Carving out the nervous system.

Carving out the nervous system

An important aspect of development is the whittling down of the brain: A newborn has far more nerve cells and nerve cell connections than does an adult. Recent research gives an indication of the magnitude and modes of this whittling. Pasko Rakic of Yale University School of Medicine reports that in some brain areas during the first weeks of life an infant loses as many as two nerve cells each second. He suggests that which cells are eliminated is influenced by the cells' activity and environment. His observations help explain the versatility of the primate brain, including its ability to compensate for physical abnormalities.

The overproduction of nerve cells has been demonstrated in several brain areas. For example, in the corpus callosum, a bundle of fibers connecting the right and left hemispheres, a newborn has 200 million axons (nerve cell output fibers) and the adult only 50 million. In a smaller connecting bundle, the hippocampal commisure, the newborn has 1.2 million axons and the adult 200,000, Rakic reports. Competition between fibers connecting to the same target appears to be the key to the selective elimination of connections and then of cells.

Several groups of scientists have demonstrated that if one eye of a monkey fetus is removed, axons from the remaining eye spread over the surface normally innervated by both eyes. Fewer axons from the lone eye are eliminated than would have been eliminated if both eyes were present. In recent work Rakic demonstrated a drop in nerve cell elimination following the loss of a competing area within the brain. Two years after removing part of the visual cortex of a fetal monkey, he finds that the area of the brain that normally shares target cells with the visual cortex is twice the normal size.

When the overabundance of fibers is not eliminated, can the extra fibers function? Would they be beneficial or detrimental to the brain's activities? To investigate these questions, Yale scien tists removed one eye of a monkey fetus and asked whether the remaining eye, with its extra connections, would perform better or worse than one eye of a normal monkey. They found that on an acuity test--in which the monkey determines whether lines are close on a TV screen--the lone eye of the experimental animal did slightly better than the better eye of a normal monkey. Therefore the extra fibers do not hinder brain processing of visual information. Rakic reports that the extra input is not suppressed by the normal input; the extra cells are metabolically active as indicated by uptake of 2-deoxyglucose.

These findings suggest that the extra-keen sensitivity a blind or a deaf person seems to have in the remaining senses may reflect extra input and connections in the relevant brain areas. "The mammalian brain,' Rakic concludes, "is more malleable than we had thought.'