The Brain Builds Itself

The adult human brain is built of about 100 billion nerve cells (neurons), most of which were made before we were born. But not all of these neurons were created equal. As the fertilized human egg divides, it generates many types of primitive neurons, each of which is destined to play a particular role in the brain. Having been assigned their approximate fates by a process that turns on and off specific genes, the primitive neurons migrate to their designated places guided by chemical signals that they selectively respond to. When they get there, they start building connections with other neurons to form the neuronal circuits and networks that are the basis of all our behavior.

To build these connections, the neurons make branches called dendrites to receive signals, and other branches called axons to send signals. Dendrites are short and studded with spines. Axons can be long enough to reach other neurons anywhere in the brain, and to embrace them with clusters of little nerve endings, called boutons. Signaling between boutons of one neuron and dendrites of another occurs at structures called synapses.