The mind and body interact in powerful ways that affect a person's health. The digestive system is profoundly controlled by the mind (brain), and anxiety, depression, and fear dramatically affect the function of this system (see Overview of the Digestive System). Social and psychologic stress can trigger or aggravate a wide variety of diseases and disorders, such as diabetes mellitus, high blood pressure, and migraine headache. However, the relative importance of psychologic factors varies widely among different people with the same disorder.
Most people, on the basis of either intuition or personal experience, believe that emotional stress can cause or alter the course of even major physical diseases. How these stressors do this is not clear. Emotions obviously can affect certain body functions, such as heart rate, blood pressure, sweating, sleep patterns, stomach acid secretion, and bowel movements, but other relationships are less obvious. For example, the pathways and mechanisms by which the brain and immune system interact are only beginning to be identified. It is remarkable that the brain can alter the activity of white blood cells and thus an immune response because white blood cells travel through the body in blood or lymph vessels and are not attached to nerves. Nevertheless, research has shown that the brain does communicate with the white blood cells. For example, depression may suppress the immune system and make a person more susceptible to infections such as the common cold.
Stress can cause physical symptoms even though no physical disease may be present because the body responds physiologically to emotional stress. For example, stress can cause anxiety, which then triggers the autonomic nervous system and hormones such as epinephrine to speed up the heart rate and to increase the blood pressure and the amount of sweating. Stress can also cause muscle tension, leading to pain in the neck, back, head, or elsewhere.
The mind-body interaction is a two-way street. Not only can psychologic factors contribute to the onset or aggravation of a wide variety of physical disorders, but also physical diseases can affect a person's thinking or mood. People with life-threatening, recurring, or chronic physical disorders commonly become depressed. The depression may worsen the effects of the physical disease and add to a person's misery.