Historicalconcept Of Stress

Life exists by maintaining a complex of dynamic equilibrium or homeostasis that is constantly challenged by internal and external adverse stimuli;[1] often these stressful conditions are too demanding for the animal to adapt. However, animals have evolved mechanisms that enable them to adapt to the numerous stressors in their lives. An animal can initiate several types of biological responses to alleviate stress. These responses often result in shifts or alterations in biological resources that are normally used for other basal functions. Thus, under most circumstances the biological cost (in terms of biological function) is minimal for acute stressors, but during prolonged stress the cost is significant, thus leading to a prepathological or pathological state.[2] The stress response elicited by a stressor protects the animal and restores homeostasis, thus enhancing the probability of survival.

The stress response initiated by a stressor results in the release of neurotransmitters and hormones that serve as the central nervous system's (CNS) messengers to other parts of the body. The CNS obtains information from the external environment and signals to the organism that a particular danger or threat to homeostasis has been perceived. The perception of the threat is mostly related to prior experience and the physiological state of the animal (Fig. 1). Once the threat has been perceived, adaptive responses are initiated by evoking well-orchestrated defenses that include behavioral and physiological adjustments. Neuroendocrine changes are initiated to meet energy requirements for behavioral responses and to maintain homeostasis. It is the final stage of the stress response that determines whether the animal is simply experiencing a brief disruption in ho-meostasis with no significant consequences or experiencing extreme difficulty, which may lead to the development of disease. Oftentimes, the consequences of the stress response are adaptive in nature. However, if the animal reaches a state in which the intensity and duration of the stressor is severe and uncontrollable, compromising health and reproduction, this condition may lead to development of a prepathological state or pathology.

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