Saturday, December 26, 2009

Roger W. Sperry Nerve regeneration

Roger W. Sperry
The Nobel Prize in Physiology or Medicine 1981

In experiments with fish, frogs, and salamanders (chosen because they have great powers of regeneration), Sperry demonstrated that individual nerve fibers (which are actually different cells) behave as if each is chemically different from every other, and these chemical differences are matched in the brain. The result is that in an animal whose optic nerves are severed and then allowed to regenerate, the thousands of individual fibers that make up each optic nerve grow back into the brain and there make the same connections they had before. The animal is then able to see as if the nerves had never been severed. Proof that no adaptive reorganization of the neural circuits is involved in regeneration consisted of showing that if an eye whose optic nerve is severed is also rotated in its socket, the world seen by the eye after regeneration is still upside down and backwards. Furthermore, as in the case of the rat with the crossed nerves, no amount of retraining makes it see correctly: the animal invariably strikes to the left when it sees a worm on its right.

The conclusion that the circuitry of the brain is fixed in early development is supported by much more evidence than I can summarize here. It has given rise to a field of research focused on "axonal guidance". Sperry's result concerning the chemical individuality of each nerve fiber has been confirmed by modern molecular methods. It is a result that is loaded with meanings at many levels - from immediate consequences for neurosurgery to large and still not fully explored implications for evolution and development, and even for social-political questions. It raises other fascinating and still unsolved questions. For example, the capacity to learn obviously implies some neural plasticity. But given the basic determinism of the brain that Sperry uncovered, what does learning actually consist of at the cellular and chemical level? These and other questions posed by his findings are now being studied, and no doubt they will continue to be worked on for a long time in the future.

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