Autism spectrum disorder

Autism spectrum disorder, often shortened to ASD and simply called autism by many people, is a neurological disorder, meaning it involves the nervous system. It's also a developmental disorder: it has to do with how the brain takes shape as a person grows. It changes the way someone relates to other people, communicates, learns, and behaves, and its symptoms usually show up in the first two years of life.

To see what's going on inside, you have to look at genes, the instructions the body uses to build itself. Many genes linked to ASD are involved in building the brain itself. Some influence how many neurons get made; neurons are the cells that carry the brain's messages. Others influence synapses, the contact points where one neuron connects to another to pass its message along. Picture a city under construction: some blueprints set how many houses go up, and others set how the wiring runs between them.

Studies indicate that in some people with ASD, extra neurons appear while the brain is forming, and the cortex grows too much. The cortex is the brain's outer layer, and this overgrowth happens in its frontal part, behind the forehead, and in its temporal part, along the sides of the head. This is thought to explain the differences in how people with ASD socialize and communicate. Notice the words "thought to": this is a proposed explanation, not a pathway that has been proven step by step.