منتدى استراحات زايد - نداء وأجتماع لطلاب الثاني عشر ( علمي

Stress is the consequence of the failure to adapt to change, specifically the inability to respond appropriately to emotional or physical threats to the organism, whether actual or

imagined.[1] Common stress symptoms include irritability, muscular tension, inability to concentrate and a variety of physical reactions, such as headaches and accelerated heart rate.[2]

The term stress was first used by the endocrinologist Hans Selye in the 1930s to identify physiological responses in laboratory animals. He later broadened and popularized the concept to include the perceptions and responses of humans trying to adapt to the challenges of everyday life. Stress, in Selye's terminology, refers to the reaction of the organism, and stressor to the perceived threat.

Stress in certain circumstances may be experienced positively. Eustress, for example, can be an adaptive response prompting the activation of internal resources to meet challenges and achieve goals.
Hans Selye researched the effects of stress[3] on rats and other animals by exposing them to unpleasant or harmful stimuli. He found that all animals presented a very similar series of reactions, broken into three stages. He describes this universal response to the stressors as the General Adaptation Syndrome, or GAS, in 1936.[4][5]

Alarm
This is the 1st stage. When the threat or stressor is identified or realized, the body's stress response is a state of alarm. During this stage adrenaline will be produced in order to bring about the fight-or-flight response. There is also some activation of the HPA axis, producing cortisol.
Resistance
This is the 2nd stage. If the stressor persists, it becomes necessary to attempt some means of coping with the stress. Although the body begins to try to adapt to the strains or demands of the environment, the body cannot keep this up indefinitely, so its resources are gradually depleted.
Exhaustion
This is the 3rd stage. In the final stage in the GAS model, all the body's resources are eventually depleted and the body is unable to maintain normal function. At this point the initial autonomic nervous system symptoms may reappear (sweating, raised heart rate etc.). If stage three is extended, long term damage may result as the capacity of glands, especially the adrenal gland, and the immune system is exhausted and function is impaired resulting in decompensation. The result can manifest itself in obvious illnesses such as ulcers, depression, trouble with the digestive system or even cardiovascular problems, along with other mental illnesses.

[edit] Lazarus
Richard Lazarus published in 1974 a model dividing stress into eustress and distress.[6] Where stress enhances function (physical or mental, such as through strength training or challenging work) it may be considered eustress. Persistent stress that is not resolved through coping or adaptation, deemed distress, may lead to anxiety or withdrawal (depression) behavior. The difference between experiences which result in eustress or distress is determined by the disparity between an experience (real or imagined), personal expectations, and resources to cope with the stress. Alarming experiences, either real or imagined, can trigger a stress response.[7] Therefore, Lazarus's model argues that cognitive processes of appraisal are central in determining whether a situation is potentially threatening or harmful.[8]

[edit] Zajonc
Robert B. Zajonc (1984), somewhat in opposition to the Lazarus model of stress, argued that emotional reactions occur before cognitive reactions, and in fact, may be at odds with cognitive responses. This belief was consonant with the previous James-Lange hypothesis (1890, 1922), which held that the body's emotional reaction to stress occurred prior to and resulted in conscious responses.

The debate has underscored the existence of two modes of reactivity, one conscious and under volitional control, and the other automatic and uncontrollable. Scholars such as Aldwin have argued for a simultaneous, parallel processing approach rather than a sequential neurological processing model where emotions come first followed by cognition, or vice versa.[8]

[edit] Neurochemistry and physiology
The neurochemistry of the stress response is now believed to be well understood, although much remains to be discovered about how the components of this system interact with one another, in the brain and throughout in the body.

In response to a stressor, corticotropin-releasing hormone (CRH) and arginine-vasopressin (AVP) are secreted into the hypophyseal portal system and activate neurons of the paraventricular nuclei (PVN) of the hypothalamus. The locus ceruleus and other noradrenergic cell groups of the adrenal medulla and pons, collectively known as the LC/NE system, also become active and use brain epinephrine to execute autonomic and neuroendocrine responses, serving as a global alarm system.[9]

The autonomic nervous system provides the rapid response to stress commonly known as the fight-or-flight response, engaging the sympathetic nervous system and withdrawing the parasympathetic nervous system, thereby enacting cardiovascular, respiratory, gastrointestinal, renal, and endocrine changes.[9]

The so-called hypothalamic-pituitary-adrenal axis (HPA), a major part of the neuroendocrine system involving the interactions of the hypothalamus, the pituitary gland, and the adrenal glands, is also activated by release of CRH and AVP. This results in release of adrenocorticotropic hormone (ACTH) from the pituitary into the general bloodstream, which results in secretion of cortisol and other glucocorticoids from the adrenal cortex. These corticoids involve the whole body in the organism's response to stress and ultimately contribute to the termination of the response via inhibitory feedback.[9]

Stress can significantly affect many of the body's immune systems, as can an individual's perceptions of, and reactions to, stress. The term psychoneuroimmunology is used to describe the interactions between the mental state, nervous and immune systems, as well as research on the interconnections of these systems.

Chronic stress has also been shown to impair developmental growth in children by lowering the pituitary gland's production of growth hormone, as in children associated with a home environment involving serious marital discord, alcoholism, or child abuse.[10