Octopuses are not like humans – they are invertebrates with eight arms and closely related to clams and snails. Despite this, they have evolved complex nervous systems with as many neurons as canine brains, which has allowed them to display a wide range of complex behaviours.
This makes it an interesting topic for researchers such as Melina Hill, Ph.D., William Rennie Harper Professor of Biology of Organisms and University Vice Chancellor at University of Chicagowho want to understand how alternative nervous system structures can perform the same functions as those in humans, such as sensing limb movement and controlling movement.
In a recent study published in Current BiologyThen, Hill and her colleagues discovered a surprising new feature of the octopus’ nervous system: a structure that allows neuromuscular cords (INCs), which help the octopus sense its arm movement, to connect to the arms on either side of the animal.
The startling discovery provides new insights into how invertebrate species independently evolved complex neuronal species. It could also provide inspiration for robotic engineering, such as new autonomous underwater devices.
“In my lab, we study mechanosensation and proprioception — how limb movement and position are sensed,” Hill said. “These INCs have long been thought to be sensory, so they have been an interesting target to help answer the kinds of questions our lab is asking. To date, not much work has been done on them, but previous experiments have indicated that they are important for arm control.”
Thanks to cephalopod research support provided by the Marine Biological Laboratory, Hill and her team were able to use baby octopuses for the study, which were small enough to allow the researchers to image the base of the eight arms at once. This allows the team to track the INCs through the tissue to determine their trajectory.
“These octopuses were about the size of a nickel or maybe a quarter, so it was a process of gluing the specimens in the right direction and getting the right angle while slicing [for imaging]said Adam Koospalo, senior research analyst at UChicago and lead author of the study.
Initially, the team was studying the larger axonal nerve cords in the arms but they began to notice that the INCs did not stop at the base of the arm, but rather continued out of the arm into the animal’s body. Realizing that little work had been done exploring the anatomy of INCs, they began tracing the nerves, expecting them to form a loop in the octopus’ body, similar to axonal nerve cords.
Through imaging, the team determined that in addition to running the length of each arm, at least two of the four cylinders extend into the octopus’s body, where they bypass adjacent arms and fuse with the INC of the third arm. This pattern means that all arms are connected symmetrically.
However, it was difficult to say how the pattern would hold up across all eight arms. “As we were filming, we realized they weren’t all coming together like we expected, they all seemed to be going in different directions, and we were trying to figure out how if the pattern is consistent for all the arms, how does it work?” Hill said. “I even brought one of these kids’ toys—the Spirograph—to play with what it would look like, and how everything would connect in the end. It took a lot of filming and playing with the graphics as we racked our brains over what could happen before it became clear how it all fit together.” .
The results were not at all what the researchers expected to find.
“We think this is a novel limb-based nervous system design,” Hill 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 of the older research papers have shared interesting insights,” Hill said. One study from the 1950s showed that when you manipulate an arm on one side of an octopus with damaged brain regions, you will see the arms responding on the other side. Therefore these nerves could allow for decentralized control of reflexive response or behaviour. However, we also see that fibers exit from nerve cords into muscles along their tracts, so they may also allow continuity of allergic reactions and motor control along their lengths. “
The team is currently conducting experiments to see if they can gain insights into this question by analyzing the physiology and unique mapping of INCs. They are also studying the nervous system of other cephalopods, including cuttlefish and cuttlefish, to see if they share similar anatomy.
Ultimately, Hill believes that in addition to shedding light on unexpected ways invertebrate species may design a nervous system, understanding these systems could help develop new engineering technologies, such as robotics.
“Octopuses could be a biological inspiration for the design of autonomous devices under the sea,” Hill said. “Think of their arms — they can bend anywhere, not just at the joints. They can twist and extend their arms and operate their suckers, all independently. The function of an octopus’s arm is much more complex than ours, so understanding how octopuses integrate sensorimotor information and control their Movement can support the development of new technologies.”
Reference: “Multiple nerve cords connect the arms of octopuses, providing alternative pathways for signaling between the arms” By Adam Koospalo, Samantha Cuddy, and Melina E. Hill, Nov. 28, 2022, Available Here. Current Biology.
The study was funded by the United States Office of Naval Research.
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