Your central nervous system consists of your brain and your spinal cord. Your peripheral nervous system includes the nerves throughout your body. Together they form your nervous system—both an internal network of connections and your sensory link to the external world.

Your brain develops rapidly in infancy and adolescence, but it continues to change throughout your life. The connections formed by your brain's neural pathways, which connect one part of your nervous system with another, continue to change as well. These pathways are very important for understanding depression.