Organization of the neuronal circuits in the central nervous system during development

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 homeotic genes. After a prosencephalic phase with the formation of the hemispheres, the neurons prolipherate to number about 100 billion halfway through gestation. They also migrate to their final positions in an inside-outside fashion with the newly formed neurons at the outer layer of the cortex, followed by synaptogenesis, programmed cell death and organization of the neuronal 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".

Development of the neuronal circuits in the CNS can be viewed as epigenetic. i.e. many different components must come together at the right time and place.