| side of the spine but also refined our conception of the lag between the activation of adjoining segments. This delay gses from the neurons that reach along the cord and inhibit segments in the tailward direction. These connections ensure that the overal level of excitation will typically be highest at the head end of the animal's condition that leads to delayed activation of the segments, one after the other, all along the animal's body. We also found that the normal pattern could be reversed by increasing the excitability in the most tailward part of the spinal cord, thereby enabling backward swimming. The "hardwired" spinal network of the lamprey thus retains a considerable degree of flexibility. For the most part, we considered computer simulations that mimic only the lamprey's neural activity. But recent efforts led by Ekeberg have succeeded in modeling the entire lamprey, from the muscle fibers controlling the different segments to the viscous properties of the surrounding water. The neural-control circuits we had previously charted provided everything this virtual lamprey needed to swim. |
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| VIRTUAL SWIMMING by a simulated lamprey suggests that neural models developed in the laboratory can portray how the real creature maneuvers itself through the water. These computer-generated images show the lamprey swimming straight(a), turning (b) rolling to one side (c) and pitching downward (d). | ||
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Crawling Out of the Water We can now be satisfied that the lamprey's capacity for locomotion can be undustood in turms of the interactions of spinal nerve cells. But how certain is it that these mechanisms operate in higher forms of life? The lamprey diverged from the main vertebrate line quite early during the course of evolution, about 450 million years ago, at a time when vertebrates had not yet developed limbs. So it was not immediately obvious whether our results were relevant to other animals. |
other vertebrates as well, from fish to primates, the overall neural organizaton is arranged along a similar plan. Discrete regions of the brain stem initiate locomotion, and the spinal cord processes the signals with specialized circuits. Yet the cellular mechanisms used for locomotion in these other animals are still largely unknown. Researchers have shown that pattern generators are present and have probed some of their neural conaponents, but so far it has not been possible to unravel their inner architecture. During the past few years, however, new techniques have been developed to isolate the spinal cords of the other classes of vertebrates (mammals, birds and reptiles), and it seems likely that in the next few years investigators may uncover how these animals |
control walking, running and flying. Because the earliest vertebrates used only undulatory swimming for locomotion, the networks that later evolved to control fins, legs and wings may not be all that different from what my colleagues and I have already studied. Evolution rarely throws out a good design but instead modifies and embellishes on what already exists. It would be most surprising to discover that there were few similarities between lampreys and humans in the organization of control systems for locomotion. Scientists may yet devise ways to map out and to activate dormant pattern-generating circuits in people with severed spinal cords. indeed, such miraculous medical advances might not be that far away: a turbocharged Ferrari is, after all, just another kind of car. |
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STEN GRILLNER recieved an M.D.-Ph.D. degree in 1969 from the UIniversity of Goteborg in Sweden, where he then joined the faculty. In 1975 Grillner moved to the department of physiology at the Karolinska Institute in Stockholm. He joined the Nobel Insitute for Neurophysiology in 1987 and now served both as chairman of the department fo neuroscience at Karolinska and as director of the Nobel Committee for Physiology or Medicine since 1987. |
NEUROBIOLOGICAL BASES OF RHYTHMIC MOTOR ACTS IN VERTEBRATES. S. Grillner in Science, Vol. 228, pages 143-149; April 12, 1985. |
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SCIENTIFIC AMERICAN January 1996 |
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