The brain and childhood depression. Illustration of the structures of the limbic system in the brain of a depressed child. The limbic system is the complex network of nerve pathways involved with expression of human instinct, mood, and the stress response. These regions, found deep inside the brain, include the reticular formation (purple), the thalamus(green), the hypothalamus, the cingulate cortex, the mammillary body, the amygdala, and the hippocampus (all red). Activities governed by the limbic system include self-preservation, reproduction, emotional life, and the formation of long-term memories.