Organization of the neuronal circuits in the central nervous system duringdevelopment

The human brain is a product of genetic instructions, cellular interactions and influences of innate activity and external stimulation. The formation of the neural tube and the patterning of the brain are determined by homeot ic genes. After a prosencephalic phase with the formation of the hemisphere s, the neurons prolipherate to number about 100 billion halfway through ges tation. They also migrate to their final positions in an inside-outside fas hion with the newly formed neurons at the outer layer of the cortex, follow ed by synaptogenesis, programmed cell death and organization of the neurona l circuits. This phase is probably determined not only by genes but also by innate activity, which for example has been detected in the foetal retina: "Cells that fire together wire together while those which don't won't". Conclusion: Development of the neuronal circuits in the CNS can be viewed a s epigenetic, i.e. many different components must come together at the righ t time and place.