Hierarchical Rules in the Specification of Neural Connections in the Central Nervous System

Abstract

The brain embodies the biological bases of mental and behavioral phenomena, and for these to have adaptive value, it is necessary that the development of the brain proceeds normally. What are the mechanisms guiding the establishment of precise nervous connections among millions of neurons? Using the interhemispheric connection through the corpus callosum as an experimental model, evidence was obtained that retinal activity, even that generated spontaneously before eye opening, plays a major role in the normal development of the visual interhemispheric connection. Surprisingly, it was also found that if retinal activity is abolished, interhemispheric linkages are able to form, but according to an abnormal plan. These results suggest that there are several mechanisms capable of specifying different patterns of nervous connections, and that these mechanisms are hierarchically organized. Thus, in the absence of retinal activity, an abnormal pattern of interhemispheric connections develops probably under the guidance of gradients of chemical substances. However, if retinal activity is present, it would supersede the chemical influences and impose the establishment of a normal pattern of interhemispheric connections. These ideas suggest that abolishing or altering mechanisms operating at different hierarchical levels may cause the development of anomalies in the nervous system.