Stretch and pressure, the keys to eels’ remarkable locomotive abilities, inform development of new robot

A spinal cord injury in most vertebrates likely inhibits locomotion and induces paralysis—not so in eels. They not only possess the ability to move through water, and surprisingly, across land when intact, but can also continue to swim even if their spinal cord is severed.

The neural mechanisms behind these incredible abilities have long remained a mystery. Revealing more about the control mechanisms behind eel-like locomotion could radically improve the development of robots to better enable them to navigate diverse and challenging environments.

An international group of researchers has done exactly this by integrating two types of sensory feedback into a neural circuitry model for eel-like elongated fish and testing it with computer simulation and experiments with a real robot. Their findings revealed that eels rely on signals from their bodies—like the feeling of stretch and pressure on the skin—to adjust to different environments. These signals, together with the nervous system’s built-in rhythm, may be enough to keep movement coordinated even after a serious spinal cord injury.

Details of the findings are published in Proceedings of the National Academy of Sciences.

“Our findings will help design highly adaptive robots capable of navigating complex and unpredictable environments,” explained Kotaro Yasui, an assistant professor at Tohoku University’s Frontier Research Institute for Interdisciplinary Science (FRIS), and lead author of the study.

Yasui and colleagues first set out to find the control principle behind eels’ movement. They first developed a mathematical model of a neural circuit that integrates two sensory feedbacks: stretch and pressure. The model assumed that each body segment has its own neural circuit, like a Central Pattern Generator, which produces movement rhythms regulated by these sensory signals.

They tested their model with computer simulations and robotic experiments, where the model quickly produced stable swimming thanks to sensory feedback. This same neural circuit also enabled the robot to crawl on land and navigate around obstacles, with the stretch feedback being vital for pushing against obstacles to generate forward thrust.

To explore how eels maintain movement after spinal cord injury, the group conducted spinal cord transection experiments with real eels and corresponding simulation and robot experiments using the neural circuitry model with the stretch and pressure feedback. Simulation and robot experiments revealed that the combination of multisensory feedback and the circuits’ own intrinsic rhythm-generating ability allows the body to synchronize its movements across the injury site, even without input from the brain.

The study had the added benefit of furthering our evolutionary understanding of locomotion.

“The discovery that a swimming neural circuit also supports movement on land suggests that vertebrates may not have needed an entirely new neural circuit when they transitioned to land; rather, flexible swimming circuits were repurposed, reducing the need for complex top-down control while enabling effective movement across different environments,” explained Akio Ishiguro, a professor at Tohoku University’s Research Institute of Electrical Communication (RIEC) and co-author of the paper.