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nervous system composed of compara- tively few cells (only about 1,000 in each segment of the spinal cord), making it ideal for our purposes. The lamprey also suited us because Carl M. Rovainen of Washington University had shown that the fish's central nervous system could be maintained in a glass dish and studied for several days after it is removed from the animal. Moreover, motor networks in the isolated nervous system remain active. The strategy of choosing a simple but relevant experimental animal for study has yielded key insights into many different biological processes. For example, exam- ination of invertebrate nerve cells, such as those of the squid and lobster, provided the first important clues to how nerve impulses are generated and how networks of nerve cells function. |
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| PATTERN GENERATORS, separate neural networks that control each limb, can interact in different ways to produce various gaits, such as the amble, trot and gal of a home In ambling (top) the animal must move the fore and hind leg of one flank in parallel. Trotting (middle) requires movement of diagonal limbs (front right and back left, or front left and back right) hi unison. Galloping (bottom) involves the forelegs, and then the hind legs, acting together. |
A Hardwired Fish From the beginning of our studies r with the lamprey in the late 1970s, MY colleague Peter Wallen and 1, along with a number of collaborators, have concentrated on understanding the fundamental features features of the animal's swimming. Like other fish, the lamprey propels itself forward through the water by contracting its muscles in an undulating wave that passes along the creature's body from head to tail. To produce a propulsive wave, the animal must generate bursts of muscle activity that bend each section of the spine toward one side and then the other in rhythmic alternation. But the lamprey also needs to coordinate the contractions of consecutive segments along its body so that a smooth wave forms. We soon discovered that the neural controls for both these abilities are distributed throughout the spinal cord. If a lamprey's spinal cord is isolated and separated into several pieces, each length can be made to show the characteristic alternating pattern, and within any given portion the activity between adjoining segments stays coordinated. Further observations showed that the lag between activation of adjacent segments remains fixed during a given wave, as the undulatory motion propagates down the body of the lamprey. But the lag time changes with the fish's speed, so that the overall period of that wave (the time it takes for the wave to travel the entire length of the body) can vary from about three seconds during very slow swimming to as little as one tenth of a second for sudden sprints. Exactly, the same characteristic contractions occur in reverse order when the fish swimming backward. To understand how the lamprey nevous |
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brain, called basal ganglia, connect (Other directly or through relay cells) to target neurons in the brain stem that in turn can initiate different "motor programs." Under resting conditions, the basal ganglia continuously inhibit the brain's sundry motor centers so that no movements occur. But when the active inhibition is released, coordinated motions may begin. The basal ganglia thus function to keep the various motor programs of the nervous system under strict control. This suppression is essential: renegade operation of a motor program could be disastrous for most any animal. In humans, for instance diseases of the basal ganglia can cause involuntary facial expressions and hand or limb movements. Such hyperkinesis occurs commonly in cerebral palsy and Huntington's disease and as a side effect of some medications. Other diseases of the basal ganglia can lead to the opposite situation, with more inhibition than desired being applied; victims then have difficulty initiating movements. The best known example of such a disability is Parkinson's disease. Although medical researchers keenly desire to understand how such neurological disorders arise and what might be done to correct them, progress has been difficult to achieve because the |
human nervous system (which houses nearly a trillion neurons) is extraordinarily complicated. It is not yet feasible to examine the neural circuits in humans, or indeed in any mammal, in much detail. My colleagues and I have therefore focused our studies on much simpler vertebrates. We sought an experimental animal with the same basic neural organization as humans but with far fewer components. Our fundamental approach has been similar to something an imaginary researcher from outer space might undertake to deduce the basic mechanics of an automobile. An extraterrestrial scientist would hue best by becoming such an analysis with a Model T Ford (if one could be obtained), because that vintage vehicle has all the essential components of a car--internal-combustion engine, transmission brakes and steering-manufactured from a simple design and arranged for easy inspection. Investigations that began by directly probing a more advanced model, such as a modern turbocharged Ferrari might prove far more frustrating. One presumes that knowledge of a Model T would serve as the foundation needed to understand me anatomy of the more elegant and sophisticated car. We investigated several possible subjects before settling finally on me lamprey-an elongate, jawless fish with a large mouth adapted for sucking. The lamprey is a primitive vertebrate with a |
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66 SCIENTIFIC AMERICAN January 1996 |
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