a Fish Story

What mysteries about movement can the lamprey help solve?

By Steve Schultz, Comm '98

How does the spinal cord control movement? Dr. James T. Buchanan’s pioneering research in neurophysiology is shedding light on that mystery and may one day lead to new therapies for curing spinal cord diseases and injuries.

Buchanan’s research focuses on the neuronal networks and cellular structures that control locomotion in simple vertebrate organisms. He works with the simplest of such organisms, the lamprey, a primitive fish that has survived for millions of years relatively unchanged. Compared to higher vertebrates (such as mammals), the lamprey has a manageable number of cells to study.

A lamprey is a long, thin fish that looks something like an eel. A remarkable physical feature is the lamprey’s jawless mouth that is a sucker filled with dozens of small teeth used for attaching to a fish. The lamprey will then feed off of the fish until it is sated or the fish dies.

Buchanan places segments of the lamprey spinal cord — each comprising 1,000 nerve cells — into a physiological solution that keeps them alive and then uses micromanipulators to insert the tips of microelectrodes into the nerve cells. The electrodes allow Buchanan to record the electrical activity of individual nerve cells or to stimulate them by injecting current through the electrodes.

Like higher vertebrates, the lamprey spinal cord can generate locomotor activity even without sensory feedback or descending input from the brain. This locomotor activity in the isolated spinal cord, in the absence of muscles and movement, is called fictive locomotion. In other words, the cellular machinery for locomotor rhythm generation is located in the spinal segments.

“The locomotor network is active when I’m studying the isolated spinal segments, and the nerve cells are generating rhythmic activity that would normally cause the muscles to propel the fish in the water,” Buchanan explains. “I then use a variety of methods to understand how these nerve cells and their synaptic connections produce this rhythmic locomotor activity.”

Buchanan and his colleagues have identified classes of spinal nerve cells that are major components of the locomotor network. They also have been able to characterize the electrical properties (how they process synaptic inputs) of the spinal nerve cells and the properties of their synapses (neurotransmitter and whether they are excitatory or inhibitory), and they have identified their synaptic connections with other spinal nerve cells.

“We have revealed a network of nerve cells that can account for many aspects of lamprey swimming

activity. The similarities in the network we have characterized in lamprey with the locomotor networks that are being studied in frogs and mice give us confidence that we are getting at the fundamental core of the locomotor network in all vertebrates,” he says.

Although the spinal locomotor network can function without sensory feedback and descending input from the brain, both are important to the proper functioning of the network. Therefore, Buchanan’s most recent studies are focused on the interactions of the brain and spinal cord with an objective of discerning how the brain controls the locomotor network and what feedback information the spinal cord transmits to the brain about its activity.

“The brain/spinal cord interactions are more simply organized in lamprey than in higher vertebrates, making the system easier to study,” he says. “But it is our belief that in the lamprey we are investigating a fundamental core of the brain/spinal cord interaction that will be found in all vertebrates.”

Buchanan has published extensively on his understanding of the lamprey spinal cord. While his published lamprey model does not provide a complete understanding, it is the best understood locomotor network in an adult vertebrate. Improving the model is an ongoing challenge, one that he explains — appropriately enough — drawing an analogy to fishing.