THE AUTONOMIC NERVOUS SYSTEM
The nervous system of human beings is hard-wired to respond to dangers or threats. These responses are not subject to conscious control and are the same in humans as in lower animals. They represent an evolutionary adaptation to animal predators and other dangers to which all animals, including primitive humans, had to cope.
The most familiar reaction of this type is the fight-or-flight reaction to a life-threatening situation. When people have fight-or-flight reactions, the level of stress hormones in their blood rises. They become more alert and attentive, their eyes dilate, their heartbeat rate increases, their breathing rate increases, and their digestion slows down, making more energy available to the muscles.
This emergency reaction is regulated by a part of the nervous system called the autonomic nervous system, or ANS. The ANS is regulated by the hypothalamus, a specialized part of the brainstem that is among a group of structures called the limbic system. The limb system controls human emotions through its connections to glands and muscles; it also connects to the ANS and higher brain centers, such as parts of the cerebral cortex.
One problem with this arrangement is that the limbic system cannot tell the difference between a real physical threat and an anxiety-producing thought or idea. The hypothalamus may trigger the release of stress hormones from the pituitary gland even when there is no external danger.
When individuals respond to a real danger, their body relieves itself of the stress hormones by facing up to the danger or fleeing from it. In modern life, however, people often have fight-or-flight reactions in situations in which they can neither run away nor lash out physically. As a result, their bodies have to absorb all the biochemical changes of hyperarousal rather than release them. These biochemical changes can produce anxious feelings as well as muscle tension and other physical symptoms of anxiety.
Anxiety can be a symptom of certain medical conditions. For example, anxiety is a symptom of certain endocrine disorders that are characterized by overactivity or underactivity of the thyroid gland. Cushing’s syndrome, in which the adrenal cortex overproduces cortisol, is one such disorder.
Other medical conditions that can produce anxiety are respiratory distress syndrome, mitral valve prolapse, porphyria, and chest pain caused by inadequate blood supply to the heart (angina pectoris).
MEDICATIONS AND SUBSTANCE USE
Numerous medications may cause anxiety-like symptoms as a side effect. They include birth control pills, some thyroid or asthma drugs, some psychotropic agents, corticosteroids, antihypertensive drugs, nonsteroidal anti-inflammatory drugs (such as flurbiprofen and ibuprofen), and local anesthetics. Caffeine can also cause anxiety-like symptoms when consumed in sufficient quantity.
Withdrawal from certain prescription drugs—primarily beta-blockers and corticosteroids—can cause anxiety. Withdrawal from drugs of abuse, including LSD, cocaine, alcohol, and opiates, can also cause anxiety.