The neurotransmitters serotonin, norepinephrine, and dopamine are produced in several places deep in the brain. From their production sites they travel throughout the brain, as shown in simplified form here. Serotonin (red),which helps adjust body temperature and influences sleep and mood, and norepinephrine (blue), associated with the fight-or-flight response, go to the limbic system as well as throughout the cortex and into the cerebellum at the back of the brain. Dopamine (green) is also found in the limbic system and frontal cortex and in the basal ganglia, which help control skeletal muscles. It is mainly produced in an area called the substantia nigra. In Parkinson's disease, these cells are lost, resulting in a dopamine deficiency.