13 March 2007

Wie man Salamandern das Laufen beibringt.

Schwimmen und Laufen sind schon zwei ziemlich unterschiedliche Sachen. Nicht nur für Menschen, auch für Salamander. Wie haben sich diese beiden unterschiedlichen Bewegnungsformen entwickelt, wie werden sie reguliert? Eine nette Geschichte aus Science.
Salamandra robotica is a triathlete. She walks. She crawls. She swims. One of very few robots capable of multiple modes of locomotion, this salamanderlike machine has demonstrated that it may have been relatively easy for early animals to take their first steps on land. [...] Thanks to Salamandra, he and his colleagues have shown that merely changing the strength of the brain signal driving locomotion can determine whether an animal walks or swims. Once the neural networks for moving legs were in place, little additional neural circuitry was required, Ijspeert and his colleagues report. [...]

Salamanders are a lot like the first land-based tetrapods. These amphibians swim in the same manner as primitive fish, such as lampreys, and they waddle, with legs splayed out, like alligators and their ancient relatives. The salamander "represents therefore a key animal to understand the evolutionary transition from swimming to walking," [...]

They focused on two networks of nerve cells, called central pattern generators, located along the spine. When a network is activated, its individual nerve cells alternate between firing and being quiet, causing rhythmic muscle contractions. Both the lamprey and the salamander have one network to drive the body musculature. For swimming, this network sends waves of muscular contractions down the body, repeatedly creating S-shaped waves that move tailward. Amphibians have a second one, which controls the limbs.

In 2003, Cabelguen and his colleagues discovered the region of the salamander's midbrain that fires off signals to these two central pattern generators. When the researchers gently stimulated this part of the brain electrically, they caused the limbs to move as if walking. As they gradually increased the applied current, neural activity in the limbs sped up until finally the nerve cells shut down. At this point, the amphibian's limbs stopped moving and the body started undulating much faster, as in swimming.

Ijspeert's group developed a mathematical model of this transition, from which they concluded that the limbs' central pattern generator interfered with the other neural network's ability to set up the S-waves. This interference produced the slower body bending necessary for walking. Only when the limb's central pattern generator was shut down was the salamander's other network of nerve cells free to fire as fast as needed to generate swimming or, on land, crawling.

Ijspeert then built Salamandra robotica to test the mathematical model's predictions.[...]

As in Cabelguen's experiments, less intense signals caused the robot to walk. With stronger signals, the legs sped up. But with the strongest signals, the legs stopped moving and Salamandra began slithering. "This close correspondence suggests that the researchers may have accurately recreated some of the actual neural control mechanisms salamanders use," says John Long, a biomechanist at Vassar College in Poughkeepsie, New York. The results, Long and others say, suggest that early animals didn't need to invent completely new neural pathways to expand their locomotor repertoire.


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