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| UNDULATORY SWIMMING in the eelike lamprey constitutes a relatively simple form of vertebrate locomotion that neuroscientists can examine effectively, in response to signals emitted by the brain, wave after wave of muscle contraction | (red ) and extension (green ) pass from head to tail down the body of a fish propelling it forward through the water (left ). Similar waves traveling from tail to head can drive the creature backward (right ). |
| system could orchestrate such motions, my colleagues and I needed to determine exactly which nerve cells contributed to locomotion and how they interacted. So we devised experiments using electrodes with tips that were less than a thousandth of a millimeter wide. With these sensors we could map out distant connections by placing one electrode inside an individual cell in the brain stem and, at the same time, using another electrode to probe various target cells in the spinal cord. To find a pair of nerve cells that could communicate with each other among the hundreds of possibilities required considerable skill and patience. But the diligent labor of many people finally made it possible to identify the neurons that controlled locomotion and to trace how they were wired together. We ultimately discovered that nerve cells in the brain stem have long extensions (axons) that are in contact with the neurons involved with locomotion throughout the spinal cord In response to signals from the brain, local networks of cells within discrete parts of the cord generate bursts of neural activity. These networks act as specialized circuits, exc- iting the neurons on one side of a given segment of the lamprey's body while suppress g similar nerve cells on the |
| SEA LAMPREY (Petromyzon marinus), which can be up to a has served as an ideal experimental animal ~'s studies of vertebrate locomotion. Because the system is comprised of relatively few cells, the brain stem (brown) and spinal cord (beige) can be isolated and probed in the laboratory (inset). |
SCIENTIFIC AMERICAN January 1996 |
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