In organisms, the term stress refers to a heightened physical or
mental state produced in an organism by a change in its internal or external environment. Even lower organisms such as yeast exhibit stress. Yeast responds to temperature increases, for example, by producing protective proteins. In evolution, stress may result in the appearance of better adapted species. Human stress is discussed here.
In humans, physical stress
results from injuries or illnessesÑfor example, bone fractures or insulin shock. Psychological stress results from perceived or anticipated threats. The stress may be acute, as in response to immediate danger, or chronic, as when an individual is experiencing an unhappy life situation. In either case the body mechanisms are similar.
chronic physical illness is almost always accompanied by significant psychological effects. Long-lasting psychological stress, in turn, often leads to debilitating changes. Milder stresses, on the other hand, are considered a normal part of life. A number of themÑphysical exercise, sexual intercourse, competitive mental performances, and creative acts such as dancingÑare generally highly regarded.
The common denominator of the stress response is the hypothalamus, a part of the base of the
brain that overlies the pituitary gland. The
hypothalamus receives impulses from the rest of the brain. Thus, when a threatening object is interpreted by the higher cortical centers as dangerous, they send signals not only to the motor cortex (to cause the individual to begin running) but also to the hypothalamus. This structure contains many nucleiÑcollections of nerve cellsÑthat control vital activities such as respiration and heart rate. To a large extent this is done by stimulating the adrenal medulla through the sympathetic nervous system and causing it to secrete adrenaline (epinephrine) and noradrenaline (norepinephrine). These hormones augment an increase in heart rate by the sympathetic nervous system, widening the pupils of the eye and even causing the liver to break down glycogen into glucose for a quick supply of energy.
Whether a stress arises from a cause perceived by sense organs that relay the danger to the brain or from mental actions such as worry, the course of events can be the same, even though occurring at different levels of intensity.
The hypothalamus synthesizes corticotropin releasing hormone, which goes directly to the anterior pituitary gland. There it results in the manufacture of adrenocorticotropic
hormone (ACTH).€TH stimulates the adrenal cortex to produce cortisol, a glucocorticoid. This hormone affects carbohydrate metabolism, protein metabolism, and fat
metabolism and is a necessary part of the chronic stress reaction.
Responses to stress may be modulated by psychotropic drugs (see psychopharmacology) or by psychotherapy. The best results occur, however, when the cause of the stress can be removed or ameliorated.
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