Octopuses are not like humans – they are invertebrates with eight arms and are more closely related to clams and snails. Nevertheless, they have evolved complex nervous systems with as many neurons as a dog’s brain, allowing them to exhibit a wide range of complex behaviors.
That makes them an interesting topic for researchers like Melina Hale, Ph.D., William Rainey Harper Professor of Organic Biology and Vice Provost at the University of Chicago, who want to understand how alternative structures in the nervous system can perform the same functions with those in humans, such as the sensation of limb movement and the control of movement.
In a recent study published in Current BiologyHale and her colleagues discovered a new and surprising feature of the octopus’s nervous system: a structure that allows the intermuscular nerve cords (INCs), which help the octopus sense its arm’s movement, connect the arms on opposite sides of its animal.
The surprising discovery provides new insights into how invertebrates have independently evolved complex nervous systems. It may also provide inspiration for robotic engineering, such as new autonomous underwater devices.
“In my lab, we study mechanosensation and proprioception — how movement and position of limbs is sensed,” 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. Until now, not much work has been done on them, but previous experiments have shown that they are important for arm control.”
Thanks to cephalopod research support from the Marine Biological Laboratory, Hale and her team were able to use young octopuses for the study, which were small enough to allow the researchers to image the base of all eight arms at once . This allowed the team to trace the INCs through the web to determine their course.
“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 right angle during the cut. [for imaging]said Adam Kuuspalu, Senior Research Analyst at UChicago and lead author of the study.
Initially, the team was studying the larger axonal nerve chords 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 to locate the nerves, expecting them to form a ring on the octopus body, similar to axonal nerves.
Through imaging, the team determined that in addition to 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 design means that all arms are connected symmetrically.
However, it was difficult to determine how to hold the design in all eight hands. “As we were imaging, we realized that 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 was true for all the arms, how would that work?” Hale said. 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 visualization and playing with drawings, racking our brains about what could be going on before it became clear how everything fits together.”
The results were not at all what the researchers expected to find.
“We think this is a new blueprint for a limb-based nervous system,” Hale said. “We haven’t seen this in other animals.”
Researchers don’t yet know what function this anatomical design might serve, but they have some ideas.
“Some older papers have shared interesting ideas,” Hale said. “A study from the 1950s showed that when you operate a hand on one side of the octopus with damaged brain areas, you will see the hands respond on the other side. So it could be that these nerves allow decentralized control of a reflex response or behavior. That said, we also see that fibers exit the nerve cords to the muscles along their entire tracts, so they could also allow a continuum of proprioceptive feedback and motor control along their entire length.”
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 arrangement. They are also studying the nervous systems of other cephalopods, including squid and cuttlefish, to see if they have similar anatomy.
Ultimately, Hale believes that in addition to illuminating the unexpected ways an invertebrate species can design a nervous system, understanding these systems can help develop new technologies, such as robots.
“Octopuses can provide biological inspiration for the design of autonomous underwater devices,” said Hale. “Think about their arms — they can bend anywhere, not just at the joints. They can twist, stretch their arms, and manipulate their clowns, all independently. The function of an octopus arm is much more complex than ours, so understanding how octopuses integrate sensorimotor information and control movement can support the development of new technologies.”
Citation: “Multiple nerve cords connect octopus arms, providing alternative routes for signaling between arms” by Adam Kuuspalu, Samantha Cody and Melina E. Hale, 28 Nov 2022 Current Biology.
The study was funded by the United States Office of Naval Research.
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