During development, neurons are generated at the rate of 250,000 per minute. By birth, the neurons are organized into major networks of communication. Information in the form of electrical impulses flows through these networks at lightning speed. The impulses trigger the release of chemical neurotransmitters at the synapses, or junctions between an axon and another cell. The transmitter then passes to the target cell.
Genes establish the major pathways between parts of the brain.
More precise and detailed connections depend on environmental stimulation-or how you use your brain. In studies of the visual system in animals, for example, some pathways emerge in the fetus, while others come later. For instance, certain neurons respond to impulses from both eyes in newborn cats and monkeys, but in adults, those neurons respond to one eye only. Apparently, the neurons withdraw some branches and grow others, a process of fine tuning which is partly due to visual stimulation.