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STRESS AND STRESS SYNDROME- DEFINITIONS AND PHENOMENOLOGY

All vital physiologic systems of the body are inherently programmed, through rigorous fine-tuning achieved during evolution, to preserve a predefined steady state (homeostasis or eustasis), which is essential for life and well-being [1-3]. This optimal equilibrium is constantly challenged by adverse forces which are intrinsic or extrinsic, real or even perceived, and are described as stressors [1]. Thus, stress is defined as a state of disharmony (cacostasis or allostasis) and is counteracted by an intricate repertoire of physiologic and behavioral responses which aim to maintain/reestablish the threatened homeostasis (adaptive stress response) [1]. This adaptive stress response is mediated by a complex and interconnected neuroendocrine, cellular, and molecular infrastructure which constituents the stress system and is located in both the central nervous system (CNS) and the periphery [1, 2]. The adaptive response of each individual to stress is determined by a multiplicity of genetic, environmental, and developmental factors. Changes in the ability to effectively respond to stressors (e.g. inadequate, excessive and/or prolonged reactions) may lead to disease. Moreover, highly potent and/or chronic stressors can have detrimental effects on a variety of physiologic functions, including growth, metabolism, reproduction, and immune competence, as well as on behavior and personality development. Of note, prenatal life, infancy, childhood, and adolescence are critical periods in the process of forming the matrix of the adaptive stress response, characterized by high plasticity of the stress system and increased vulnerability to stressors.

The stress system receives and integrates a great diversity of neurosensory (i.e. visual, auditory, somatosensory, nociceptive, and visceral), blood-borne, and limbic signals which arrive at the various stress system centers/stations through distinct pathways. Acute stress system activation triggers a cluster of time-limited changes, both behavioral and physical, which are rather consistent in their qualitative presentation and are collectively defined as the stress syndrome [1-4]. Under normal conditions these changes are adaptive and improve the chances of survival. Initially, the stimulation of the stress system components follows a stressor-specific mode; however, as the potency of the stressor(s) increases the specificity of the adaptive response decreases in order to eventually present the relatively nonspecific stress syndrome phenomenology which follows exposure to potent stressors.

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Behavioral adaptation includes enhanced arousal, alertness, vigilance, cognition, focused attention, and analgesia, whilst there is concurrent inhibition of vegetative functions, such as feeding and reproduction. In parallel, physical adaptation mediates an adaptive redirection of energy and body resources. As such, increases in the cardiovascular tone, respiratory rate and intermediate metabolism (gluconeogenesis and lipolysis) work in concert to promote this redirection of vital substrates, while energy consuming functions (e.g. digestion, reproduction, growth, and immunity) are temporally suppressed. Thus, oxygen and nutrients are primarily shunted to the CNS and to stressed body site(s) where they are needed the most.

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In addition to the adaptive stress response, restraining forces are also activated during stress to prevent a potential excessive response of the various stress system components [1-4]. The ability to timely and precisely develop restraining forces is equally essential for a successful outcome against the imposed stressor(s), since prolonging the mobilized adaptive stress response can turn maladaptive and contribute to the development of disease.

Interestingly, the mobilization of the stress system is often of a magnitude and nature that allows the perception of control by the individual. Under such conditions, stress can be rewarding and pleasant, or even exciting, providing positive stimuli to the individual for emotional and intellectual growth and development [5]. Thus, it is not surprising that the stress system activation during feeding and sexual activity, both sine qua non functions for survival, is primarily linked to pleasure.

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