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Within a single segment of the lamprey's spinal cord lies an intricate net work of interconnected nerve cells. Groups of neurons (boxes ) on the left and right sides of the cord are excited by signals sent form the animal's brain stem. Specialized neurons within these groupings respond by sending either excitatory (red ) or inhibitory (purple ) signals to neighboring cells Neurons known as E cells (for excitatory on one side of We spinal segment will activate motoneurons (M) that in turn cause the muscles on that side of the fish to contract. These E cells also induce inhibitory (1) neurons to reduce the level of excitation in the group of neurons on the opposite side of the spine, ensuring that the opposing muscles relax. The bursts of excitation that cause one side to contract are terminated in a number of ways. Certain stretch receptor neurons (purple triangles ) on the opposite side of the spine emit signals that inhibit the contraction. At the same time, other activated stretch receptors (red triangles ) excite the neurons on the extended side to initiate contraction there. In addition large (L) inhibitory nerve cell on the contracting side can be induced by the brain stem to inhibit the I cells. This allows the opposite side to become active and send inhibitory signals back. Finally, there an several electrochemical mechanisms inside cells can force a pulse of excitation to subside helping to control the timing of the network. Although these local spinal cord circuits can operate autonomously, they normally feed back information to the brain about the ongoing network activity. These signals can then be combined with other forms of sensory input, such as cues from vision or from the balance system in the inner ear, to modify the animal's movements. |
neural networks extend axons along the spine. Special inhibitory cells in each segment send signals through these axons in the direction of the tail for as much as one fifth of the length of the spine. So-called excitatory cells contain somewhat shorter axons that extend in both directions. Thus, the activity at one location on the spinal cord can affect adjacent regions. But how exactly might signals linking different segments create the characteristic wave like motion? After much thought, we proposed that nerve signals could excite the leading segment (near the lamprey's head) so that the contractions there alternate back and forth faster than the spine would otherwise tend to oscillate. The second section behind the head would follow the quickened motions of the first (because the two segments are coupled by nerve cells), but with a slight lag as the inherently slower section tried to catch up with the leader. By similar reasoning, the third section should then follow the second with a slight delay-and so forth down the line. The series of incremental delays, we surmised, allowed the lamprey to produce a uniform wave.
Even with our newly developed wiring diagrams and a mass of other detailed information about the properties of the different types of nerve cells involved, we were long challenged to make more than modest, general statements about how these complex neural circuits operated. To test whether the information we had gleaned truly explained how the lamprey could swim, my colleagues and I joined with Anders Lansner and Orjan Ekeberg of the Royal Institute of Technology in Stockholm to create various computer models of the process. |
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opposite side. Thus, when one flank of a given section becomes active, the other is automatically inhibited. Other specialized nerve cells, called motoneurons, link the nerves of the spinal cord to the muscle fibers that actually do the job of moving the lamprey through water.
But these spinal networks are not simply passing signals sent down from the brain of the animal. Although the brain stem issues the overall command for the fish to swim, it delegates the task of coordinating the muscle movements to these local teams, which can process incoming sensory data and adjust their own behavior accordingly. In particular, they react to specific "stretch receptor neurons" that sense the bending |
of the lamprey's spine as it swims. As one side of the body is contracting, the other is extending-and it is this extension that triggers the stretch receptors. These activated nerve cells then take one of two complementary actions: they either excite neurons on the extended side (inducing muscles there to contract), or they inhibit neurons on the opposite side, causing them to halt contraction. By such processes (as well as several rather complex cellular mechanisms), the fundamental oscillatory movements of the lamprey's neuromuscular system are maintained. As we further followed the neural circuitry of the lamprey's spinal cord, we determined that some of the local |
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68 SCIENTIFIC AMERICAN January 1996 |
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