Spinal cord findings could help explain origins of limb control

We might have more in common with a lamprey than we think, according to a new Northwestern University study on locomotion. At its core, the study of transparent zebrafish addresses a fundamental evolution issue: How did we get here?

Neuroscientists Martha W. Bagnall and David L. McLean have found that the circuits that produce body bending in swimming are more complicated than previously thought.

Vertebrate locomotion has evolved from the simple left-right bending of the body exemplified by lampreys to the appearance of fins in bony fish to the movement of humans, with the complex nerve and muscle coordination necessary to move four limbs.

Bagnall and McLean report that differential control of an animal's musculature—the basic template for controlling more complex limbs—is already in place in the spinal networks of simple fish. Neural circuits in zebrafish are completely segregated: individual neurons map to specific muscles.

Specifically, the that drive muscle movement on the dorsal (or back) side are separate from the neural circuits activating muscles on the ventral (or front) side. This is in addition to the fish being able to separately control the left and right sides of its body.

Ultimately, understanding more about how fish swim will allow scientists to figure out how humans walk.

"Evolution builds on pre-existing patterns, and this is a critical piece of the puzzle," McLean said. "Our data help clarify how the transition from water to land could have been accomplished by simple changes in the connections of spinal networks."

The findings will be published Jan. 10 in the journal Science. McLean, an assistant professor of neurobiology in the Weinberg College of Arts and Sciences, and Bagnall, a postdoctoral fellow in his research group who made the discovery, are authors of the paper.

"This knowledge will put us in a better position to devise more effective therapies for when things go wrong with neural circuits in humans, such as ," McLean said. "If you want to fix something, you have to know how it works in the first place. Given that the fish spinal cord works in a similar fashion to our own, this makes it a fantastic model system for research."

McLean and Bagnall studied the motor neurons of baby zebrafish because the fish develop quickly and are see-through. They used state-of-art imaging techniques to monitor and manipulate neuronal activity in the fish.

"You can stare right into the nervous system," McLean said. "It's quite remarkable."

The separate circuits for moving the left and right and top and bottom of the fish allow the animal to twist its body upright when it senses that it has rolled too far to one side or the other.

"This arrangement is perfectly suited to provide rapid postural control during swimming," Bagnall said. "Importantly, this ancestral pattern of spinal cord organization may also represent an early functional template for the origins of limb control."

Separate control of dorsal and ventral muscles in the fish body is a possible predecessor to separate control of extensors and flexors in human limbs. By tweaking the connections between these circuits as they elaborated during evolution, it is easier to explain how more complicated patterns of motor coordination in the limbs and trunk could have arisen during dramatic evolutionary changes in the vertebrate body plan, the researchers said.

"We are teasing apart basic components of locomotor circuits," McLean said. "The molecular mechanisms responsible for building spinal circuits are conserved in all animals, so this study provides a nice hypothesis that scientists can test."

More information: The paper is titled "Modular Organization of Axial Microcircuits in Zebrafish." www.sciencemag.org/content/343/6167/197.abstract

Related Stories

Watching fish swim

Jan 07, 2014

As fish go, the lamprey has to be one of the most repulsive. Its eel-like body culminates in a tooth-encrusted sucker mouth straight out of a sci-fi horror film. Yet it turns out the lamprey, the most primitive ...

Spinal nerve connections develop using simple rules

Jan 09, 2014

(Medical Xpress)—Repairing spinal injuries with stem cells may be a step closer thanks to scientists at the Universities of Bristol and Plymouth. A new study, published today in the Journal of Neuroscience, employ ...

Recommended for you

Emotional adjustment following traumatic brain injury

Oct 24, 2014

Life after a traumatic brain injury resulting from a car accident, a bad fall or a neurodegenerative disease changes a person forever. But the injury doesn't solely affect the survivor – the lives of their spouse or partner ...

New ALS associated gene identified using innovative strategy

Oct 22, 2014

Using an innovative exome sequencing strategy, a team of international scientists led by John Landers, PhD, at the University of Massachusetts Medical School has shown that TUBA4A, the gene encoding the Tubulin Alpha 4A protein, ...

User comments