10.1016/j.cub.2022.11.007

The study was funded by the US Office of Naval Research.