Studying Neural Circuits in Crayfish

By Marlene Cimons, National Science Foundation

One of the most enduring mysteries within the field of neurobiology is how animals coordinate the movements of their body parts. Since each limb has a separate neural circuitry, how do they manage to work in synch? How, for example, do humans use their arms and legs to effortlessly stroll down a street?

The crayfish, a lobster-like crustacean that uses four abdominal appendages known as swimmerets to propel itself through streams and lakes, may provide some of the answers.

Scientists believe that the crayfish, with its simple neural circuitry and nerve cells easily accessible to impulse-recording electrodes, is the perfect creature in which to study the basis of movement. To be sure, crayfish do not have spinal cords, but they have something similar, a central nerve cord, that is close enough, and which is easy to study.

“You can take the equivalent of a spinal cord out of a crayfish, put it in a dish, and it will produce the same motor activity that would drive the coordinated beating of the swimmerets when the animal is swimming forward,” said Brian Mulloney, professor of biological sciences in the University of California, Davis department of neurobiology, physiology and behavior.  “I can do the same kinds of cellular experiments that we need to do to mammals to understand the circuits. We can work with these animals’ nervous systems under conditions where we can take them apart.”

The crayfish system “shows all the same characteristics, all the same complexity and coordination, as that in humans,” he added.

Mulloney and his colleague, Timothy Lewis, associate professor of mathematics in the UC Davis College of Letters and Science, are merging mathematical theory with biological experiments to build a comprehensive model of the entire neural circuitry of the crayfish’s swimmerets. The scientists will then examine how this system interacts with nerve cells and synapses to produce stable, coordinated movement.

“The things controlling the muscles within one limb are independent of the things controlling the muscles in the other limb. How do you coordinate that?” he said.

The goal of the research is to provide a foundation to better understand the workings of the spinal cord and brain stem in all organisms, including humans.

Their work ultimately could benefit the field of robotics—creating walking machines that are well-coordinated—as well as in clinical health settings, such as in the treatment of mid-level spinal cord injuries, including the development of new electronic products that potentially could help patients. “If we succeed in our efforts, that will bring us many steps closer to finding treatments for a range of neural system diseases and disorders,” Mulloney said.

The research, which thus far has created four new jobs, is funded by the National Science Foundation as part of the American Recovery and Reinvestment Act of 2009. The researchers are receiving about $200,000 a year for five years.

Based on their own earlier work, the scientists already have some of the answers. “We know which neurons do this coordination. We know for each swimmeret which neurons export information about the swimmerets’ movement to other parts of the nervous system, and we know the neurons they talk to,” Mulloney said. “We’ve got all the key players.”

The next step is to formally describe in mathematical terms “what the coordinating system is doing when you stand back and look at it as a set of neural circuits that are coordinated to produce an affected behavior,” Mulloney said.

“How do we describe it?” he continued. “Each output produces a step. How are they described mathematically, and how do you tie them together to get the kind of stable coordination necessary for the movement? Mathematics will allow us to step back from the specifics of the swimmeret system and describe how limb coordination is being achieved.”

The results could provide the field of robotics with “new ways to think about coordination,” for building walking machines that walk, Mulloney said. In humans, “it would be wonderful to understand how normal behavior in health people is accomplished,” in order to find treatments to help those who have lost leg function.

“When human circuits are isolated from brain stem and cortical control, they can still, in principle, function,” he said. The challenge will be “to find effective ways to turn them on, to reinforce the coordinating information that is left in the spinal cord,” he added. “We are trying to figure out in general how this class of circuits works, then, possibly, you can look for specific points at which to restore their function in people.”

While likely a long time in the future, “we hope to be able to see applications that will result in long term good for patients who need it,” Mulloney said.

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