Stress may be defined as a nonspecific response to perceived environmental threats (called stressors). But a particular environmental change (a demand or an event) may be perceived by one person as stressful and by another as benign. An examination is, for example, likely to be less stressful for a student who has mastered all homework assignments than it is for a student who waits to cram the night before the test. The generalized feeling of fear and apprehension associated with a stressor is called anxiety. Anxiety is typically accompanied by activation of the sympathetic nervous system and increased physiological arousal, which causes rapid breathing, increased heart rate, sweating, and dilation of the pupils.
Fight or flight. In the 1920s, Walter Cannon recognized that the autonomic nervous system is activated in response to stress and suggested that stress mobilizes the body's responses in readiness for either attacking ( fight) or fleeing ( flight) an enemy or threatening situation. Although such responses may have promoted survival when they evolved in human history, they are not productive given the longer periods of stress exposure common in modern life. Such enterprises as keeping a job, going to school, and playing on the soccer team require more complex responses.
The general adaptation syndrome. Hans Selye is credited with identifying the body's reaction to stress with a syndrome he called the general adaptation syndrome, which has three phases, as evidenced by the level of stress hormones (Figure 1).
The General Adaptation Syndrome
alarm: The body first organizes physiological responses (similar to fight‐or‐flight responses) to threat.
resistance: Stress‐activated responses continue, stabilizing the body's adaptations to stress.
exhaustion: The body has depleted its reserves and can no longer maintain responses to the stressors.
During the alarm phase, when the body is first aroused, the hypothalamus sends signals to the pituitary gland. This endocrine gland in turn secretes adrenocorticotrophic hormone (ACTH), which travels via the bloodstream to the cortex (outer layer) of the adrenal glands, where corticosteroids are released. The hypothalamus also activates the adrenal medulla, the central part of the adrenal gland, which causes adrenaline (epinephrine) to be released and the activation of the sympathetic nervous system. After maintaining high levels of the hormones for a long time, the body loses its ability to do so and exhausts its resources. Selye is credited with identifying the fact that the incidence of certain types of diseases (stress‐related diseases, such as some types of coronary disorders) increases during this stage of exhausted body resources and that a second stressor introduced during the resistance phase or the exhaustion phase further increases that incidence.
Evidence demonstrates that prolonged stress also affects the ability of the immune system to function adequately and can affect the release of other neurotransmitters such as serotonin. Stress may also affect the release of endorphins, chemicals similar in structure to morphine and other opiate drugs used in the modulation of pain.