Octopuses are not like humans: they are eight-armed invertebrates and are most closely related to clams and snails. Despite this, they have evolved complex nervous systems with as many neurons as in the brain of dogs, allowing them to exhibit a wide range of complex behaviors.
This makes them an interesting topic for researchers like Melina Hale, Ph.D., William Rainey Harper, Professor of Biology of Organisms and Vice Chancellor of the University of Chicagowho want to understand how alternative structures of the nervous system can perform the same functions as those in humans, such as detecting limb movement and controlling movement.
In a recent study published in current biologyHale and his colleagues discovered a surprising new feature of the octopus nervous system: a structure that allows intramuscular nerve cords (INCs), which help the octopus sense the movement of its arms, to connect arms on opposite sides of the animal.
The startling discovery provides new insights into how invertebrate species have independently developed complex nervous systems. It can also serve as inspiration for robotics engineering, such as new autonomous underwater devices.
“In my lab, we study mechanosensation and proprioception — how movement and position of the limbs are detected,” Hale said. “These INCs were long 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 indicated that they are important for arm control.”
With support for cephalopod research from the Marine Biological Laboratory, Hale and his team were able to use young octopuses for the study, which were small enough to allow researchers to image the base of all eight arms at the time. 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 get the specimens in the right orientation and get the angle right while cutting. [for imaging]said Adam Kuuspalu, a 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 outside 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, hoping they would form a ring on the octopus’s body, similar to axial nerve cords.
Through 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’s body, where they bypass the two adjacent arms and merge with the INC of the third. arm. This pattern means that all the arms are connected symmetrically.
However, it was a challenge to determine how the pattern would hold up across all eight arms. “As we were taking the images, we noticed that they weren’t all coming together the way we expected, they all seemed to be going in different directions, and we were trying to figure out how if the pattern held for all the arms, how would that work?” Hale said. “I even took out one of those children’s toys, a Spirograph, to play with what it would look like, how it would all connect in the end. It took a lot of pictures and playing with drawings while we racked our brains over what might be going on before it was clear how it all fit 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.”
The researchers don’t yet know what function this anatomical design might serve, but they do have some ideas.
“Some older documents have shared interesting insights,” Hale said. “A study from the 1950s showed that when you manipulate an arm on one side of an octopus with damaged areas of the brain, you will 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 fibers exiting nerve cords into muscles along their tracts, so they could also allow for a continuity of proprioceptive feedback and motor control along their lengths.”
The team is currently running experiments to see if they can gain insight into this question by looking at the physiology of the INCs and their unique design. They are also studying the nervous systems of other cephalopods, including squids and cuttlefish, to see if they share similar anatomy.
Ultimately, Hale believes that in addition to illuminating the unexpected ways in which an invertebrate species could engineer a nervous system, understanding these systems can help in the development of new engineering technologies, such as robots.
“Octopuses may be a biological inspiration for the design of autonomous underwater devices,” Hale said. “Think about your arms – they can bend anywhere, not just at the joints. They can twist, extend their arms, and operate 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 movement control may aid the development of new technologies.”
Reference: “Multiple nerve cords connect the arms of octopuses, providing alternate pathways for signaling between arms” by Adam Kuuspalu, Samantha Cody, and Melina E. Hale, November 28, 2022, current biology.
The study was funded by the United States Office of Naval Research.
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