Octopuses are not like humans – they are eight-armed invertebrates and are more closely related to oysters and snails. Despite this, they developed complex nervous systems with as many neurons as dogs’ brains, allowing them to exhibit a wide variety of complex behaviors.
This makes them an interesting subject for researchers like Melina Hale, Ph.D., William Rainey Harper Professor of Organismic Biology and Associate Dean. University of ChicagoThose who want to understand how alternative nervous system structures can perform the same functions as in humans, such as sensing limb movements and controlling movement.
In a recently published study Current BiologyHale and colleagues have discovered a surprising new feature of the octopus nervous system: a structure that allows the octopus to sense arm movement, a structure that allows intramuscular nerve cords (INCs) to connect arms on opposite sides of the animal.
The surprising discovery provides new insights into how invertebrate species independently evolved complex nervous systems. It could also provide inspiration for robotic engineering, such as new autonomous underwater devices.
“In my lab, we work on mechanosensation and proprioception — how movement and positioning of limbs are perceived,” Hale said. “These INCs have long been thought to be proprioceptive, so they are an interesting target to help answer the kinds of questions our lab is asking. So far they haven’t been studied much, but past experiments have shown they are important for arm control.”
Thanks to support for cephalopod research offered by the Marine Biology Laboratory, Hale and his team were able to use young octopuses for the study, which were small enough to allow researchers to image the bases of all eight arms at once. This allowed the team to trace INCs throughout 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 stick the samples in the right direction and get the right angle during sectioning. [for imaging]said Adam Kuuspalu, Senior Research Analyst at UChicago and lead author of the study.
The team was initially examining 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 instead continued from the arm down the animal’s body. Realizing that little work had been done to explore the anatomy of INCs, they began tracing the nerves and expected them to form a ring in 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 it bypasses the two adjacent arms and merges with the INC of the third arm. This pattern means that all arms are connected symmetrically.
However, it was difficult to determine how the model would stand on all eight arms. “While we were imaging, we noticed that they didn’t all come together as we expected, they all seem to be going in different directions, and we were trying to figure out what it would be like if the model was valid for all arms. Work?” said Hale. “I even pulled out one of those kids’ toys – a Spirograph – to play with how it would look and how it would eventually connect. It took a lot of viewing and playing with the drawings as we blew our brains over what was going to happen until it became clear how it all came together.”
The results were not what the researchers had hoped to find.
“We think this is a novel 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 old papers shared interesting information,” Hale said. “A study done in the 1950s showed that when you manipulate an arm on one side of an octopus with lesions in the brain regions, you will see the arms on the other side responding. So these nerves may be allowing for a reflex response or decentralized control of its behavior. we also see that they go to the muscles along their path, so they can allow for continuity of proprioceptive feedback and motor control along their length.
The team is currently conducting experiments to see if they can gain insight into this question by analyzing the physiology and unique arrangement of INCs. They’re also examining the nervous systems of other cephalopods, including squid and cuttlefish, to see if they have similar anatomy.
Ultimately, Hale believes, in addition to illuminating unexpected ways in which an invertebrate species might design a nervous system, understanding these systems could help develop newly designed technologies such as robots.
“Octopuses could be a biological inspiration for the design of autonomous submarine devices,” Hale said. “Think of their arms – they can bend everywhere, not just at the joints. They can bend independently, extend their arms and operate their suction cups. The function of an octopus arm is much more complex than ours, so understanding how octopuses integrate sensory-motor information and movement control could support the development of new technologies. ”
Reference: Adam Kuuspalu, Samantha Cody, and Melina E. Hale, November 28, 2022, “Multiple nerve cords connect the arms of octopuses, providing alternative pathways for interbranch signaling”, Current Biology.
The study was funded by the United States Office of Naval Research.
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