The endocrine system: While the nervous system communicates with the rest of the body through electrical signals, the endocrine system communicates through chemical messengers. There are two basic types of chemical messengers: 1) Hormones travel throughout the entire body through the blood stream. 2) Neurotransmitters are chemical messengers that work in the brain.
The adrenal glands are part of the endocrine system. When the SNS is activated, it causes the adrenal glands to release two hormones called adrenaline and noradrenaline. These two chemicals provide the extra fuel the body needs as it "revs-up" for action. These two chemicals are very similar. They work to increase heart rate and blood pressure. Because these chemicals are also found in the brain, adrenaline and noradrenaline are considered neurotransmitters as well.
In addition to these two hormones, other hormones also activate various mechanisms throughout the body in response to stress. One stress-related hormone is corticotropin-releasing hormone (CRH). So far, most research on CRH has been conducted with animals (specifically rodents). This research suggests that high levels of CRH are associated with anxiety-related behaviors. Therefore, it has been hypothesized that dysfunction, or dysregulation of CRH may cause increased anxiety in human beings as well. In addition, CRH helps turn-on the hypothalamic-pituitary-adrenocortical (HPA) axis. The HPA is another part of the neuroendocrine system that is also involved in the stress response. It has been linked to anxiety and mood disorders such depression. While these are promising areas of investigation, this research is highly complex and beyond the scope of this article. Nonetheless, our ever-increasing understanding of the human brain has advanced our understanding of psychiatric disorders, including anxiety disorders.
The second important type of chemical messenger is neurotransmitters. Neurotransmitters are the brain's communication system. They cause one nerve (neuron) to communicate with another, telling it what to do. It is believed that some symptoms of psychiatric disorders are created by imbalances, or improper amounts of these neurotransmitters. The neurotransmitter that is often associated with anxiety is serotonin. Serotonin is also known for its impact on mood, appetite, and sleep. It is thought that people with anxiety disorders have decreased levels of serotonin. That is why medications that are commonly prescribed for anxiety disorders are Selective Serotonin Reuptake Inhibitors (SSRIs). These medications serve to increase the levels of available serotonin. More information about these medications can be found in the treatment section on pharmacologic treatments.
In addition to serotonin, GABA, or gamma-aminobutyric acid, has also been linked to anxiety. GABA works to slow neural transmission down and "calm" the brain. This in turn relaxes the body. People who experience chronic anxiety may have a GABA deficiency. It has been proposed that people with this deficiency might experience more anxiety because their body is already in a heightened state of arousal and vigilance, thus producing a biological vulnerability to increased levels of stress. A class of medication known as benzodiazepines (such as Ativan®, Xanax®, and Klonopin®), which are often prescribed for anxiety disorders, are thought to increase the release of GABA thus causing a relaxation effect. The use of medication in the treatment of anxiety disorders is discussed another section.