Scientists have been fascinated by the lamprey, a type of jawless fish, for its impressive ability to heal from spinal cord injuries for almost half a century. A recent study suggests that lampreys may have a method to regain their swimming ability, even though their neural regeneration is limited.
A team of researchers, led by Christina Hamlet of Bucknell University and including Jennifer R. Morgan of the Marine Biological Laboratory (MBL), has developed a mathematical model that sheds light on how lampreys are able to regain their ability to swim even after suffering severe spinal injuries. Their findings, published in the Proceedings of the National Academy of Sciences, could have important implications for the development of new therapeutic approaches for human patients or algorithms for soft robots.
According to Morgan, the study’s key takeaway is that it is possible to restore locomotion in lampreys, even in the absence of descending command across a spinal lesion, by boosting sensory feedback. Unlike humans and other mammals, lampreys are able to recover fully and relatively quickly after suffering severe spinal cord injuries high up in the spine. Although neural regeneration does play a role in their recovery, it is not the only factor at play.
Morgan’s previous research had already shown that the restoration of only a small percentage of neurons and neuronal connections across a spinal injury was insufficient to explain the lamprey’s remarkable recovery. This led the team to hypothesize that the lamprey must be using some other mechanism to regain its swimming abilities, which the new study helps to elucidate.
“I had all of these questions about how that could possibly work. How could you get a functioning nervous system with a few small sparse connections?” Morgan asked.
The scientists had previously speculated that lampreys might use proprioception or kinesthesia, which is feedback from the body, to navigate their movements in addition to the descending neural connections in the spinal cord. Jennifer R. Morgan reached out to her old friend, Eric Tytell, who is an Associate Professor of Biology at Tufts University and a former MBL Whitman Center Investigator, to discuss this idea. Eric was already working with Lisa Fauci, a Mathematics Professor at Tulane University, and Christina Hamlet, a co-mentored postdoc at Tulane, on the same topic.
Eric Tytell, Lisa Fauci, and Christina Hamlet were working on creating mathematical models to imitate the swimming movements of lampreys. They collaborated to investigate how sensory feedback affects the swimming behavior of lampreys, with the aim of creating models that could replicate these effects. Hamlet, who is now an Assistant Professor of Mathematics at Bucknell University, explained that their goal was to see whether they could simulate the impact of sensory feedback on lamprey locomotion.
The team of researchers simulated various scenarios involving lampreys with spinal injuries, using mathematical models that factored in sensory feedback. They tested both plausible and implausible situations, assuming no neural regeneration across the spinal cord lesion. The advantage of using models, according to Hamlet, is that they can break things that cannot be broken in biology. The model accounted for the curves and stretches in the body above the lesion, transmitting this information to the rest of the body through muscles, rather than the spinal cord.
Surprisingly, even with moderate sensory feedback, the biologically plausible models showed a remarkable recovery of swimming patterns. Greater sensory feedback led to even better improvement. Lampreys, which do regenerate some of their neurons after a lesion and therefore have descending command from the brain to control movement, may require even less sensory feedback than the model suggested. The researchers plan to integrate neuronal regeneration into the model to explore how it impacts movement and interacts with sensory feedback.
“If you have a good computational model, you can go through so many more scenarios of manipulations than is practical with experimentation,” said Morgan.
The researchers believe that their study and upcoming research will aid in the development of treatments for people suffering from spinal cord injuries and other conditions that affect movement. They anticipate that future developments in brain machine interfaces and stimulator devices will incorporate sensory feedback from the body to produce more fluid movements following an injury. Their research could help determine the quantity and type of feedback required to achieve optimal outcomes for people with these conditions.
“Whether you’re an animal like a lamprey that [recovers] spontaneously or a human that needs to be given a drug or an electrical stimulator device, getting to the point where you have a few things in the right place and then reuse what’s already there should be more achievable than trying to recapitulate the identical original pattern of synaptic connections and growth,” said Morgan.
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