Return to CPN Home Page
Neural Networks
|
||
|---|---|---|
|
are controlled by specialized neural networds. For a jawless fish known as the lamprey, the circuitry has been worked out |
||
|
|
||
| It is difficult to grasp how the human brain is able to keep up with the re- quirements of running or even walk- ing: deciding what joint needs to be moved, exactly when it should bend and by how much, and then sending the proper series of impulses along nerves to activate the appropriate combination of muscles. The dexterity that even low- ly creatures display as they swim, fly, run or otherwise propel their bodies through their surrounds is truly marvelous. Even the most sophisticated mobile robots perform poorly in comparison. Although many mysteries of animal locomotion are yet unsolved, scientists are beginning to comprehend the way vertebrates (creatures with backbones, including humans) can almost effort- lessly coordinate complicated movements that may involve hundreds of muscles. The formidable task of manag- ing the body's various motions is sim- plified by a remarkable form of neural organization, one that distributes the re- sponsibility for coordinating such acts to distinct networks of nerve cells. Some of these specialized circuits, such as the one that keeps a person constantly breathing, are ready to operate flawlessly from birth. others, such as those that control crawling, walking or running, can take time to mature. The neural networks that govern specific oft-repeated motions are sometimes called central pattern generators. They can steadfastly execute a particular action over and over again without the need for conscious effort. The key neural-control circuits that humans use for breathing, swallowing, chewing and certain eye movements are contained within the brain stem, which surrounds the uppermost spinal cord. Oddly enough, the circuits for walking and running (as well as some protective reflexes) are not located in the brain at all but reside in the spinal cord itself. |
Since the late 1960s, my colleagues and I have been attempting to unravel the design of the neural systems that coordinate locomotion in various experimental animals in hopes that this research will help scientists understand some of the intricacies of the human nervous system. Much is yet to be learned, but we have finally produced a blueprint for the neural networks responsible for movement in a simple vertebrate, a type of jawless fish known as a lamprey.
Scientists have deduced much about the organization of the human central nervous system from studies of laboratory animals. Appreciation for the significance of the spinal cord to locomotion first came just after the turn of the century, when pioneering British neurophysiologists Charles S. Sherrington and T. Graham Brown observed that mammals with severed spinal cords could produce alternating leg movements even though the connection to the brain had been cut. Much later my colleagues and I were able to show definitively that such motions corresponded to those movements used for locomotion. Thus, we could conclude that the essential nerve signal patterns for locomotion am generated completely with in the spinal cord. Yet it remained a question how the brain controls these circuits and chooses which should be active at a given instant. Much insight into this process came during the 1960s, when the Russian investigators Grigori N. Orlovski and Mark L. Shik, then at the Academy of Science in Moscow, demonstrated that the more that specific parts of the brain stem of a cat were activated, the faster the animal under study would move. With increasing stimulation, the |
cat would proceed from a slow walk to a trot and finally to a gallop. A very simple control signal from a restricted area of the brain stem could thus generate intricate patterns involving a large number of muscles in the trunk and limbs by activating the pattern generators for locomotion housed within the animal spinal cord. Beyond providing clues to the interactions between brain and spinal cord, this experiment helped to explain how animals can move about even after much of their brain is surgically removed. Some mammals (such as the common laboratory rat) can have their entire forebrain excised and are still able to walk, run and even maintain their balance to some extent. Although they move with a robotic stride, without making any attempt to avoid obstacles placed in their path, these animals are fully able to operate their leg muscles and to coordinate their steps. The details of how the brain activates neural networks in the spinal cord took years to lay out. It is now known that large groups of nerve cells in the fore- LOCOMOTION for humans, like all vertebrate animals, is orchestrated by the central nervous system. Specialized neural circuits in the forebrain (red ) select among an array of "motor programs" by activating specific parts of the brain stem (blue.) The brain stem in turn initiates locomotion and controls the speed of these movements by exciting neural networks (called central pattern generators) located within the spinal cord (purple). These local networks contain the necessary control circuitry to start and stop the muscular contractions involved in locomotion at the appropriate times. Networks of neurons in the brain stem also control breathing, chewing, swallowing, eye movements and other he quently repeated motor patterns. |
64 SCIENTIFIC AMERICAN January 1996 |
||
|
|
||
|
|
|
|
Internet Services Provided by
